Hurricanes (Facts on File Dangerous Weather Series)

Dangerous Weather

Hurricanes Revised Edition

Michael Allaby ILLUSTRATIONS by Richard Garratt

For Ailsa —M.A. To my late wife, Jen, who gave me inspiration and support for almost 30 years —R.G. Hurricanes, Revised Edition Copyright © 2003, 1997 by Michael Allaby All rights reserved. No part of this book may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage or retrieval systems, without permission in writing from the publisher. For information contact: Facts On File, Inc. 132 West 31st Street New York NY 10001 Library of Congress Cataloging-in-Publication Data Allaby, Michael. Hurricanes / Michael Allaby.—Rev. ed. p. cm.—(Dangerous weather) Includes bibliographical references and index. ISBN 0-8160-4795-2 1. Hurricanes. I. Title. QC944 .A44 2003 551.55′2—dc21

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Facts On File books are available at special discounts when purchased in bulk quantities for businesses, associations, institutions, or sales promotions. Please call our Special Sales Department in New York at (212) 967-8800 or (800) 322-8755. You can find Facts On File on the World Wide Web at http://www.factsonfile.com Text design by Erika K. Arroyo Cover design by Nora Wertz Illustrations by Richard Garratt Printed in the United States of America VB Hermitage 10

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Contents Preface: What is a hurricane? Introduction

WHY HURRICANES ARE TROPICAL What happened when Mitch struck? Convection Lapse rates and stability Where hurricanes happen Intertropical convergence and the equatorial trough Jet stream Hurricane and storm tracks Weather fronts Global wind systems

AIR AND SEA Ocean currents and sea-surface temperature General circulation of the atmosphere Trade winds and doldrums George Hadley and Hadley cells Potential temperature Warming, convection, and low pressure Adiabatic cooling and warming Air pressure, highs, and lows Storm clouds How clouds are classified Latent heat and dewpoint Evaporation, condensation, and the formation of clouds

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1 1 4 6 9 11 16 16 19 22

24 24 26 33 35 36 37 41 43 46 47 50 52

INSIDE THE STORM How a hurricane begins Christoph Buys Ballot and his law Vortices The Coriolis effect Conservation of angular momentum What happens inside a hurricane Why the wind blows Wind force and Admiral Beaufort

57 57 59 64 69 71 72 73 78

HURRICANES, TYPHOONS, AND CYCLONES

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Hurricanes in the United States and Caribbean Hurricanes that reach Europe Air masses and the weather they bring Depressions and the jet stream Asian typhoons and cyclones Monsoon Arctic and Antarctic hurricanes

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WHAT A HURRICANE CAN DO Hurricane damage Kinetic energy and wind force Saffir/Simpson Hurricane Scale Daniel Bernoulli and how hurricanes can lift roofs Storm surges Historic hurricanes

LIVING WITH FIERCE STORMS How hurricanes are named and tracked How hurricane damage is predicted Will global climate change bring more hurricanes? El Niño The solar spectrum Protection and safety

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Appendixes Hurricanes in history Cyclone names SI units and conversions

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Bibliography and further reading

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Index

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Preface What is a hurricane? It begins far out at sea, way out to the east, over the central Atlantic. At first it amounts to nothing much, merely a body of air in which the atmospheric pressure is lower than in the surrounding air. This is a depression. There are no fronts associated with it, so it has not formed at the boundary between two large air masses with different temperatures and pressures, but for all that it is unexceptional. Had it formed farther north, say at around 50° N, it would be no different from the many depressions that each year drift eastward across the Atlantic. Some fill, as air flows into them, increasing the pressure until it is the same as that in the surrounding air, and no more is heard of them. Others bring dull weather, with rain or snow, to northwestern Europe. If there is a large difference in pressure between the depression and the surrounding air, they may also bring wind to drive the rain and make sure anyone caught outdoors without rain gear is thoroughly soaked. Depressions are unpleasant, but they cause no harm. They are not dangerous. But this is not 50° N. It is the Tropics, and already the depression is being watched carefully by scientists alert for the first sign of trouble. They study photographs of the clouds forming inside the disturbance, transmitted by a satellite high above. These allow the scientists to trace how the depression develops and track its movements. Ships and aircraft passing through the system radio measurements of the winds, pressure, and air temperature around them to the weather center. Perhaps the depression will fill. In that case it and the clouds it produces will vanish from the satellite pictures. This depression does not fill. The pressure at its center falls further. After a couple of days it has dropped by 20 mb. Again, a drop in pressure of this magnitude is not uncommon in middle latitudes, but in the Tropics it is unusual. At this point the meteorologists start to pay even closer attention.

Starting to turn As air pressure falls, the depression begins to turn counterclockwise, rotating majestically around its own center, and the prevailing easterly winds start to carry it westward. Air is drawn into the region of low pressure, its speed proportional to the difference in pressure inside and outside the depression. Falling pressure causes wind speeds to increase.

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Hurricanes Clouds grow around the center, building into great, towering masses. Seen from space, they form a spiral shape. Beneath them, the rain falls in torrents. When the wind speed exceeds about 25 MPH (40 km/h), the weather system is officially classified as a tropical depression and identified by a number, for example TD14. When the wind speed exceeds about 40 MPH (64 km/h), it becomes a tropical storm and may be given a name. Tropical storms are fairly common during the summer and fall. Forecasters considered 2001 an average year. There were 15 named tropical storms over the Atlantic in that year, of which nine grew into hurricanes. Four of those were classed as major hurricanes—Saffir/Simpson category 3 or higher. In 2000 there had been 15 tropical storms. Forecasters expected 2002 to be another average year for Atlantic storms. They predicted there would be 11 named tropical storms, of which six would develop into hurricanes, two of them major. Pressure continues to fall and wind speed continues to increase. When it exceeds 74 MPH (119 km/h), the tropical storm becomes a hurricane. Of the 15 tropical storms in 2000, eight grew into hurricanes. Still it moves westward and still it is strengthening. Now the spiral of cloud is about 125 miles (202 km) in diameter. As it enters the Caribbean and approaches the first of the inhabited islands, it begins to swing north.

Lashing rain, screaming wind It arrives with lashing rain and a screaming wind. Trees are uprooted and thrown about like sticks. Flimsy buildings are demolished. Roofs are lifted from more substantial houses, cars and trucks are overturned, windows are shattered, and flying debris adds to the damage caused by the wind itself. Then the sea arrives. Driven by the wind, huge waves sweep over lowlying coastal areas. Winds of more than 100 MPH (160 km/h) can raise 15-foot (4.5-meter) waves and these may combine with the tide to produce a massive storm surge, with spray and foam blowing still farther inland. By now the hurricane has reached its maximum ferocity. Its winds are much stronger on the right-hand side than on the left-hand side. As the diagram shows, this is because the hurricane itself is moving in a generally westward direction. To the right of its center, the speed of its own movement adds to the wind speed. To its left, the wind blows in the opposite direction to that in which the hurricane is traveling, so wind speed is reduced. It turns north, heading now toward the coast of the United States and devastating the islands in its path. In September 1999, Hurricane Floyd struck the Bahamas with 155-MPH (250 km/h) winds, then moved northward along the U.S. coast as far as New York and New Jersey. It caused at least 57 deaths and meant 2.3 million people had to be evacuated from their homes in Florida, Georgia, and the Carolinas. Estimates of the cost of the damage it caused to property ranged from $3 billion to more than $6 billion. Better preparedness and highly efficient emergency services mean hurricanes are less deadly than they used to be. In 1900, a hurricane caused 8,000 deaths in Galveston, Texas.

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direction of storm movement (northwest) speed 55 mph 230

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Wind speeds around a hurricane. The movement of the whole storm means the wind speed is higher on one side than on the other. Once it moves inland, however, the hurricane is doomed. It needs water to sustain it, and away from the sea it is starved. Slowly it weakens and eventually dies, although it may retain enough force to cause substantial damage as far north as Pennsylvania. A hurricane releases no radiation, of course, but it may have as much energy as a one-megaton hydrogen bomb, and it can be as destructive. It is the largest, fiercest storm our atmosphere is capable of producing.

Introduction Several years have passed since the first edition of this book was published. Much has happened during those years, and the decision to update the book for a second edition gives me a welcome opportunity to report at least some of them. In doing so, I have substantially altered, expanded, and in some places rewritten the original text. There have been more hurricanes, of course. I wrote the first edition before Hurricane Mitch devastated Central America in 1998. That was one of the most savage storms of the century. Overall, though, hurricanes are not growing fiercer or more frequent. Indeed, in some parts of the world they are now much rarer than they used to be. All the same, no matter how the climates of the world may change in years to come, there is little chance that hurricanes will disappear entirely. Climate research has intensified in recent years. Concern over the possibility that we may be altering the global climate has stimulated funding agencies to increase the resources available for evaluating the likelihood of global warming and its consequences. If we are to understand the extent of this threat—if it is a threat—scientists need to learn much more about the ways the Sun, atmosphere, and oceans interact to produce our day-to-day weather. New discoveries are now being made at an unprecedented rate, and, although there is still a long way to go before the global climate is fully understood, we are learning more about it almost every day. This new edition takes account of the most recent relevant findings. Updating the text has also given me an opportunity to expand it in order to provide more detailed explanations. Mitch deserves, and receives, a chapter to itself. I have retained the use of sidebars to display detailed explanations or interesting items of information without interrupting the main flow of the text. This edition contains more sidebars than were found in the first edition. These explain concepts from atmospheric science, such as potential temperature, the link between the jet stream and midlatitude depressions, why the wind blows at all, and the conservation of angular momentum—the principle that explains the way hurricane winds accelerate as air spirals inward. Measurements are given in familiar units, such as pounds, feet, miles, and degrees Fahrenheit, throughout the book, but in each case I have added the metric or scientific equivalent. All scientists now use standard international units of measurement. These may be unfamiliar, so I have added them, with their conversions, as an appendix. I have also

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Hurricanes added appendices that list some of the most notorious hurricanes of the past and the set of names that are used to identify tropical cyclones in different parts of the world. The first edition contained no suggestions for further reading. These have been added in this edition. The sources include a number of books that you may find useful, but a much larger number of Web addresses. If you have access to a computer, these will allow you to learn more about hurricanes and about climate generally quickly and free of charge. We have decided to omit the photographs from the first edition and instead to increase the number of diagrams and maps. These provide more useful information than photographs of such subjects as flying roofs and wind-bent palm trees. My friend and colleague Richard Garratt has drawn all of the illustrations. As always, I am deeply grateful to him for his skill in translating my crude drawings into such accomplished artwork. I am grateful, too, to Frank K. Darmstadt, my editor at Facts On File, for his hard work, cheerful encouragement, and patience. If this “new, improved” edition of Hurricanes encourages you to pursue your study of the weather further, it will have achieved its aim and fulfilled my highest hopes for it. I hope you enjoy reading the book as much as I have enjoyed writing it for you. — Michael Allaby Tighnabruaich Argyll, Scotland www.michaelallaby.com

What happened when Mitch struck?

WHY HURRICANES ARE TROPICAL What happened when Mitch struck? It helps to know the storm is approaching. You can bolt the doors, seal the windows, and take shelter or simply evacuate. Nowadays the warnings from the National Weather Service give people ample time to prepare. That is why hurricanes cost fewer lives today than did those that people endured a century ago. But there are exceptions. Very rarely there is a storm so ferocious and so big that for many people there can be no escape. A storm like that happened in 1998. That was the year the Caribbean was hit by the deadliest storm to strike the region since the Great Hurricane of October 1780. It disrupted communications to such an extent that a week passed before the outside world received the first news of the scale of the devastation. Mitch was born on October 8, as a disturbance called a tropical wave in the airflow over southern West Africa. The wave crossed the African coast, then traveled over the Atlantic, where the west-southwesterly highlevel winds prevented it from developing further. It passed through the eastern Caribbean Sea on October 18, and by October 20 satellite images showed that an organized cloud pattern was developing. On October 21, what was then a tropical depression developed in the southern Caribbean Sea. By the following day the depression had intensified. When the wind speed around its center exceeded 25 MPH (40 km/h), the depression was reclassified as a tropical storm. It had become Tropical Storm Mitch, its cloud pattern marking it clearly on the photographs being transmitted to the storm watchers on the ground by the U.S. GOES-8 satellite, stationary over the equator. Mitch was hardly moving, but already it was vigorous. Its winds were blowing at about 45 MPH (72 km/h)—a fresh gale on the Beaufort scale of wind force—and strengthening fast. The pressure at the center had dropped to 29.5 inches of mercury (1,000 millibars). This is only slightly lower than the average sea-level pressure, of 29.9 inches of mercury (1,013.25 mb). At that stage Mitch was far out at sea, approximately due south of the western tip of Jamaica and due east of central Nicaragua. During October 23, Mitch drifted a little to the northeast, farther away from the Central American mainland, and it continued to intensify. Its core pressure fell to 29.4 inches of mercury (997 mb). The drop

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Rain and mud Mitch moved very slowly during the last few days before it crossed the coast. Inflowing air gradually filled the depression, and as the core

What happened when Mitch struck?

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Track of Hurricane Mitch, October –November 1998 pressure rose, so the winds slackened. Convection continued, however (see sidebar). Convection produced and sustained the towering storm clouds— the clouds that make all hurricanes so clearly visible in satellite images. The storm clouds produced torrential rain. The wind may have weakened, but the rain was incessant. In total, Mitch is estimated to have dumped about 75 inches (1,900 mm) of rain onto Central America. In places the rain was sometimes falling at the rate of 12–24 inches (305–610 mm) a day.

Convection When a fluid—a gas or liquid—is heated, its molecules move faster and farther between collisions. Consequently, the fluid expands. Expansion means that a given volume of the fluid contains fewer molecules than it did before the fluid expanded. This must be so, because the same amount of fluid is now taking up more space. Because it contains fewer molecules, the expanded fluid is less dense than fluid that has not expanded. At this point gravity takes over. Volume for volume, a dense fluid weighs more than one that is less dense. Place the two side by side and the dense fluid will sink downward, displacing the less dense fluid. This pushes the less dense fluid upward. So the dense fluid sinks and the less dense fluid rises. It is not that the less dense fluid rises of its own accord, simply because it has become less dense. For that to happen some kind of antigravitational force would have to be acting on it. It does not rise by itself, but is pushed upward by the denser fluid sinking beneath it.

Suppose that a fluid is heated from below. This is the way the atmosphere is heated, by contact with the land and sea surface. The air in contact with the surface grows warmer, so it expands and becomes less dense. Denser air sinks to displace it, and so the warm air rises. As it rises, the air moves farther and farther from the source of heat, and so it cools and as it does so its density increases (see sidebar, page 41). Meanwhile, the air that sank to displace it is now in contact with the source of heat and is being warmed. So it now expands, denser air displaces it, and the process continues. This is convection, one of the three processes by which heat is transferred from one place to another (the others are radiation and conduction). Once the pattern is established, fluid circulating vertically by convection forms what is called a convection cell. The diagram shows what happens. Air in contact with the surface of the warm, tropical ocean is heated and rises by convection. Convection is the mechanism that sustains a hurricane.

warm water rises, cools at surface, then sinks

Convection

heat

What happened when Mitch struck? As Mitch approached Honduras, the first place to suffer was the popular vacation island of Guanaja, where tourists enjoy sailing and diving in ordinarily tranquil tropical waters. Guanaja, 11 miles long and 3.5 miles wide (18 × 6 km), about 40 miles (64 km) from the coast, took the full force. The island was devastated, and for a time it lost all contact with the outside world. Then the hurricane reached the mainland of Honduras, close to its border with Nicaragua. Honduras is a mountainous country, and as Mitch moved inland it was forced to rise as it crossed the mountains. This made the air even more unstable (see sidebar), squeezing still more water out of the sky. Mitch crossed Honduras, then attacked Guatemala, Belize, and El Salvador. By November 2, the storm had weakened to nothing more alarming than a tropical depression. That is when, still drifting northwestward, it crossed the Yucatán Peninsula in Mexico and reached the warm waters of the Bay of Campeche. This reinvigorated the storm, and Mitch was once more classified as a tropical storm. It changed track, now heading northeastward, across the western side of the Yucatán Peninsula and back into the Gulf of Mexico. Now it was heading straight for Florida. It reached Key West on November 4, traveling at 26 MPH (42 km/h) and with winds gusting to nearly 80 MPH (130 km/h). During the rest of that day and the next Mitch dropped between 6 and 8 inches (150–200 mm) of rain over the southern part of the state. It also generated several tornadoes. One traveled from Marathon to Key Largo, injuring at least seven people. The tornadoes injured a total of 65 people and damaged 645 homes. Another tornado caused damage at Miramar, north of Miami. Around 100,000 people lost their electrical power. Two people died in Monroe County when a fishing boat capsized. The cost of the damage caused by Mitch in the United States was estimated at $40 million. On November 5, Mitch moved away from Florida toward the Bahamas. By this time it had been downgraded to an extratropical storm. It left the Tropics and finally dissipated.

Honduras Of all the countries Mitch touched, Honduras suffered the most. At its most intense, the hurricane was producing more than 4 inches (102 mm) of rain an hour. The town of Choluteca received 18.37 inches (467 mm) in a single day and 35.89 inches (912 mm) over the entire period of the storm, from October 25 through 31. This was the heaviest rainfall, but La Ceiba was not far behind, with 11.19 inches (284 mm) on a single day and a storm total of 34.52 inches (877 mm). To put this in perspective, in an average year New York City receives 3–4 inches (76–102 mm) of rain a month; about 17.5 inches (445 mm) falls between January 1 and May 31.

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Lapse rates and stability Air temperate decreases (or lapses) with increasing height. The rate at which it does so is called the lapse rate. When dry air cools adiabatically, it does so at about 5.5°F for every 1,000 feet (10°C per km). This is known as the dry adiabatic lapse rate (DALR). When temperature of the rising air has fallen sufficiently, its water vapor will start to condense into droplets. This temperature is known as the dewpoint temperature, and the height at which it is reached is called the lifting condensation level. Condensation releases latent heat, which warms the air. Consequently, thereafter the air cools at a slower rate, known as the saturated adiabatic lapse rate (SALR). The SALR varies, but averages 3°F per 1,000 feet (6°C per km). The actual rate at which the temperature decreases with height is calculated by comparing the surface temperature, the temperature at the tropopause (in middle latitudes about –67°F; –55°C), and the height of the tropopause (about 7 miles;

11 km in middle latitudes). The result is called the environmental lapse rate (ELR). If the ELR is less than both the DALR and SALR, rising air will cool faster than the surrounding air, so it will always be cooler and will tend to subside to a lower height. Such air is said to be absolutely stable. If the ELR is greater than the SALR, air that is rising and cooling at the DALR and later at the SALR will always be warmer than the surrounding air. Consequently it will continue to rise. The air is then absolutely unstable. If the ELR is greater than the DALR but less than the SALR, rising air will cool faster than the surrounding air while it remains dry, but more slowly once it rises above the lifting condensation level. At first it is stable, but above the lifting condensation level it becomes unstable. This air is said to be conditionally unstable. It is stable unless a condition (rising above its lifting condensation level) is met, whereupon it becomes unstable.

SALR

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Lapse rates and stability. If the environmental lapse rate (ELR) is less than both the dry (DALR) and wet (SALR) adiabatic lapse rates, the air is absolutely stable. If the ELR is greater than the SALR, the air is absolutely unstable. If the ELR is less than the SALR but greater than the DALR, the air is conditionally unstable.

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What happened when Mitch struck? The devastation affected the entire country and was so great that President Carlos Flores issued an appeal to the international community: “We will get back on our feet . . . May the Lord illuminate us and give us all strength. We are making an urgent and anguished appeal to the international community, to all countries, to international financial organizations and to aid organizations so that they heed this SOS. Our capacity for suffering and pain was never before put to such a hard test.” The official estimate was that 5,657 Hondurans had been killed, 8,052 were missing and unaccounted for, and 11,762 were injured. More than 400,000 people were living in temporary shelters. Altogether 1.9 million people were affected. More than 25 small villages had been washed away completely. More than a week after the hurricane had passed there were still survivors clinging to rooftops. Supplies had to be carried by helicopter, because most of the country’s secondary roads had been destroyed and more than 90 bridges had been wrecked or damaged. In the capital city, Tegucigalpa, some buildings that were more than 350 years old were washed away, and one-third of all the buildings in the city were damaged by flooding. One entire neighborhood was demolished, and many of the damaged buildings that remained standing were structurally unsound, in danger of falling. In the country as a whole, at least 70,000 houses were damaged or destroyed. Out in the countryside, at least 70 percent of all farm crops were destroyed, including 80 percent of the banana crop. Coffee warehouses and storage facilities were flooded. The value of the lost crops was estimated at $900 million. The total damage throughout Honduras was estimated at more than $5 billion.

Nicaragua On October 29 and 30, the rain bands associated with Mitch became stationary over the western part of the country. During that time the land was lashed by tens of inches of rain. The Malacatoya River rose by more than 50 feet (15 m), overflowing roads and destroying them. On Friday, October 30, a lake in the crater of Mount Casita, a dormant volcano, overflowed and part of the crater wall collapsed. This released a mudslide that moved in a southwesterly direction, eventually covering an area 10 miles (16 km) long and 5 miles (8 km) wide. At least four villages between Casita and the town of Posoltega were completely buried in mud. More than 2,000 people died in the mudslide, and some survivors were stranded for several days, until the mud dried sufficiently for rescuers to be able to walk across it. Entire landscapes were restructured. Rivers widened and in some places adjacent rivers merged. New lakes appeared and hills were washed away. The death toll for the entire country was about 3,800, with up to 7,000 missing. More than half a million people lost their homes. Farm crops were devastated. The total cost of all the damage was estimated at about $1 billion.

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El Salvador The west of the country suffered more severely than any other region. Mitch drew in moisture from the Pacific, and that is where the storm dropped it as rain. Estimates of the numbers of dead and missing varied. About a week after the hurricane passed, the National Emergency Committee of El Salvador estimated that 239 people had died and there were 135 missing. The Red Cross estimated there were 400 deaths and 600 people were missing. It is unlikely that the true number of deaths will ever be known. In all, almost 56,000 people were affected and around 10,000 houses were destroyed. Many more were damaged.

Guatemala Mitch arrived on Sunday, November 1, and moved northwestward across the country. The authorities had time to evacuate nearly 6,000 people from the path of the storm, but on November 4 the Red Cross estimated that 27,000 Guatemalans were still housed in shelters, unable to return home. The floods destroyed or severely damaged approximately 19,000 homes, 32 bridges, and 40 roads. The hurricane also destroyed about 95 percent of the country’s banana crop, 50–60 percent of the other farm crops, and almost onethird of the cattle. Officials estimated that 258 people lost their lives and 120 were missing.

Belize The authorities knew Mitch was coming and evacuated more than 75,000 people from Belize City and the offshore islands. They were accommodated in temporary shelters in the capital, Belmopan. Flood damage was extensive to houses and farm crops, but the timely evacuation saved many lives. Mitch left 11 people dead or missing.

Costa Rica Although Costa Rica lay far to the south of the hurricane’s track, nevertheless heavy rain caused flooding along the whole of its Pacific coast. Many people had to be evacuated, and up to seven people were killed.

Mexico The country was well prepared. There were plans to evacuate threatened areas, and the Mexican Red Cross positioned emergency teams in the Yucatán Peninsula. Nine people were killed, five of them in a car that was washed from the road near Tapachula, in the southwest of the country close to the Guatemala border.

Where hurricanes happen

Recovery Mitch struck with such ferocity and caused such widespread destruction that it will be years before the worst affected countries recover fully. The United Nations, as well as U.S. AID and many nongovernmental organizations responded with help. The United States provided $80 million in assistance. After visiting the region, former Presidents George Bush and Jimmy Carter called for the restructuring and scaling back of the international debt owed by Honduras and Nicaragua. Spain contributed $105 million, and Sweden said it would donate $100–200 million over three years. The humanitarian organizations flew in food and emergency goods and materials. Recovery will take time and the sorrow of bereavement will take time to heal. Mitch demonstrated the power our atmosphere is capable of wielding and the vulnerability of our towns and cities, roads, bridges, and crops. The scale of the disaster was appalling. Yet, in time, there will be a full recovery. Catastrophe draws people together and forges links that are not quickly broken when the worst of the pain has passed.

Where hurricanes happen Hurricanes begin as tropical depressions. These are areas where the atmospheric pressure is just a little lower than it is in the air surrounding it, and they are strictly a tropical phenomenon. It would be impossible for a hurricane to form over Minnesota, for example, or over Europe, although a hurricane traveling from the Tropics might reach such places. It would have greatly weakened during its long journey away from the sources of the energy it needs to sustain it (and meteorologically it would no longer be classed as a hurricane, because some of its essential characteristics would have changed). Hurricanes form in the Tropics because it is only in the Tropics that the necessary conditions ever occur. The map shows where they form and the directions in which they move from their “breeding grounds.” What we in the Western Hemisphere call a hurricane goes by different names in other parts of the world, although nowadays the name hurricane is tending to replace the other names—if you call it a hurricane, everyone will know what you mean. If it forms in the Bay of Bengal, however, its traditional name is cyclone (which, confusingly, is also the meteorological term for a depression in middle latitudes). Over most of the Pacific it is called a typhoon, although near Indonesia and the Philippines it is sometimes called a baguio. Baguio is the name of a town in Luzon, Philippines, unfairly blamed for the catastrophically violent weather that so often assaults the islands. If it occurs near Australia, it is a typhoon, but some people call it a willy-willy (or willy-willi or willynilly). Willy-willy also describes a dust storm or desert whirlwind, and

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Hurricane-prone areas. The map shows the areas where hurricanes develop and their direction of travel.

although you will still find the name in books, meteorologists no longer use it. These are all the same, despite their different names. Southerly winds along the western coast of Mexico generated by a hurricane in the southeastern North Pacific Ocean are known as cordonazo de San Francisco, or the “lash of St. Francis.” The name refers to the fact that these winds are most likely to occur around October 4, which is the Feast of St. Francis. Meteorologists call all of them tropical cyclones, although they, too, are tending increasingly to use the name hurricane. The word hurricane is from the Spanish huracán, derived in turn from Hurakán, the Caribbean god of stormy weather. Typhoon has two derivations. The Greek typh on ¯ means whirlwind and, more probably in this case, the Cantonese tai fung means “big wind.” A cyclone is an area of low atmospheric pressure; its opposite is an anticyclone, which is an area of high pressure. As the name suggests, a tropical cyclone is one that forms in the Tropics. Like cyclones elsewhere in the world, it is a distinct area of low pressure. In the Tropics it becomes much more intense than in higher latitudes, but otherwise it is similar.

How it begins For a tropical cyclone to develop, first there must be a fall in atmospheric pressure over a fairly large area. The difference need not be great. A drop

Where hurricanes happen

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of a mere 0.6 inch of mercury, or 0.3 lb. per square inch (20 mb) over about two days is sufficient. This is enough to produce only a fairly weak depression. A fall of this magnitude is common in temperate latitudes, although it is unusual in the Tropics, where the air pressure is fairly constant over very large areas. Under certain circumstances this small fall in pressure is enough to trigger the development of a more persistent tropical depression from which a hurricane (tropical cyclone) may grow. There are several events that can cause this gentle drop in pressure. Depressions much deeper than this are constantly developing, traveling eastward, and filling in middle latitudes. A pocket of low-pressure air can sometimes become detached from the edge of one of these middle-latitude weather systems. It can then spill over into the Tropics as a tongue of low pressure (called a trough) that extends toward the equator at a high altitude. Alternatively, a low-pressure system on land may drift out over the sea, or a wave may develop along the equatorial trough (see sidebar). This

Intertropical convergence and the equatorial trough The trade winds blow toward the equator in both hemispheres. Consequently, there is a region close to the equator where the winds meet. When two bodies of air flow toward each other they are said to converge. The meeting of the trade winds is therefore the intertropical convergence, and the region where it happens is the intertropical convergence zone (ITCZ). Because the ITCZ forms a boundary between air from the two hemispheres, it is sometimes called the intertropical front (ITF), although it is not a front strictly comparable to those in middle latitudes between polar and tropical air.

The ITCZ is more strongly developed over the oceans than over the continents, and even over the oceans it is evident only as an average. The convergence of the trade winds varies in strength and disturbances form in it and then travel westward. The ITCZ rarely occurs in the areas affected by the doldrums (see the sidebar on trade winds and doldrums in the section on ocean currents and sea-surface temperatures, page 33). The position of the ITCZ changes through the year. The map shows its approximate location in February and August, as this is revealed by bands of clouds that are clearly visible on satellite images. As (continues) A

30°N A

A equator F

F February

A

A

F

F

F F F

F

30°S

ITCZ. The map shows the approximate position of the ITCZ and equatorial trough in February and August.

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(continued) the map shows, the ITCZ is more often to the north of the equator than to the south, and it seldom coincides with the equator itself. Instead, its position coincides with the thermal equator. This is the region where the surface temperature is highest. Any change in the sea-surface temperature is likely to cause the ITCZ to move. The highest sea-surface temperature also produces the most highly developed convection, with the formation of convective clouds and heavy rain.

Both convergence and convection cause air to rise. This reduces air pressure near the surface and produces a region of high pressure in the upper air, where air diverges. The diagram shows what happens. The low surface pressure is known as the equatorial trough. The trough does not coincide precisely with the ITCZ, but is a short distance from it on the side farthest from the equator.

high

direction of air flow

Convergence. Air converges and rises, producing low surface pressure and high pressure in the upper troposphere.

low

will produce a depression that detaches itself and moves away from the equatorial trough, which reforms behind it. No matter what causes it, once it has formed the depression moves westward as an easterly wave (“easterly” because it comes from the east and “wave” because it produces a kink or wave in the otherwise regular flow of air).

Need for warmth and Coriolis Minor depressions can form anywhere, but one will grow into a hurricane only if it crosses a large expanse of very warm sea, where the surface

Where hurricanes happen temperature is at least 76°F (24°C); hurricanes are most likely to form where the sea-surface temperature is about 80°F (27°C). At any lower temperature convection is just not vigorous enough to generate a storm the size of a hurricane. This confines the birthplace of hurricanes to the Tropics. In latitudes higher than about 20° the sea-surface temperature is usually too low. Close to the equator the sea is often warm enough to start a hurricane, but hurricanes never form in latitudes lower than 5°, because this close to the equator moving air follows paths that are very gentle curves—almost straight lines, in fact. There is nothing to set the circulation turning. What is needed is the Coriolis effect, to swing the air moving toward the lowpressure area into a circular path. There is no Coriolis effect at the equator (see sidebar, page 69), and within 5° of the equator it is not strong enough to cause the necessary swing. Vorticity (see the section on vortices, page 64) will cause moving air to swing into a curved path and eventually start it rotating. After that the conservation of its angular momentum (see sidebar, page 71) will accelerate the air as it converges into an ever smaller radius. Without assistance from the Coriolis effect, however, this will not generate winds of hurricane force. For a hurricane to form closer than 5° from the equator by vorticity alone, air would need to converge on the low-pressure region from such a vast area that there is simply not enough air available. Even at 5° latitude, air within a radius of more than 300 miles (480 km) would need to contract to a radius of about 20 miles (32 km) to produce winds of about 100 MPH (160 km/h). In contrast, at latitude 20°, air contracting from a radius of about 90 miles (145 km) to 20 miles would produce 100-MPH winds. These areas are calculated without taking account of friction, which reduces angular momentum and slows the rate at which wind speed increases. Allow for friction, and the areas must be increased. Taken together, the need for a high sea-surface temperature and a sufficiently strong Coriolis effect confine the region in which hurricanes can form to a belt over the oceans between latitudes 5° and 20° in both hemispheres. They also restrict the time of year. The ocean needs time to warm up after the winter, even in the Tropics, and it is usually only in late summer and fall that the water is warm enough, although occasionally hurricanes develop outside these seasons.

Hurricanes occur in the west Some tropical cyclones form in the east of the North Pacific, but the great majority do not develop until a tropical depression has crossed to the western side of an ocean. This is also due, indirectly, to the Coriolis effect. Over the Tropics, air moves vertically in Hadley cells (see sidebar, page 35). As it moves away from the equator at a great height, the Coriolis effect causes the air to swing to the right in the Northern Hemisphere and to the

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Hurricanes left in the Southern. In both hemispheres, this is a swing to the east, and it makes the layer of high-level air moving away from the equator deeper on the eastern side of the Hadley cells than on the western side. This highlevel air sinks while still over the Tropics and warms adiabatically (see sidebar, page 41). This limits the upward movement of warm air rising from the surface by convection, because the rising air meets a layer of air that is less dense than itself and can rise no farther. A layer of warm air overlying cooler air is called a (temperature) inversion, and air rising from below cannot usually penetrate it. This one is known as the trade wind inversion. Sometimes the convection currents are vigorous enough to break through the trade wind inversion, but it is much easier for them to do so on the western side of the Hadley cells than on the eastern side. This concentrates areas where tropical cyclones develop on the western side of oceans. It is only there that condensation in the rising air can reach a high enough altitude to fuel the storm. Depressions can form over land or sea, in dry or moist air. For it to develop into a hurricane, however, a tropical depression must gather enough water vapor to provide it with a layer of very moist air. This layer must be deep enough to supply the depression with the latent heat of condensation it needs to maintain the instability of the air, which is what sustains it (see sidebar, page 6). The only way a tropical depression can find the water it needs is by traveling a long distance over a warm sea. If the depression moves over a continental land mass, it will not gain moisture and therefore it will not develop into a hurricane. Many tropical depressions end in this way. Those that do grow into hurricanes have already crossed an ocean—and since tropical weather systems travel from east to west, driven by the prevailing easterly winds, that is another reason why most hurricanes begin on the western side of oceans.

Equatorial trough and the jet stream There is one more difficulty. The equatorial trough rarely lies exactly at the equator. It moves north and south with the seasons, and hurricanes develop only within about 60 miles (96 km) of it. In the South Atlantic the trough never moves south of 5°. This is too close to the equator for the Coriolis effect to exert a strong enough influence on moving air, and so hurricanes never occur in the South Atlantic. It is not only the equatorial trough that moves with the seasons. So does the jet stream (see the sidebar, page 16). In winter it is quite far to the south, crossing northern Florida, but in summer it moves north. The map shows its approximate positions in January and July. In fact, there are two jet streams, the subtropical and polar, but the subtropical is by far the more constant, and so the term jet stream usually refers to the subtropical jet stream, which is the one shown here. The position of the jet stream is important, because a tropical cyclone cannot develop unless air is able to spiral upward unhindered. This requires that the wind speed and direction are more or less the same at all

Where hurricanes happen

15

heights. (The technical term is to say that there is little or no wind shear.) Often they are not, a fact pilots use by studying the winds at different heights to decide the best altitude at which to fly. The spiraling air will be dissipated if it encounters a strong wind, and for this reason tropical cyclones cannot develop directly beneath the jet stream. It is very unlikely, therefore, that a hurricane could develop over the Caribbean Sea in winter, because the jet stream is then much too close (and it also blows more strongly in winter than in summer). In summer, however, any southward movement by the jet stream can encourage the development of tropical cyclones. It will never come so close as to interfere with their spiraling structure, but the edge of it may carry away air from the top of the cyclone, which will intensify the storm by drawing more air upward. By late summer and fall, the jet stream is starting to move south, so this is a favorable time for tropical cyclones to grow. The area in which hurricanes can be born, therefore, is quite strictly confined. They develop on the western side of oceans. Their requirement for warm water restricts them to latitudes lower than 20°, but the need for the Coriolis effect to set them turning means they are unable to develop within 5° of the equator. They must also be between 5° and 10° of the equatorial trough (to its north in the Northern Hemisphere and to its south in the Southern) when the trough itself is more than 5° away from the equator. Many begin to form in or close to the doldrums (see sidebar, page 33), where the air is moving only slowly and they can grow

Jet stream. The map shows the approximate position of the mean jet stream in January and July, and the regions where wind speed is greatest.

July position January position Areas of maximum wind speed

30°N

equator

30°S

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Hurricanes

Jet stream During World War II, when high-altitude flying was new, aircrews sometimes found their journey times radically different from those they had calculated prior to takeoff. The effect was not reliable enough to predict, but when flying from west to east they could find their airspeed dramatically increased, and when flying in the opposite direction they were just as dramatically slowed. They had discovered a narrow, wavy, ribbon of wind blowing at speeds comparable to those of their aircraft. They called it the jet stream. If they approached the jet stream from above or below, pilots found the wind speed increased by about 3.4–6.8 MPH for every 1,000 feet of altitude (18–36 km/h per 1,000 m). If they approached from the side, it increased by the same amount for every 60 miles (100 km) of distance from the core of the jet stream. At the center of the stream the wind speed averages about 65 MPH (105 km/h) but it sometimes reaches 310 MPH (500 km/h). There are several jet streams. The polar front jet stream is located between about 30° N and 40° N in winter and about 40° N and 50° N in summer. There is an equivalent jet stream in the Southern Hemisphere. The subtropical jet stream is located at

about 30° throughout the year in both hemispheres. These jet streams blow from west to east in both hemispheres. In summer there is also an easterly jet at about 20° N extending across Asia, southern Arabia, and into northeastern Africa. This jet stream blows from east to west. The jet streams are thermal winds. That is to say they are generated by the sharp difference in temperature across the front separating two air masses. This difference is greatest close to the tropopause, which is why the jet streams occur at high altitude—the polar front jet stream at about 30,000 feet (9,000 m) and the subtropical jet stream at about 40,000 feet (12,000 m). The polar front jet stream is associated with the polar front, separating polar air and tropical air. The temperature difference responsible for the subtropical jet stream occurs only in the upper troposphere, on the high-latitude side of the Hadley cells. The polar jet stream is quite variable, and often it is not present at all. The subtropical jet stream is more constant. Consequently, the term jet stream often refers simply to the subtropical jet stream, and this is the one that is usually shown on maps.

undisturbed. They develop in late summer and fall, when the sea has warmed, and when both the equatorial trough and the jet stream have started to move south.

Hurricane and storm tracks On average, two-thirds of all the tropical cyclones that occur each year happen in the Northern Hemisphere, and half of those begin over the western side of the North Pacific Ocean. They are Asian typhoons, and many of them scream into the China Seas to assault the coasts of China and Japan. Hurricanes, affecting the Caribbean Sea and western North Atlantic Ocean, account for only one-sixth of the Northern Hemisphere

Hurricane and storm tracks total and only a little more than one-tenth of the world total. About one in 10 of all the tropical cyclones that occur in the Northern Hemisphere are cyclones—the name given to storms that develop in the northern Indian Ocean (those in the southern Indian Ocean are typhoons). Tropical cyclones of all kinds occur mainly in the late summer and fall. Atlantic hurricanes and Asian typhoons are most likely to strike between July and October, baguios in the region of Indonesia and the Philippines between September and November. In the Southern Hemisphere, most tropical cyclones develop between December and March. Typhoons in the Indian Ocean usually form near Madagascar and South Pacific typhoons near Australia. In summer, the intertropical convergence zone (ITCZ) and equatorial trough move north. The map in the sidebar on page 11 shows their approximate positions in February and August. As the ITCZ crosses the Bay of Bengal in May or June, cyclones develop close to it and in September, as it moves south again, there is a second cyclone season.

Carried by the wind Tropical cyclones develop in air that is moving westward. This reflects the trade-wind air movement in the Tropics (see the sidebar, page 33). Should they leave the Tropics, the storms enter a region where the prevailing winds blow in the opposite direction, from west to east, and they are affected by that movement. Air pressure is generally low in the Tropics, due to the ITCZ and equatorial trough, but around 30° from the equator lies the edge of a subtropical belt of mainly high pressure. Tropical cyclones form and travel not far from this region, and many reach the edge of a local high-pressure system (an anticyclone). In the Northern Hemisphere air circulates in a clockwise direction around areas of high pressure (and counterclockwise in the Southern Hemisphere). This anticyclonic circulation will carry with it any storm that approaches closely enough to be caught. It is the entire storm that moves, of course, and its anticyclonic movement around an area of high pressure has no effect on the speeds of the winds blowing cyclonically (counterclockwise in the Northern Hemisphere, clockwise in the Southern) around its low-pressure center. As a hurricane approaches one of the subtropical anticyclones on the side nearest the equator, it swings around the edge on a track that carries it away from the equator. Most hurricanes are then traveling at 10 to 15 MPH (16 to 24 km/h), but as they enter higher latitudes they sometimes accelerate to twice that speed. Storms that form in middle latitudes are also carried by the prevailing winds, which in this case are westerlies. As the map shows, storms that form in lower latitudes, over the Gulf of Mexico and off Cape Hatteras, are immediately caught in the subtropical anticyclonic circulation. This carries them northward and into the prevailing westerly winds.

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Storm tracks across North America. In middle latitudes, storms travel from west to east, carried by the prevailing westerlies. Storms that form in the Gulf of Mexico travel north and then east.

Hurricanes

Colorado o

Gul

These North American storms form over land or in coastal waters, and they can be severe, with thunderstorms and winds of gale force. In places they can produce tornadoes. Tornadoes often form beneath the eyewalls of tropical cyclones, too, but there the similarity ends. Apart from covering a much larger area and being a great deal fiercer, tropical cyclones grow from slight disturbances in air that is otherwise at almost exactly the same temperature and pressure over a large area. Midlatitude storms, on the other hand, form in association with weather fronts—the boundaries between masses of air at different temperatures and pressures (see sidebar, page 19). Over the world as a whole, the total force of all the westerly winds is balanced by an equal force of easterly winds (see sidebar, page 22). This must be so. There is friction between moving air and the surface of land and sea. This slows the wind—which is why the wind is usually stronger in air that is well clear of the ground and winds over the sea, where there is less friction, are stronger than those over land, where there are hills, trees, buildings, and other obstructions to cause friction. Friction slows the wind, but the uneven surface that gives rise to it also provides purchase for the wind, so the wind tends to push the Earth in the direction it is blowing. This is the force—due to friction—that drives surface sea waves and ocean currents. The effect on land is very slight, but it is not zero. If the

Hurricane and storm tracks

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winds blowing from west to east were always stronger than those blowing from east to west, over millions of years that drag would accelerate the rotation of the Earth, because it would amount to a force acting in the same direction. Eventually we would notice the days growing shorter. If the east-to-west winds were stronger, the Earth’s rotation would be slowed

Weather fronts During World War I, a team of meteorologists led by the Norwegian Vilhelm Bjerknes (1862–1951) discovered that air forms distinct masses. Because each mass differs in its average temperature, and therefore density, from adjacent masses, air masses do not mix readily. He called the boundary between two air masses a front. Air masses move across the surface of land and sea, and so the fronts between them also move. Fronts are named according to the temperature of the air behind the front compared with that ahead of it. If the air behind the advancing front is warmer than the air ahead of it, it is a warm front. If the air behind the front is cooler, it is a cold front. Fronts extend from the surface all the way to the tropopause, which is the boundary between the lower (troposphere) and upper (stratosphere) layers of the atmosphere. They slope upward, like the sides of a bowl, but the slope is very shallow. Warm fronts have a gradient of 1° or less, cold fronts of about 2°. This means that when you first see, high in the sky, the cirrus clouds marking the approach of a warm front, the point where the front touches the surface is about 350 to 715 miles (570 to 1,150 km) away. When you see the first, high-level sign of an approaching cold front, the front is at the surface about 185 miles (300 km) away. Cold fronts usually move across the surface faster than warm fronts, so cold air tends to undercut warm air, raising it upward along the cold front. If the warm air is already rising, it will be raised even faster along the front separating it from cold air. The front is then called an ana-front, and there is usually thick cloud and heavy rain or snow associated with it. If the warm air is sinking, an advancing

cold front will raise it less. This is a kata-front, usually with only low-level cloud and light rain, drizzle, or fine snow. The diagram shows these frontal systems in cross-section, but with the frontal slopes greatly exaggerated. After a front has formed, waves start to develop along it. These are shown on weather maps, and as they become steeper, areas of low pressure form at their crests. These are frontal depressions, which often bring wet weather. Just below the wave crest, there is cold air to either side of a body of warm air. The cold front moves faster than the warm front, lifting the warm air along both fronts until all the warm air is clear of the surface. The fronts are then said to be occluded, and the pattern they form is called an occlusion. Once the fronts are occluded and the warm air is no longer in contact with the surface, air to both sides of the occlusion is colder than the warm air. Occlusions can still be called cold or warm, however, because what matters is not the actual temperature of the air, but whether air to one side of a front or occlusion is warmer or cooler than the air behind it. In a cold occlusion the air ahead of the front is warmer than the air behind it, and in a warm occlusion the air ahead is cooler, but both of these are cooler than the warm air that has been lifted clear of the surface. The diagram shows this in cross section. As the warm air is lifted, clouds usually form and often bring precipitation. Eventually; the warm and cold air reach the same temperature, mix, and the frontal system dissipates. Often, however, another similar system is following behind, so frontal depressions commonly occur in families. (continues)

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(continued) direction of movement

A t o p o au se

warm air

direction of movement

B t o p o au se

cold air

warm air

Weather fronts. A) At an ana-front air is rising along both fronts. B) At a kata-front air is subsiding along both fronts.

Hurricane and storm tracks

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direction of movement

C tr o p o p a u s e

warm air

cool air

cold air

direction of movement

D tr o p o p a u s e

warm air

cool air

cold air

warm front cold front occluded front

Occluded fronts. C) A cold occlusion, with the air behind the occlusion cooler than the air ahead of it. D) A warm occlusion, with the air behind the occlusion warmer than the air ahead of it. In both cases the air to either side of the occlusion is cooler than the warm air that has been lifted clear of the surface.

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Hurricanes and the days would grow longer. This does happen from time to time, but the change in day length amounts to no more than a few thousandths of a second so we do not notice it, and the change does not accumulate. A temporary increase in winds from one direction is soon balanced by an increase in those from the opposite direction.

Where do they go? Most of the tropical cyclones forming off the west coast of Central America follow tracks that take them westward into the middle of the Pacific, where they die without causing harm. Unfortunately, these are the exception. The great majority of tropical cyclones form on the western side of oceans, not far from continents, and move along tracks that carry them directly toward inhabited regions. Cyclones in the Indian Ocean swing north and head for the subcontinent. Bangladesh is especially vulnerable to them. Asian typhoons also swing north and head for Indonesia, the Philippines, China, and Japan.

Global wind systems Rising air produces low atmospheric pressure at the surface, into which air flows. Subsiding air produces high surface pressure, with air flowing outward. According to the three-cell model of atmospheric circulation, air rises at the equator, producing a region of low pressure into which trade winds blow from north and south. The winds converge at low level, in the intertropical convergence zone (ITCZ), a narrow belt to either side of the equator (see sidebar, page 11). In the Tropics, therefore, the prevailing winds are the trades, blowing from an easterly direction in both hemispheres. Air descending on one side of the low-latitude Hadley cells produces high pressure at around latitude 30° in both hemispheres. Near the surface, the subsiding air diverges, some flowing back toward the equator and some flowing toward the poles. The air moving toward the poles is deflected by the Coriolis effect, to the right in the Northern Hemisphere and left in the Southern (see sidebar, page 69). In both hemispheres this produces prevailing westerly winds—southwesterly in the Northern Hemisphere and northwesterly in the Southern Hemisphere.

There is a second region of low pressure at about 60° in both hemispheres. This is where the westerly winds flowing toward the poles encounter air spilling toward the equator from the high-pressure region over the poles, where air is subsiding and diverging. The two types of air meet at the polar front and rise. Air flowing away from the polar high-pressure regions is also deflected by the Coriolis effect. Deflection to the right in the Northern Hemisphere and left in the Southern produces prevailing easterly winds. Over the world as a whole, the prevailing surface winds form three belts in each hemisphere. Winds are easterly between 0° and 30°, westerly between 30° and 60°, and easterly between 60° and 90°. The diagram shows the pattern. At higher levels, the prevailing winds in the Tropics become westerly. The midlatitude winds are westerly at all altitudes. High-level polar winds are westerly. In midlatitudes, these are the directions from which winds blow most commonly, but on any particular day they may blow from a different direction due to the passage of weather systems.

Hurricane and storm tracks

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polar easterlies 60° N polar front midlatitude westerlies 30° N

trades 0°

L trades

H

30° S

midlatitude westerlies L

polar front

H

polar easterlies

60° S prevailing pressure H high

L low

Caribbean and Atlantic hurricanes head for the islands of the Caribbean, then for the Gulf of Mexico, or Florida and the United States mainland. Although they start weakening as soon as they leave the warm, tropical ocean, tropical cyclones can travel a considerable distance over land while retaining enough energy to cause serious damage. Their tracks continue to swing, however, and this may carry them back out to sea, where they are once more in contact with warm water. Renewed evaporation and convection often reinvigorates them. Atlantic hurricanes occasionally cross the ocean and reach Europe, although they weaken greatly as they pass over the cold North Atlantic water. At the same time, the air inside the eye becomes cool rather than warm. Since a warm eye is one of the defining characteristics of a tropical cyclone, strictly speaking the storms that reach Europe are no longer hurricanes. With the exception of the early summer cyclone season in the Bay of Bengal, tropical cyclones form in late summer and fall over the oceans throughout the Tropics. They travel westward, then curve away from the equator. Those that survive long enough continue to follow curved tracks that eventually carry them in an easterly direction. Because almost all of them start life on the western side of oceans, a short westward journey is enough to take them over land and through inhabited areas. It is where they form and the direction in which they move that makes them so dangerous.

Global wind belts. There are three wind belts in each hemisphere. Overall, the force exerted on the Earth by the westerlies is exactly balanced by the force of the easterlies.

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AIR AND SEA Ocean currents and sea-surface temperature

Plane of the ecliptic. The plane of the ecliptic is the imaginary disk with the Sun at its center and the Earth’s orbital path as its edge.

Our planet is warmed by the Sun, but it is not warmed evenly. The equator faces the Sun directly. To either side of the equator, the Tropics at 23.5° N and S (Cancer in the Northern Hemisphere and Capricorn in the Southern Hemisphere) mark the limits of the region where the Sun is directly overhead at noon on at least one day in the year. These latitudes are equal to the angle by which the rotational axis of the Earth is tilted from the plane of which the Earth’s orbit around the Sun marks the edge (called the plane of the ecliptic). The diagram explains this. The Tropics receive much more energy than the polar regions, where on at least one day each year the Sun never rises above the horizon and on at least one day each year it does not descend below it (see sidebar, page 26). The Arctic and Antarctic Circles are at 66.5° N and S (90 – 23.5 = 66.5). It is not surprising, therefore, that the Tropics enjoy a warm climate and the Arctic and Antarctic are cold.

Ocean currents and sea-surface temperature

What difference do the atmosphere and oceans make? If the Earth had no atmosphere and no liquid water at its surface, these differences would be much greater, and the difference in temperature between day and night would be as extreme as it is on the Moon. There the maximum daytime temperature is about 230°F (110°C) and the minimum nighttime temperature about –275°F (–170°C). The Moon has no atmosphere to speak of (there is a very thin atmosphere, amounting to a few gas molecules in every cubic inch of space close to the surface). On Earth, however, the Sun warms the surface of land and sea, and the air is heated by contact with the warmed surface. Air and water can move, and in doing so they transport warmth away from the equator, warming the cold places and cooling the hot places. When it is warm or cold, when the Sun shines or it is cloudy, and when it rains or snows, naturally enough we assume it is the air which brings us our weather. This is true, of course, but the oceans also play a very important part. Indeed, although it is the air that produces our dayto-day weather, to a large extent it is the oceans that give us our climates. If you doubt this, compare the climate of London, England, at about 51.5° N and St. John’s, Newfoundland, at 47.5° N. In January, the average daytime temperature in London is 43°F (6°C), and the average July daytime temperature is 71°F (22°C). The January and July daytime temperatures in St. John’s are 29°F (–2°C) and 68°F (20°C)—yet St. John’s is four degrees farther south than London. What makes this difference? The North Atlantic Ocean does. The climatic influence of the oceans is due to certain remarkable properties of water. The first of these is its thermal capacity. This is the amount of heat energy that must be applied to a given mass of a substance to raise its temperature by a specified amount. Water has a very high thermal capacity. At 59°F (15°C), which is the average temperature at the surface of the Earth, it requires 51 calories of heat to raise the temperature of one ounce of water by one degree Fahrenheit (4.1855 joules per gram per kelvin; 1 kelvin (K) = 1°C). This means that water absorbs a large amount of heat before its temperature changes at all, and then it warms only slowly. Its thermal capacity varies at different temperatures, but only very slightly. Dry soil, with a thermal capacity of about 10 calories per ounce per degree Fahrenheit (0.84 J g–1 K–1), warms much more quickly than water. Air, passing from land to sea in summer, is warmed by contact with the land, then cooled as it passes over the sea. Their different thermal capacities also mean that water is much slower than dry land to give up its heat. In winter, air may be warmed as it passes over the ocean and cooled when it crosses land. Water moderates climates because of its high thermal capacity. It slows the rate of summer warming by absorbing heat with little change in

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Hurricanes

General circulation of the atmosphere The Tropics, of Cancer in the north and Capricorn in the south, mark the boundaries of the belt around the Earth where the Sun is directly overhead at noon on at least one day in the year. The Arctic and Antarctic Circles mark the boundaries of regions in which the Sun does not rise above the horizon on at least one day of the year and does not sink below the horizon on at least one day in the year. Imagine a beam of sunlight just a few degrees wide. As the drawing shows, this beam illuminates a much smaller area if the Sun is directly overhead than it does if the Sun is at a low angle in the sky. The amount of energy in each beam is the same, because they are of the same width, so energy is spread over a smaller area directly beneath the Sun than it is when the Sun is lower. This is why the Tropics are heated more strongly than any other part of the Earth and the amount of heat we receive from the Sun decreases the farther we are from the equator. Solar energy warms the surface of land and water. The air is warmed by contact with the surface. As it is warmed, the air expands. This makes it

less dense than the air immediately above it, so it is lifted by denser air flowing beneath from a colder region. This air is heated in its turn. Where the surface is heated strongly and air in contact with it is expanding, there will be a region of low atmospheric pressure at the surface. The equatorial belt is a region of generally low pressure. At high altitude, the air moving away from the equator subsides at about latitude 30°. Its subsidence is not due to its temperature or density (see sidebar, page 36), but to general atmospheric movements, including the flow of low-level air toward the equator. The high-level air is very dry, having lost its moisture as it rose and cooled adiabatically. Where the sinking air reaches the surface, the atmospheric pressure will be high. The edges of the Tropics and the subtropics, where equatorial air is sinking, are regions of generally high pressure, one in each hemisphere. Although the air is very cold while it remains at a great height, it warms adiabatically as it sinks and is compressed, so air in the tropical–subtropical regions is warm. Over the poles, very cold air sinks to the surface. This produces generally high pressure.

its own temperature and it slows the rate of winter cooling by gradually releasing the heat it spent the summer absorbing. By late fall, the sea-surface temperature in the northern North Atlantic is usually a few degrees higher than the air temperature over adjacent land. Oceans cover about 70 percent of the Earth to an average depth of rather more than 12,000 feet (3,660 m). Their total volume is more than 325 million cubic miles (1,354 km3). That is a very large amount of water spread over a very large area. Consequently, its moderating effect on climate is huge.

Transporting heat This is only one way the oceans influence the global climate. They also move heat from low to high latitudes by themselves, independently of the air.

Ocean currents and sea-surface temperature

Between the low-latitude high pressure and the high-latitude high pressure there is, in each hemisphere, a belt of generally low pressure. Air movements carry warm air away from the Tropics and cool air away from polar regions. This distributes the warmth we receive from the Sun more evenly than would be possible if the Earth had no atmosphere. Although the Earth is heated most strongly in the Tropics, all parts of the planet receive some

27

warmth from the Sun, and land and water respond differently. Land warms and cools much faster than water. As air moves, it is warmed or cooled by the surface over which it travels. Together, the transport of heat from low to high latitudes and the difference in the effect of heating on land and water generate the general circulation of the atmosphere. It is this circulation that produces regional climates and our day-today weather.

high p

Sun low pressure high pressure low pressure high pressure

Sunshine and pressure. The Sun’s radiance is much stronger in the Tropics than in higher latitudes. The general circulation of the atmosphere produces latitudinal belts of high and low pressure.

Water near the equator is strongly warmed by the Sun and flows north and south away from the equator as warm currents, its place being taken by cold currents flowing away from the poles. Air passing over a warm current is warmed by it. The difference in average temperatures in Britain and Newfoundland, mentioned above, occurs in part because the British coast is bathed by the extension of the warm Gulf Stream known as the North Atlantic Drift, while Newfoundland is washed by the cold Labrador Current, flowing south from the Arctic. The Gulf Stream and North Atlantic Drift form part of a gyre, a largescale system of currents flowing in a clockwise direction around the North Atlantic. The map shows the principal components of this system. It begins just north of the equator as the westward-flowing North Equatorial Current. This turns north off the North American coast, where it is joined by the Equatorial Counter Current (bringing water across the equator) and becomes the Antilles Current (not named on the map), and

28

Hurricanes then the Florida Current. As it passes the Gulf of Mexico it becomes known as the Gulf Stream, with the still waters of the Sargasso Sea bordering it to the east. At about 40° N, in the latitude of Spain and Portugal, it turns east across the ocean and then south, returning to the Tropics as the Canary Current to become the North Equatorial Current again. At about 40° N the Gulf Stream divides and the North Atlantic Drift (also called the North Atlantic Current) heads away in a northeasterly direction, washing the coast of northwestern Europe and passing around the north coast of Norway, where it becomes the Norwegian Current flowing into the Arctic. The Arctic is warmer than the Antarctic because it is covered mainly by sea fed by warm currents. The huge continent of Antarctica dominates the Antarctic climate.

The Great Conveyor This gyre strongly affects the climates of lands bordering the North Atlantic, and it forms part of what used to be called the Atlantic Conveyor when it was thought to affect only the North and South Atlantic. It is now known to be an even bigger system that influences climates over much of the world. Consequently it has been renamed the Great Conveyor. The Great Conveyor begins near the Arctic Circle, where ice forms on the sea surface. What happens next is due to two more remarkable properties of water. As the temperature of water falls, its molecules lose energy and move more slowly. This causes them to crowd together more closely, so that a given volume of water contains more of them. The water then weighs more as its density increases, reaching a maximum at 40°F (4°C). Below this temperature, ice starts to form as water molecules arrange themselves into crystals. Ice crystals are open at the center, and because they are open, freezing makes water expand and its density decreases. Ice is less dense than water that is just above freezing temperature, which is why ice floats. If this were not so, ice would accumulate on the bottom of lakes and the sea rather than on top, with disastrous consequences for the animals living on and in the bottom sediments. At the edge of the sea ice and just beneath it, water is just a little above freezing temperature, and so it is denser than the warmer water around it. At the same time, as ice crystals form and link together in salt water, the water molecules bond to one another directly. This breaks the bonds they had with the sodium and chlorine of salt. Common salt is sodium chloride (chemical symbol NaCl); a salt molecule comprises one atom of sodium (Na) joined to one atom of chlorine (Cl). The sodium atom carries a positive charge (Na+) and the chlorine atom a negative charge (Cl–). The water molecule (H2O) is also charged, because the two hydrogen atoms (H+) are both on the same side of the molecule as the oxygen atom (O–). When salt dissolves in water, its Na+ and Cl– separate. The Na+ joins its opposite charge on the O–, and the Cl– joins the H+. NaCl is the commonest of the salts found in seawater, but the

Ocean currents and sea-surface temperature

60° W

30° W

0°

29

30° E

Labr

Norweg ia

80° N

ad

60° N

or

Cu r rent

ic D r Atlant North

. nC

ift

ream Gulf St

aC

.

N o rt

h E q u ato

na

20° N

ry

C.

F r id

40° N

rial Current

E q u at

o rial C o u n

Ben gu

rren

Cu

Per

B

z il ra

rent Cur

u urr nt

t

ela

20° S

Guinea C

uatorial Cur th Eq ren

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Sou

ter Current

.

0° Equator

Ca

alklan

40° S nt urre dC

ind Drift West W

F

60° S

Atlantic currents. The major currents form gyres rotating clockwise in the North Atlantic and counterclockwise in the South Atlantic.

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Hurricanes other salts dissolve in the same way. When water molecules join together to form ice crystals, the H+ of one water molecule links to the O– of another (by hydrogen bonding). This detaches—“squeezes out”—the Na+ and Cl–. Isolated, these join together once more as NaCl. The NaCl then dissociates once more as it dissolves in the water into which freezing has “pushed” it. The diagram shows the sequence of events when salt dissolves and is detached when the water freezes. The ice is made from freshwater, and the salt removed from it and then redissolved makes the adjacent water saltier. The difference is not great, but it is enough. The average salinity of North Atlantic water is

1

2

Cl-

Na+

Na+

salt O-

H+

O-

H+

water

H+

H+

Cl-

3

Na+

sodium and chlorine separate

-

O

Cl-

H+

4

Na+

Cl-

sodium and chlorine rejoin to form salt

H+ ice forming

O-

Separating of salt when water freezes. 1. Salt (NaCl) enters water (H2O). 2. Na attaches to O, Cl to H. 3. When water freezes, Na and C1 are detached. 4. Na and Cl rejoin to form NaCl.

H+

1 Salt (NaCl) enters water (H2O). 2 Na attaches to O, Cl to H.

H+

O-

H+

3 When water freezes, Na and Cl are detatched.

4 Na and Cl rejoin to form NaCl. H+

Ocean currents and sea-surface temperature 34.9‰ (parts per thousand, pronounced “per mil”) and that of water near to the edge of the ice about 35.5‰. Salt water is denser than freshwater, because a given volume contains as many water molecules as a similar volume of freshwater at the same temperature, but also the sodium and chlorine atoms of the salt dissolved in it. Colder and saltier than the surrounding water, the water at the edge of the sea ice sinks beneath it. It fills a basin on the northern side of a ridge that extends along the ocean floor between Greenland and Scotland, then spills over the ridge between Iceland and Scotland. It is then known as Iceland Scotland Overflow Water (ISOW). It is denser than the water of the deep ocean, and it sinks all the way to the floor at a depth of about 10,000 feet (3,000 m). As more water sinks behind it, the deep water forms a current that flows southward, closely following the edge of the North American continent (which is several hundred miles from the coast). It takes more than 20 years for the water to reach the equator. It has now become the North Atlantic Deep Water (NADW). As it continues south, more very salt water joins it, spilling out of the Mediterranean Sea at the Strait of Gibraltar. The NADW flows all the way to Antarctica, where it joins the West Wind Drift (also called the Antarctic Circumpolar Current). It then branches several times, its branches flowing northward into the Indian, Pacific, and finally Atlantic Oceans. It forms the cold Benguela Current, which passes the western coast of Africa, crosses the equator, then the North Atlantic to North America, and finally returns to its starting place. By that time it has risen to the surface, ready to sink once more. The map shows the route the Conveyor follows. It is a slow process. Deep-water currents flow very slowly, perhaps at no more than about 150 feet (45 m) a day, and once it has descended to the ocean floor, the water does not mix with surface water. In the Atlantic, it takes 500 to 800 years for deep water to return to the surface (and in the Pacific it takes twice as long). The Gulf Stream, in contrast, flows at about 6 MPH (10 kmh) as a set of narrow streams. Throughout the whole of human history, the Conveyor has remained stable. It has always flowed more or less as it flows now, but this was not so in the more remote past. Then it changed at intervals, and quickly. Sometimes it flowed more strongly, sometimes it ceased to flow altogether, and its variations wrought major climatic changes. About 10,000 years ago, which is the last time it departed from the behavior we now think of as normal, the North Atlantic Drift ceased to flow and within a few decades the whole of the Northern Hemisphere was plunged into near ice-age conditions. That change, and probably others that preceded it, was caused by fresh meltwater flowing into the North Atlantic, in that case from the retreat of the ice sheet covering much of North America as the most recent ice age came to an end. At other times the Conveyor was suppressed by the sudden release of large numbers of huge icebergs for reasons scientists are at present unable to explain. Being less dense, the freshwater floated above the salt water like a huge raft and prevented dense water from sinking to form the NADW. Some scientists fear that global warming

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Hurricanes

60 N

30° N

equator

30° S

60° S

The Great Conveyor. This system of currents carries warm water away from the equator and cold water toward it.

might increase the amount of rain and snow falling over high latitudes and that this might suppress the formation of the NADW, disrupting the Conveyor and causing a sharp cooling in the climate of northwestern Europe. The risk of this is quite small, however.

Driven by the wind The Great Conveyor drives the gyres that circulate in all the oceans, but it has help from the wind. Surface currents are driven by the wind. To either side of the equator, the winds blow predominantly from the east. These are the trade winds, from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere (see sidebar). It is the trade winds that drive the Equatorial Currents in both hemispheres. There are also gyres, driven by the trade winds, in the South Atlantic and the North and South Pacific and a smaller and rather more complex one in the southern part of the Indian Ocean. In the North Pacific, the Kuroshio Current is very similar to the Gulf Stream and flows at a similar speed, washing the shores of Japan. In the Northern Hemisphere all the gyres flow in a clockwise direction, and in

Ocean currents and sea-surface temperature

33

Trade winds and doldrums At the tropical margins of the Hadley cells, air is sinking. It warms adiabatically as it sinks, but gathers no moisture, so by the time it reaches the surface it is hot and dry. When it reaches the surface, the air diverges, some flowing toward the equator, some away from it. The air flowing toward the equator is swung to the right in the Northern Hemisphere and to the left in the Southern, producing north-easterly winds to the north of the equator and south-easterly winds to its south. These are the trade winds. Trade is an old German word for “track,” and the trade winds earned their name because they always blow in the same direction. They occur over nearly half the world and they are very reliable, in speed as well as direction, although they are stronger in winter than in summer.

There is not one Hadley cell, however, but several. This means that the trade winds blow from the eastern margin of each cell (in both hemispheres). They converge where they meet, near the equator, but if you picture the winds as arrows, there are gaps between their “shafts.” The winds in these gaps are light and variable, and often the air is quite still. Sailing ships could be becalmed in these regions. This was not merely inconvenient. So far as the sailors were concerned it was dangerous. They could sit, day after day, under the scorching heat of the Sun, while their supply of freshwater dwindled. They called these areas the doldrums. They were also known as the horse latitudes, because horses that were being carried as cargo would sometimes die from thirst when the water supply ran short, and the carcasses would be thrown overboard.

Tropic of Cancer doldrums doldrums equator doldrums Tropic of Capricorn

Trade winds and doldrums. The areas of light winds called the doldrums occur in particular places.

34

Hurricanes the Southern Hemisphere they flow counterclockwise. These major currents are very distinctive. There is a sharp difference in temperature between a warm or cold current and the water on either side of it. The temperature of the Gulf Stream, for example, is a fairly constant 64–68°F (18–20°C), regardless of the temperature of the adjacent ocean water. Some currents are even visible. “Kuroshio” means “black water.”

Doldrums and horse latitudes There are regions in the Tropics, however, where winds are usually light and often do not blow at all. They are places where the Sun beats down mercilessly, often in very still air. In the days of sailing ships, sailors dreaded these areas and tried to avoid them, because ships could be becalmed there for weeks on end. Sailors called these places the doldrums. In those days ships often carried cargoes of horses, which might die if the ship were becalmed long enough for freshwater to become scarce. When this happened, the dead horses were thrown overboard. The winds were most likely to fail at around latitude 30° N and S. Sailors called these the horse latitudes. The doldrums, horse latitudes, and the trade winds are all produced by the convective circulation of tropical and subtropical air in Hadley cells (see sidebar). Thousands of feet below the surface of the sea, cold, dense water flows south, and its replacement causes the general circulation of ocean water. At the surface, water is also driven by the trade winds. The resulting currents carry away water that has been heated by the tropical Sun, so although tropical waters are always warm, they are cooler than they would be were they not constantly moving and being replaced by cooler water flowing in from higher latitudes. Similarly, if warm water were not fed into them constantly, the oceans in the far north and south would be even colder than they are. In the doldrums, however, the lack of wind means the surface water is not moving so fast (see sidebar, page 33). It remains exposed to the hot Sun for longer, and from time to time patches of it become very warm indeed. If the sea-surface temperature rises above about 80°F (27°C) over a large area, the scene may be set for a hurricane to develop in the air warmed by contact with the sea. An area of warm sea in the right place is not the only condition that is necessary for a hurricane to form (see “Where hurricanes happen,” page 9), but it is probably the most important one.

Hurricane seasons There are no seasons at the equator, but there are in the Tropics to its north and south, and it is during the tropical summer that the sea is heated most strongly. Because of the high thermal capacity of water, the sea warms slowly through the summer and temperatures high enough to trigger the development of hurricanes are not reached until mid to late

Ocean currents and sea-surface temperature

35

summer, so hurricanes are most likely later in the summer and in the fall. In the western North Atlantic and Caribbean, and also in the western Pacific, July to October is the peak season for them, although they can occur as early as May and as late as December. They are most frequent from September to November in southeastern Asia and between April and June and again in October and November in the Indian Ocean. December to March is the hurricane season throughout the Southern Hemisphere, but hurricanes never develop in the Atlantic south of the equator.

George Hadley and Hadley cells When European ships began venturing far from their home ports, into the Tropics and across the equator, sailors learned that the trade winds are very dependable in both strength and direction. They made use of them, and by the end of the 16th century their existence was well known. Many years passed, however, before anyone knew why the trade winds blow so reliably. Like many scientific explanations, this one developed in stages. Edmund Halley (1656–1742), the English astronomer, was the first person to offer an explanation. In 1686 he suggested that air at the equator is heated more strongly than air anywhere else. The warm equatorial air rises, cold air flows in near the surface from either side to replace it, and this inflowing air forms the trade winds. If this were so, however, the trades either side of the equator would flow from due north and south. In fact, they flow from the northeast and southeast. There the matter rested until 1735. In that year George Hadley (1685–1768), an English meteorologist, proposed a modification of the Halley theory. Hadley agreed that warm equatorial air rises and is replaced at the surface, but said that the rotation of the Earth from west to east swings the moving air, making the winds blow from the northeast and southeast. Hadley was right about what happened, but not about the reason for it. This was discovered in 1856 by the American meteorologist William Ferrel (1817–91), who said the swing is due to the tendency

of moving air to rotate about its own axis, like coffee stirred in a cup. In accounting for the trade winds, Hadley had proposed a general explanation for the way heat is transported away from the equator. He suggested that the warm equatorial air moves at a great height all the way to the poles, where it descends. This vertical movement in a fluid, driven by heating from below, is called a convection cell and the cell Hadley described is known as a Hadley cell. The rotation of the Earth prevents a single, huge Hadley cell from forming. What really happens is more complicated. In various equatorial regions, warm air rises to a height of about 10 miles (16 km), moves away from the equator, cools, and descends between latitudes 25° and 30° N and S. These are the Hadley cells. When it reaches the surface in the Tropics, some of the air flows back toward the equator and some flows away from the equator. Over the poles, cold air descends and flows away from the poles at low level. At about latitude 50° it meets air flowing away from the equatorial Hadley cells. Where the two types of air meet is called the polar front. Air rises again at the polar front. Some flows toward the pole, completing a high-latitude cell, and some flows toward the equator until it meets the descending air of the Hadley cell, which it joins. There are three sets of cells in each hemisphere. This is called the three-cell model of atmospheric circulation by which warm air moves away from the equator and cool air moves toward the equator.

36

Hurricanes

Potential temperature Cold air is denser than warm air, because its molecules are closer together. Consequently, a given volume of cold air has a greater mass than a similar volume of warm air and so it weighs more. Warm air rises because it is less dense than the cold air above it. The cold, dense air sinks beneath the warm, less dense air, and pushes it upward. Air temperature decreases with height. If you climb to the top of a mountain, you expect the air to be colder there. High mountaintops are covered in snow, even in summer, and climbers take warm clothes with them. Why is it, then, that the cold, dense air at the top of a mountain, or at the top of the troposphere, does not simply sink to the surface? How does it manage to stay up? To answer that you must imagine what would happen to the air if it did descend. Suppose, for example, that the air is fairly dry, with no clouds in the sky, and the temperature near to ground level is 80°F (27°C). Near the tropopause, 33,000 feet (10 km) above the surface, suppose the air temperature is –65°F (–54°C). The air near the tropopause is dense, because of its temperature, but this really means it is denser than the air immediately above it. Because air is very compressible, its density also decreases with height.

If the high-level air were to subside all the way to ground level, as it descended it would be compressed and it would heat adiabatically (see sidebar, page 41). Because it is dry, the air would warm at the dry adiabatic lapse rate (DALR; see sidebar, page 00). The DALR is 5.4°F per 1,000 feet (9.8°C per 1,000 m). As the air descends 33,000 ft (10 km), its temperature will rise by 5.4 × 33 = 178.2°F (98°C). Add this increase to its initial temperature, and its temperature when it reaches the ground will be 178.2 – 65 = 113.2°F (44°C). This is much warmer than the actual ground-level temperature of 80°F (27°C). The air could not reach the ground, because it would be less dense, and therefore lighter, than the air below. The temperature air at any height above the surface would have if it were subjected to sea-level pressure of 1,000 mb (100 kPa, 29 in. of mercury) and warmed adiabatically as it was compressed is known as its potential temperature (usually symbolized by Φ, which is the Greek letter phi). Potential temperature depends only on the actual pressure and temperature of the air. Meteorologists calculate the potential temperature of air to determine its stability.

In the world as a whole, there are about 80 hurricanes in most years, 54.6 in the Northern Hemisphere and 24.5 in the Southern Hemisphere (the average from 1958 to 1977). In the Atlantic, 1995 was an exceptional year, with 11 hurricanes, compared with the 1885–1994 average of 4.9 per year. That is more hurricanes than there had been for more than 60 years and, for the first time in more than 70 years, four named tropical storms were in the Atlantic at the same time. In 2000 there were a more typical six in the Atlantic, and a further 21 in other parts of the Northern Hemisphere, making a total of 27. In 2001 there were nine Atlantic hurricanes and 20 typhoons in the North Pacific, making a total of 29. There were eight typhoons in the Southern Hemisphere in the 2000–01 season. The world total was therefore 35, making 2000 a quiet year. Forecasters predicted six Atlantic hurricanes and 21 North Pacific typhoons in 2002.

Warming, convection, and low pressure Outside the Tropics, hurricanes cannot develop, because the sea is never warm enough. They are exclusively tropical, but once formed, hurricanes can and sometimes do travel long distances. Living outside the Tropics does not guarantee protection from these fiercest of all tropical storms. Occasionally they reach northern Europe, much weakened after so long a journey, but still with enough energy to wreak considerable havoc, and made worse because Europeans do not expect them, prepare inadequately if at all, and are invariably taken by surprise.

Warming, convection, and low pressure Before airplanes were invented, there were many arguments among scientists and engineers about the feasibility of building a flying machine that is heavier than air. Some scientists said the whole idea was ridiculous, and in December 1903 the New York Times published an editorial strongly opposing the foolish waste of public funds on experiments by Samuel Pierpoint Langley (1834–1906). Langley, an American astronomer, had designed an airplane powered by a steam engine. Several pilotless models flew successfully, but it proved impossible to scale the machine up to a size large enough to carry a human. The federal government funded three attempts. It was after the third failure that the Times denounced the project, saying it would be 1,000 years before humans could fly. Nine days later, on December 17, the Wright brothers proved them wrong. “Lighter-than-air” machines were the alternative to “heavier-thanair” machines, and the two varieties competed vigorously for many years. Airships—which were lighter-than-air—entered passenger service, and in 1936 the German Hindenburg began carrying passengers between Germany and the United States. It was not until several airships crashed in the 1930s, culminating in the crash of the Hindenburg at Lakehurst, New Jersey, on May 6, 1937, that people began to think “heavier-than-air” airplanes were safer. Airships were big. The Hindenburg was 804 feet (245 m) long, powered by four 1,100-horsepower diesel engines, and carried up to 50 passengers in considerable comfort. It carried fuel for its engines and, because the North Atlantic crossing took about 65 hours, food and sleeping accommodations for its crew and passengers. Clearly, it was heavier than air, but its engines and cabins were housed in a structure below the main hull, and it was the hull that accounted for almost all of its great size. The hull was filled with hydrogen. Modern airships (for the craft are making something of a comeback) use helium. This is more expensive than hydrogen but has the great advantage of being nonflammable.

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Hurricanes Both hydrogen and helium are lighter than the nitrogen and oxygen of which air is composed, and the difference in weight between the gas in the hull and an equivalent volume of air is equal to the weight of the hull, engines, and accommodations. An airship is made from materials weighing more than air, but the “lifting gas” compensates for their weight. In effect, an airship is a balloon equipped with engines and that can be steered. Some balloons also use helium as a lifting gas, but most do not. Ballooning has become a popular sport, and the balloons that drift peacefully across the countryside in summer are kept aloft by air itself. They are “hot-air” balloons.

Buoyancy and Archimedes Below the envelope of the balloon itself is the basket in which the passengers ride, and immediately above the basket there is a powerful burner, fuelled by gas. When the burner fires, hot air rises into the envelope and the balloon rises. When the burner is not lit, the air in the envelope slowly cools and the balloon starts to descend. Like an airship, a balloon has weight, but unlike an airship, a hot-air balloon uses no lifting gas that is lighter than air. It is lifted by air itself. This is possible because when air is heated its density decreases. The principle underlying this was discovered in 1787 by the French physicist Jacques A. C. Charles (1746–1823) and confirmed more accurately in 1802 by another French physicist, Joseph L. Gay-Lussac (1778–1850), so some

expanded volume

Buoyancy. Molecules are farther apart in the hot gas than they are in the cold gas; therefore a given volume of hot gas contains fewer molecules than the same volume of cold gas. Because it contains fewer molecules, the mass of the hot gas is less than that of an equivalent volume of cold gas.

original volume

Cold gas

Hot gas

Warming, convection, and low pressure

inserted body

volume of fluid displaced

people call it Charles’s law and others Gay-Lussac’s law. It states that if a given mass of gas is held at constant pressure, its volume is proportional to its temperature. This is expressed mathematically as V = kT, where V is the volume, T is the temperature measured in kelvins (1K = 1°C = 1.8°F), and k is a constant. If the gas is heated, therefore, it will expand to occupy a larger volume of space (increase T and V must increase in proportion). It follows that the original volume will contain less of the heated gas, and, because it contains less gas, it will have less mass and so it will weigh less. The diagram on page 38 shows what happens. Archimedes (c. 287–212 B.C.E.), the Greek mathematician and physicist, discovered that when a body is immersed in a fluid, such as water or air, it displaces an amount of that fluid equal to its own volume. This reduces the weight of the body by an amount equal to the weight of the fluid it displaced. The rule describing this is called Archimedes’ principle. The diagram on page 00 illustrates what happens. The reduction in weight is like a force pushing the body upward. The force is called buoyancy. It is positive if it acts in an upward direction, but air can also experience negative buoyancy, forcing it downward. The amount of buoyancy acting on a body—or on a “parcel” of air—can be calculated if you know the mass and density of the body (or air), the density of the surrounding fluid, and the gravitational acceleration (g = 32.18 feet per second per second; 9.807 m s–2). Meteorologists determine whether air is stable or unstable (see sidebar, page 4) by calculating the buoyancy force acting on it.

What happens when air is heated Air is a mixture of gases. These exist as molecules moving freely in all directions. They collide with one another frequently and ricochet in new

39

Archimedes’ principle. When a body is immersed in a fluid it displaces an amount of fluid equal to its own volume. This reduces the weight of the body by an amount equal to the weight of the displaced fluid.

40

Hurricanes directions. The speed at which they travel depends on their temperature. Heat is a form of energy, and when gas molecules absorb it, that energy changes from heat into energy, of motion (called kinetic energy). When molecules are traveling faster, collisions between them are more violent and they ricochet farther. This causes them to move, or be “bounced,” farther apart. The distance between them increases. That is why gas expands when it is heated and why a given volume of hot gas contains fewer molecules than a similar volume of cold gas. When the burner fires, hot air expands upward into the envelope of the hot-air balloon. Any given volume of the expanding air contains fewer molecules than the cooler air around it, so the hot air rises to the top of the envelope and denser, cool air is forced out at the bottom to make room for it. After a short time, the air inside the envelope is less dense than the air outside the envelope, so it weighs less than a similar volume of outside air. Being lighter, the warm air rises like an air bubble rising through a bottle of water—just the way Archimedes said it would. It is trapped inside the envelope, however, so it lifts the entire balloon, envelope, burner, basket, passengers, and all. As with the airship, the entire structure is lighter than air because the difference between the weight of the air inside and outside the envelope is equal to the weight of the other components. Now think about what happens when the Sun warms the Earth. Radiation from the Sun has very little effect on the gases of the atmosphere. It passes through them as though they were not there and is not absorbed until it reaches the surface of land or water. There it is absorbed and the surface is warmed. The warmed surface then warms the air in contact with it. Solar radiation always warms air from below, not from above, and it is this warming from below that produces our weather. There is now a layer of air warmed by the surface with which it is in contact. Because it is warmed, it expands and, because it expands, it becomes less dense and weighs less, volume-for-volume, than the surrounding air. It has buoyancy, so it rises. Its place near the surface is taken by cooler air, and this is warmed and made buoyant in its turn. The process continues for as long as the Sun continues to warm the surface, producing a stream of rising air fed from below by inflowing cool air and distributing heat vertically through the atmosphere by convection. Once air expands and starts to rise it begins cooling again. The temperature in the atmosphere decreases with increasing height above the surface. The rate at which temperature falls is known as the lapse rate and averages 5.5°F for every 1,000 feet (10°C per km) in dry air and about 3°F per 1,000 feet (6°C per km) in moist air. Expanding, rising air cools quite independently of the temperature of the air around it. This is called adiabatic cooling (see sidebar, page 41). Eventually, the temperature of the rising air is similar to that of the surrounding air, so its density and volume-for-volume weight are also similar. When it reaches this height, it will rise no farther, because it is

Warming, convection, and low pressure

41

Adiabatic cooling and warming Air is compressed by the weight of air above it. Imagine a balloon partly inflated with air and made from some substance that totally insulates the air inside. No matter what the temperature outside the balloon, the temperature of the air inside remains the same. Imagine the balloon is released into the atmosphere. The air inside is squeezed between the weight of air above it, all the way to the top of the atmosphere, and the denser air below it. Suppose the air inside the balloon is less dense than the air above it. The balloon will rise. As it rises, the distance to the top of the atmosphere becomes smaller, so there is less air above to weigh down on the air in the balloon. At the same time, as it moves through air that is less dense, it experiences less pressure from below. This causes the air in the balloon to expand. When air (or any gas) expands, its molecules move farther apart. The amount of air remains the same, but it occupies a bigger volume. As they move apart, the molecules must “push” other molecules out of their way. This uses energy, so as the air expands its molecules lose energy. Because they have less energy they move more slowly.

When a moving molecule strikes something, some of its energy of motion (kinetic energy) is transferred to whatever it strikes and part of that energy is converted into heat. This raises the temperature of the struck object by an amount related to the number of molecules striking it and their speed. In expanding air, the molecules are moving farther apart, so a smaller number of them strike an object each second. They are also traveling more slowly, so they strike with less force. This means the temperature of the air decreases. As it expands, air cools. If the air in the balloon is denser than the air below, it will descend. The pressure on it will increase, its volume will decrease, and its molecules will acquire more energy. Its temperature will increase. This warming and cooling has nothing to do with the temperature of the air surrounding the balloon. It is called adiabatic warming and cooling, from the Greek word adiabatos, meaning “impassable.”

Adiabatic cooling and warming. Effect of air pressure on rising and sinking air. Air is compressed by the weight of air above it. A “parcel” or “bubble” of air is squeezed between the weight of air above and the denser air below. As it rises into a region of less dense air, it expands. As it sinks into denser air, it contracts.

now denser and heavier than the air immediately above it. It has lost its positive buoyancy and now possesses neutral buoyancy. If the air is heated very strongly by its contact with the surface and if the rising air is constantly replenished by more air being warmed beneath it, it may rise to a very great height. In such a case, adiabatic cooling can reduce its temperature to around –74°F (–59°C) at an altitude of about 7

42

Hurricanes miles (11 km). Solar heating is strongest in the Tropics, and there warmed air rises to about 11 miles (18 km), where the temperature is around –112°F (–80°C). Above this height, varying from about 11 miles (18 km) over the equator and 7 miles (11 km) over the poles, the air temperature no longer falls with increasing height, and at a higher level still it starts increasing as height increases. The region of the atmosphere in which this occurs is called the stratosphere, where the air forms layers (strata) with little vertical movement. The lower boundary of the stratosphere, called the tropopause, traps rising air so it can rise no farther. In equatorial regions, then, air is being heated strongly at the surface and is rising all the way to the tropopause. In fact, it is not quite so simple. Surface heating accounts only partly for the buoyancy of the rising air. Equatorial air is moist and is also heated during its rise by latent heat that is released as water vapor condenses. Also, although adiabatic cooling is the most important cause of its fall in temperature with height, there is some mixing of warm, rising air and the cooler, surrounding air, which cools the rising air.

Air pressure Wherever you stand on the surface of the Earth there is a mass of air above you in what you can imagine as a column extending all the way to the top of the atmosphere. Air has weight. This was discovered in 1644 by the Italian physicist Evangelista Torricelli (1608–47), who worked as an assistant to Galileo (1564–1642) and later succeeded him as mathematician to the court of Tuscany. Torricelli invented an instrument to help him decide whether air has weight. The instrument became known as a barometer, and it is still used today. The weight of air exerts a pressure, just as the weight of any physical object will exert pressure on whatever lies beneath it. Just how much pressure the weight of the air exerts depends on the amount of air present in the column. This can vary. If you climb to the top of a mountain, for example, the column of air above you will be a little shorter, so it will contain less air and the pressure it exerts will be smaller. Pressure can also vary at sea level and for the same reason: if the quantity of air above a particular place (that is to say the number of air molecules) decreases, then the air will weigh less and so the surface pressure will decrease. If the amount of air increases, the surface pressure will also increase. Where the surface pressure is lower in one place than in another place nearby, air will flow from the area of high pressure to the area of low pressure—rather like water flowing downhill—although it does not flow in a straight line owing to the Coriolis effect (see sidebar, page 69). This flow of air, from high to low pressure, is what we feel as wind, and the strength of the wind varies according to the difference in pressure between the two areas (see sidebar, page 43). A hurricane produces very fierce winds

Air pressure, highs, and lows When air is warmed, it expands and becomes less dense. When air is chilled, it contracts and becomes more dense. Air expands by pushing away the air around it. It rises, because it is less dense than the air immediately above it. Air flows in to replace it, lifting it upward, and in turn is warmed by contact with the surface, so it also expands and rises. If you imagine a column of air extending all the way from the surface to the top of the atmosphere, warming from below causes air to be pushed out of the column, so it contains less air (fewer molecules of air) than it did when it was cooler. Because there is less air in the column, the pressure its weight exerts at the surface is reduced. The result is an area of low surface pressure, often called simply a low. In chilled air the opposite happens. The air molecules move closer together, so the air contracts, becomes more dense, and sinks. The amount of air in the column increases, its weight increases, and the surface atmospheric pressure also increases. This produces an area of high pressure, or simply a high. At sea level, the atmosphere exerts sufficient pressure to raise a column of mercury about 30 inches (760 mm) in a tube from which the air has been removed. Meteorologists call this pressure one bar and used to measure atmospheric pressure in millibars (1,000 millibar (mb) = 1 bar = 106 dynes cm–2 = 14.5 lb in–2). Millibars are still the units quoted

in newspaper and TV weather forecasts, but the international scientific unit has changed. Scientists now measure atmospheric pressure in pascals (Pa): 1 bar = 0.1 MPa (megapascals or millions of pascals); 1 mb = 100 Pa. Air pressure decreases with height, because there is less weight of air above to exert pressure. Pressure measured at different places on the surface is corrected to sea-level pressure, to remove differences due only to altitude. Lines are then drawn, linking places where the pressure is the same. These lines, called isobars, allow meteorologists to study the distribution of pressure. Like water flowing downhill, air flows from high to low pressure. Its speed, which we feel as wind strength, depends on the difference in pressure between the two regions. This is called the pressure gradient. On a weather map it is calculated from the distance between isobars, just as the distance between contours on an ordinary map allows the steepness of hills to be measured. As the diagram shows, the steeper the gradient, the closer together the isobars are, and the stronger the wind. Moving air is subject to the Coriolis effect, which swings it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, with the result that clear of the surface, winds flow parallel to the isobars rather than across them (see sidebar, page 59).

steep gradient;

LOW 990 994

998

1002 wind direction

Pressure gradient and wind speed. A wind that blows at right angles to the isobars, like water flowing down a hillside, is called a gradient wind. Above the ground winds flow parallel to the isobars and are called geostrophic winds.

44

Hurricanes because there is a great difference in pressure between the area covered by the hurricane system and the air surrounding it.

Tropical air circulation Over equatorial regions, where the surface is being heated strongly throughout the year and air warmed by contact with it is expanding and rising, the air all the way up to the tropopause is less dense than air to the north and south. This means there is less air (fewer air molecules) over equatorial regions than there are elsewhere and so the surface air pressure is permanently low compared with the pressure elsewhere. When it reaches the tropopause and can rise no higher, the equatorial air spreads away from the equator, moving northward in the Northern Hemisphere and southward in the Southern Hemisphere. It is now extremely cold, because it has cooled adiabatically, and its density increases because it is being fed constantly by more rising air. When the cold air moving away from the equator meets air flowing toward the equator, both sink to the surface. This air is also very dry. At low levels it is moist, because water readily evaporates into it and most of the equatorial regions are covered by ocean. The amount of water vapor air can carry depends on its temperature, however, and as the rising air cools, so more and more of its water vapor condenses (see sidebar, page 52). By the time it reaches the tropopause the rising air has lost almost all of its water vapor. Cold, dry, dense air that originated over the equator sinks all the way to the surface in the Tropics. As it sinks, it warms adiabatically, so by the time it reaches the surface it is hot, but still very dry. It produces deserts throughout the Tropics in both hemispheres. It also produces a region of permanently high surface pressure. Just below the tropopause, air is constantly being fed into the flow from the direction of the equator, increasing the amount (number of molecules) of air between the surface and tropopause and, therefore, its weight and the pressure it exerts at the surface. At the surface, some of the air moves toward the equator and some moves away from the equator. The air moving away from the equator extends the deserts into higher latitudes. The air moving toward the equator replaces the warm rising air and completes the cycle. Air rises over the equator, moves away from the equator and sinks, then flows back to the equator at a low level. This pattern of movement is called a convection cell, and the huge atmospheric convection cell moving air between the equator and Tropics is called a Hadley cell. The diagram shows how it works. It is called a Hadley cell because it was discovered in the 17th century by George Hadley (see “Ocean currents and sea-surface temperature,” page 24). Wind is simply air on the move, and Hadley wanted to explain why tropical winds are so reliable. As the sidebar on page 35 explains, how-

Warming, convection, and low pressure

45

Tropopause

cold air moves away from the equator

latent heat causes more warming

dense air descends

water vapor condenses

warm air rises

equator

ever, we now know that there is not just one convection cell, as Hadley thought, but three distinct systems of cells together making up what meteorologists call the three-cell model of atmospheric circulation. The three-cell model explains how heat is carried from equatorial regions into high latitudes. It also explains the general distribution of pressure systems. Wherever air is rising, surface pressure will generally be low, and where air is sinking, surface pressure will be high. Pressure is low in equatorial regions, high in the Tropics and subtropics, and high at the poles. In midlatitudes pressure is very variable. That is where polar air and tropical air meet, at a boundary that moves north and south according to conditions close to the tropopause. This makes midlatitude weather very changeable and difficult to forecast. Nevertheless, over the region as a whole pressure is low more often than it is high. Warm, rising air is what meteorologists call unstable, because it continues to rise until it reaches a level where the surrounding air has the same density as itself. If the air is moist as well as unstable, towering clouds will form in it. These clouds can cause storms anywhere, but near the equator, where the air is moist and more unstable than it is anywhere else on Earth, they can cause hurricanes—and sometimes do.

air warms adiabatically

latitude 30°

Hadley cell. Air rises at the equator, moves away at high level, subsides in the Tropics, and returns to the equator as the low-level trade winds.

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Hurricanes

Storm clouds Rising air cools. As it does so, water vapor may condense into tiny droplets or ice crystals to form clouds. But this is only part of the story. If this were all there is to it, clouds would all be alike and, obviously, they are not. Some are small, white, and puffy, the kind of clouds seen on a fine day in summer and called fair-weather cumulus. Clouds may be in flat layers or billowing heaps. There are white clouds, black clouds, and clouds of every imaginable shade of gray. Some clouds bring fine drizzle, some showers, and these may fall as rain or snow. Other clouds bring torrential rain, snow, or hail, often with thunder and lightning. It is these storm clouds that, in some places and under certain conditions, can cause hurricanes. Clouds are not all the same, and throughout history people tried to find ways to describe the different types clearly. It is not so easy as you might think. In ancient Greece, the philosopher Theophrastus (c. 372–c. 287 B.C.E.), a student of Plato and friend of Aristotle, wrote of “clouds like fleeces of wool” and “streaks of cloud.” Theophrastus was good at classifying things, especially plants. He is often called “the father of botany.” Yet classifying clouds defeated him—the names he suggested are not very helpful. Frenchman Jean Lamarck (1744–1829) was a great naturalist and classifier of plants and animals. He also tried classifying clouds, but with little success. In his system, clouds were described using such words as “sweepings,” “bars,” “grouped,” “piled,” “veiled,” and “dappled.” It was not until 1803 that Luke Howard (1772–1864), a young apothecary (druggist) and amateur meteorologist living in London, published an article setting out his scheme for classifying clouds. This one worked and could be made to include every kind of cloud. The classification he devised formed the basis of the one used to this day. All meteorologists use it in a way that is standardized throughout the world by the World Meteorological Organization (WMO), which is an agency of the United Nations (see sidebar, page 47), so if you describe a cloud by its proper, scientific name, any meteorologist in the world will know what you mean. The system is used just like the one used to name plants and animals. Every language has its own word for “dog,” but call the animal Canis familiaris and zoologists of all nationalities will understand you. The “official” classification is contained in the International Cloud Atlas, published by the WMO. First published in 1896, the Atlas is regularly updated. It comprises two volumes. The first is a loose-leaf manual describing the way clouds and other atmospheric phenomena should be observed. The second contains 196 pages of photographs (161 in color) with captions defining cloud types. There is also an abridged, single-volume version with 72 photographs, some in color and some in black and white, of all the cloud types and a variety of other phenomena, such as fog and waterspouts, with a text describing cloud observation.

Storm clouds

47

How clouds are classified There is an internationally agreed scheme for classifying clouds on the basis of their appearance and structure. Under the scheme, clouds are grouped into 10 distinctive types, called genera (singular genus). The genera are subdivided into species (singular and plural) and species into varieties. There are also accessory clouds (small clouds seen in association with a bigger cloud of a different type) and supplementary cloud features (extensions or protrusions from a cloud). Genera and species names, which are in Latin, have standard abbreviations. Variety names are usually written in full, as are the names of accessory clouds and supplementary cloud features. Stratocumulus (Sc), for example, may form in an almond or lens shape, producing the species lenticularis (len), abbreviated as Sclen. If a cloud appears to consist of bands, it may be given the variety name radiatus. Cloud genera are described as low-level, medium-level, or high-level according to the height at which they most commonly form, although clouds can form at higher or lower levels. Large storm clouds, which have a low-level base but extend to a great height, are counted as low-level clouds, mainly for convenience. Most medium-level clouds have names beginning with the prefix alto- and the names of high-level clouds have the prefix cirr-. The illustration, shows the 10 cloud genera, as well as the “anvil” that often forms where the ice crystals at the top of a cumulonimbus cloud are swept out into this distinctive shape by the wind. The scientific name for the anvil is incus, and it is a supplementary cloud feature.

Cloud genera Low-level c louds Cloud base from sea level to 1.2 miles (2,000 m). Stratus (St). An extensive sheet of featureless cloud that will produce drizzle or fine snow if it is thick enough. Stratocumulus (Sc). Similar to St, but broken into separate, fluffy-looking masses. If thick enough, it also produces drizzle or fine snow.

Cumulus (Cu). Separate, white, fluffy clouds, usually with flat bases. There may be many of them, all with bases at about the same height, and they may merge into a single cloud. Cumulonimbus (Cb). Very large cumulus, often towering to a great height. Because they are so thick, Cb clouds are often dark at the base. If the tops are high enough, they will consist of ice crystals and may be swept into an anvil shape. Medium-level clouds Cloud base from 1.2–2.5 miles (2,000–4,000 m) in polar regions, 4–5 miles (6,000–8,000 m) in temperate and tropical regions. Altocumulus (Ac). Patches or rolls of cloud joined to make a sheet. Ac is sometimes called “wool-pack cloud.” Altostratus (As). Pale, watery, featureless cloud that forms a sheet through which the Sun may be visible as a white smudge. Nimbostratus (Ns). A large sheet of featureless cloud, often with rain or snow, that is thick enough to completely obscure the Sun, Moon, and stars. It makes days dull and nights very dark. High-level cloud Cloud base from 2–5 miles (3,000–8,000 m) in polar regions, 3–11 miles (5,000–18,000 m) in temperate and tropical regions. All high-level clouds are made entirely from ice crystals. Cirrus (Ci). Patches of white, fibrous cloud, sometimes swept into strands with curling tails (“mares’ tails”). Cirrocumulus (Cc). Patches of thin cloud, sometimes forming ripples, fibrous in places, and with no shading that would define their shape. Cirrostratus (Cs). Thin, almost transparent cloud forming an extensive sheet and just thick enough to produce a halo around the Sun or Moon. (continues)

(continued)

35,000

30,000

25,000

20,00

15,000

10,000

5,000

Cloud types

Storm clouds

How clouds form Although the cloud classification is detailed and quite complicated, many of the names it uses describe clouds that are seen only rarely, and then only in some places. There are only 10 main cloud types and their names are not hard to remember. All clouds form by the condensation of water vapor, but the type of cloud that results depends on the height at which it forms and what is happening in the air itself. If the air is being heated strongly from below, so it is rising and heat is moving vertically by convection, the air is said to be unstable. Heaped clouds of the cumulus type (the adjective is cumuliform) will develop in it, and the stronger the vertical movement of air the taller they will be. If there is little vertical movement, and especially if it is slowly sinking, the air is said to be stable. Layered clouds of the stratus type (the adjective is stratiform) will form in it. Both cumuliform and stratiform clouds can produce rain or snow, but stratiform clouds usually deliver steady, persistent precipitation and cumuliform clouds produce showers, which can be heavy. (Precipitation is the general name given to any kind of water falling from a cloud and includes drizzle, rain, hail, sleet, and every type of snow.)

Humidity: how moist is the air? Air may be relatively moist or dry. Clouds cannot form in very dry air, but the amount of moisture in the air varies widely from place to place and time to time. Water vapor is a gas, of course, and you cannot see it, but the amount present can be measured, as the humidity of the air. In a polar desert, where it is so cold that water vapor changes directly into ice, there may be almost no water vapor in the air, but in warm, moist air it may account for as much as 7 percent of the air by weight. Expressed in this way, as the mass of water vapor in a given mass of air not counting the water vapor, the result is known as the mixing ratio or mass mixing ratio. It is usually measured as grams of the gas in one kilogram of air without the gas. Because it is measured in units of mass, with no reference to volume, the mixing ratio is not affected by changes of temperature or pressure. An alternative measure is the absolute humidity. This is the mass of the water vapor that is present in a given volume of air, usually expressed in grams per cubic meter (1 g m–3 = 0.046 ounces per cubic yard). If atmospheric moisture is measured as the mass of water vapor in a given mass of air including the water vapor, it is called the specific humidity. Water vapor accounts for such a small proportion of the total mass of the air, however, that it makes little difference whether the total mass is measured with or without the water vapor. Consequently, for all practical purposes, the mixing ratio and specific humidity are the same, although they are calculated differently.

49

50

Hurricanes The term most commonly used, though, is relative humidity (RH). When weather forecasters state the humidity, relative humidity is what they mean. This is the amount of water vapor in the air as a percentage of the largest amount the air could hold at that temperature. When the relative humidity reaches 100 percent, the air is saturated and water will start to condense. A hygrometer is an instrument that measures the humidity of the air, either with a needle and dial to display the value or a digital display if the instrument is electronic. If you have a hygrometer at home, the value it shows will be the RH.

Stable and unstable air For water vapor to condense into droplets the air must be cooled until its relative humidity reaches 100 percent, a temperature known as its dewpoint. Tiny particles, called cloud condensation nuclei (CCN), must also be present onto which the vapor can condense (see sidebar below). As the water condenses, however, something else happens. Condensation releases heat, known as latent heat. This warms the surrounding air,

Latent heat and dewpoint Water can exist in three different states, or phases: as gas (water vapor), liquid (water), or solid (ice). In the gaseous phase, molecules are free to move in all directions. In the liquid phase, molecules join together in short “strings.” In the solid phase, molecules form a closed structure with a space at the center. As water cools, its molecules move closer together and the liquid becomes denser. Pure water at sea-level pressure reaches its densest at 39°F (4°C). If the temperature falls lower than this, the molecules start forming ice crystals. Because these have a space at the center, ice is less dense than water and, weight for weight, has a greater volume. That is why water expands when it freezes and why ice floats on the surface of water. Molecules bond to one another by the attraction of opposite charges, and energy must be supplied to break those bonds. This energy is absorbed by the molecules without changing their tempera-

ture, and the same amount of energy is released when the bonds form again. This is called latent heat. For pure water, 600 calories of energy are absorbed to change one gram (1 g = 0.035 oz; 600 cal g–1 = 2,501 joules per gram; joules are the units scientists use) from liquid to gas (evaporation) at 32°F (0°C). This is the latent heat of vaporization, and the same amount of latent heat is released when water vapor condenses. When water freezes or ice melts, the latent heat of fusion is 80 cal g–1 (334 J g–1). Sublimation, the direct change from ice to vapor without passing through the liquid phase, absorbs 680 cal g–1 (2,835 J g–1), equal to the sum of the latent heats of vaporization and fusion. Deposition, the direct change from vapor to ice, releases the same amount of latent heat. The amount of latent heat varies very slightly with temperature, so this should be specified when the value is given. The standard values given here are

Storm clouds

correct at 32°F (0°C). The diagram illustrates what happens. Energy to supply the latent heat is taken from the surrounding air or water. When ice melts or water evaporates, the air and water in contact with them are cooled, because energy has been taken from them. That is why it often feels cold during a thaw and why our bodies can cool themselves by sweating and allowing the sweat to evaporate. When latent heat is released by freezing and condensation, the surroundings are warmed. This is very important in the formation of the storm clouds from which hurricanes and tornadoes develop. Warm air rises, its water vapor condenses, and this warms the air still more, making it rise higher.

freezing melting

680 cal/g

80 cal/g

water oxygen

Warm air is able to hold more water vapor than cool air can, and the amount of water vapor air can hold depends on its temperature. If moist air is cooled, its water vapor will condense into liquid droplets. The temperature at which this occurs is called the dewpoint temperature. It is the temperature at which dew forms on surfaces and evaporates from them. At the dewpoint temperature, the air is saturated with water vapor. The amount of moisture in the air is usually expressed as its relative humidity (RH). This is the amount of water present in the air expressed as a percentage of the amount needed to saturate the air at that temperature.

sublimation (direct change between water vapor and ice)

ice

51

evaporation condensation

600 cal/g water vapor

hydrogen

Latent heat. As water changes between the gaseous, liquid, and solid phases, the breakage and formation of hydrogen bonds linking molecules releases or absorbs energy as latent heat.

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Hurricanes

Evaporation, condensation, and the formation of clouds When air rises, it cools adiabatically. If it is dry, at first it will cool at the dry adiabatic lapse rate of 5.5°F every 1,000 feet (10°C per km). Moving air may be forced to rise if it crosses high ground, such as a mountain or mountain range, or meets a mass of cooler, denser air at a front. Locally, air may also rise by convection where the ground is warmed unevenly. There will be a height, called the condensation level, at which its temperature falls to its dewpoint. As the air rises above this level, the water vapor it contains will start to condense. Condensation releases latent heat, warming the air, and once the relative humidity of the air reaches 100 percent and the air continues to rise, it will cool at the saturated adiabatic lapse rate, of about 3°F per 1,000 feet (6°C per km). Water vapor condenses onto minute particles, called cloud condensation nuclei (CCN). If the air contains CCN consisting of minute particles of a substance that readily dissolves in water, water vapor will condense at a relative humidity as low as 78 percent. Salt crystals and sulfate particles are common

examples. If the air contains insoluble particles, such as dust, the vapor will condense at about 100 percent relative humidity. If there are no CCN at all, the relative humidity may exceed 100 percent and the air will be supersaturated, although the relative humidity in clouds rarely exceeds 101 percent. Cloud condensation nuclei range in size from 0.001 µm to more than 10 µm diameter, but water will condense onto the smallest particles only if the air is strongly supersaturated, and the largest particles are so heavy they do not remain airborne very long. Condensation is most efficient on CCN averaging 0.2 µm diameter (1 µm = one-millionth of a meter = 0.00004 inches). At first, water droplets vary in size according to the size of the nuclei onto which they condensed. After that, the droplets grow, but they also lose water by evaporation, because they are warmed by the latent heat of condensation. Some freeze, grow into snowflakes, then melt as they fall into a lower, warmer region of the cloud. Others grow as large droplets collide and merge with smaller ones.

sometimes increasing its buoyancy enough to allow the denser air above it to subside and push it upward. The subsiding air then warms adiabatically, sometimes enough to evaporate the water droplets in it. Evaporation absorbs latent heat from the surrounding air, cooling it, and this may cause the air to sink even farther. If you have ever flown through cloud in an airplane, you will know that it looks just like fog and gives the impression of being nothing more than a great mass of very tiny water droplets hanging motionless in the air. In fact, though, very complicated things are going on inside any cloud (see sidebar above). In highly unstable air, conditions inside a cloud can be extremely violent. If you have ever flown through or even close to cumuliform (heaped) cloud, you may have been advised to fasten your seat belt, because the air is likely to be “bumpy” due to the strong vertical air currents that alternately lift the airplane upward and then drop it again. It is not very likely

Storm clouds

1

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condensation level

2

3

warm air

cold air

that you will have flown through a really big storm cloud. Pilots avoid them, and for good reason. The electrical fields inside them can make compasses and other instruments useless, and the vertical air currents can make the airplane uncontrollable or even tear it apart.

Why air rises. 1. Air is forced to rise over high ground (orographic lifting). 2. Air rises by convection, due to uneven heating of the ground. 3. Air is forced to rise along a weather front.

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Hurricanes

Inside a storm cloud At a certain height, known as the condensation level, rising air will cool to its dewpoint temperature and water droplets will start to form. This level marks the base of the cloud. The air is still rising strongly, and the release of latent heat warms it and makes it rise even faster. It is mainly the release of the latent heat of condensation in the towering clouds that form in very warm, very moist air that drives the low-latitude Hadley cells (see sidebar, page 35). Inside a storm cloud air is rising by convection and cooling adiabatically, its water vapor is condensing, and cloud droplets are evaporating. Eventually, the rising air cools so much that its water droplets freeze, releasing still more latent heat. As they grow larger and heavier, ice crystals start to fall, colliding with one another and splintering as they do so. At a lower level they melt, but are then carried aloft again in rising air currents, so droplets freeze, fall to a level at which they start to melt, then freeze again, collecting more water all the time. This is how hailstones form. If you could cut through a big one, you would see it is layered, like an onion. When a hailstone grows too heavy for the upcurrents to lift, it falls from the cloud, and the bigger the stones that reach the ground in a hailstorm, the stronger the upcurrents in the cloud above, and the taller the cloud. Inside the storm cloud upcurrents and downcurrents form side by side, and water is constantly changing from vapor to liquid to ice and back again. Ice particles are shattered by collisions, and large liquid droplets disintegrate. The small ice fragments and droplets acquire positive charge and drift to the top of the cloud. The large ones acquire negative charge and sink to the bottom of the cloud. Before long this starts to distribute electrical charge inside the cloud. As the diagram on page 55 shows, the upper part of the cloud becomes positively charged and the lower part negatively charged. Electricity flows to the surface constantly from the ionosphere. In that part of the atmosphere, at a height of about 55–125 miles (90–200 km), molecules absorb shortwave solar radiation, which supplies the energy to strip electrons from atomic nuclei. An atom that has lost one or more electrons is an ion and is said to have been ionized, hence the name ionosphere for the region of the atmosphere in which a significant proportion of the air is ionized. The electrons, carrying negative charge, flow downward, leaving the ionized nuclei with a positive charge and the Earth’s surface with a negative charge. Inside a storm cloud this electrical polarization becomes much stronger, and the powerful negative charge at the base of the cloud can induce local areas of positive charge on the ground.

Thunder and lightning Air is a good electrical insulator, but locally the strength of the electrical field can exceed about 300,000 volts per foot (1 million volts per meter). When this happens, a spark flies from positive to negative. This is light-

Storm clouds ning, leaping from place to place along a forking path. The lightning spark heats the air around so suddenly the air explodes. Thunder is the noise of that explosion. As the illustration on page 56 shows, lightning will spark from one place in a cloud to another in the same cloud. From the ground this appears as a white flash, or sheet lightning. It can also spark from one cloud to another and, of course, lightning can flash between a cloud and the ground. There are several stages to a flash of “forked lightning.” The first, called the stepped leader, carries negative charge—in fact, a stream of electrons— downward from the base of the cloud, jumping from place to place wherever it finds the least resistance. As it moves, it ionizes the air adjacent to it, creating an ionized path about 8 inches (20 cm) in diameter—the diameter of a lightning stroke. As it approaches the accumulated positive charge on the surface, the stepped leader is met by a return stroke, carrying positive charge upward along the same ionized path. This is the main part of the flash. The stepped leader and return stroke only partially neutralize the charge in the cloud. They are followed by a dart leader, starting at a higher

55

Charge separation in a storm cloud. Small ice fragments and water droplets carry positive charge upward. Bigger fragments and droplets carry negative charge downward. This separates charge inside the cloud. Eventually, lightning sparks between positive and negative.

56

Types of lightning. Lightning may spark between two points inside the same cloud, between one cloud and another, or between a cloud and the ground.

Hurricanes

level inside the cloud, and a second stroke. This also triggers a return stroke. Further flashes follow until the cloud is completely neutralized. A lightning stroke typically consists of three or four flashes, about 50 milliseconds apart, and the entire flash lasts about 0.2 second. The flash carries a current of about 10,000 amperes. Thunderstorms can develop only in really huge, towering clouds. Often they extend from a height of 1,000 feet or so all the way to the tropopause. This much height is needed for water to exist as liquid in some places and ice at others and to produce strong vertical currents. Such (cumulonimbus) clouds form in unstable air or in air that becomes unstable once something has forced it to start rising (it is conditionally unstable). There are several ways in which air can be made to rise. These account for the formation of most clouds of the cumuliform (heaped) type (see sidebar, page 47), including most cumulonimbus thunderstorm clouds. Over the tropical seas, however, the convective forces inside a weather system that produces cumulonimbus may sometimes be so strong that the system as a whole becomes much more violent even than the largest thunderstorm. It may grow into a tropical storm and then into a full hurricane.

How a hurricane begins

INSIDE THE STORM How a hurricane begins On Saturday, October 6, 2001, Hurricane Iris killed three people as it crossed the Dominican Republic. The following day it lashed the coast of Jamaica, its 85-MPH (137-km/h) winds ripping trees from the ground and tearing roofs from houses. Then, as it crossed the open sea, Iris strengthened to a category 4 hurricane, with sustained winds of 140 MPH (225 km/h), and headed for the Central American mainland. It struck Belize on Monday, making landfall about 80 miles (129 km) south-southwest of Belize City, and sending people hurrying to find shelter. Then the storm died down rapidly. It was bad, and very frightening, but it could have been worse. As the winds drop, the rain eases, and the clouds start to break overhead, it is clear that the storm is dying—and when it dies, it often dies quickly. After an hour or two only the recollections of shocked survivors and the sad wreckage of property, homes, and lives remain to bear witness to what happened. That is how it dies. But how was it born?

Easterly waves Where the trade winds meet, there is a permanent area of low pressure, called the equatorial trough. This is where winds from the northeast in the Northern Hemisphere and southeast in the Southern Hemisphere converge and air rises strongly (see sidebar, page 11). The trough moves north and south with the seasons, but in a very complicated way that scientists do not fully understand. Hurricanes form either along the equatorial trough or close to it. Hurricanes are able to form only over very warm water. The sea-surface temperature must be at least 80°F (27°C) over a large area. This means a hurricane can begin only in the Tropics in late summer or autumn. Air at the surface is warmed by contact with the sea, and so it expands and rises. If the sea is warmer in a particular place than it is elsewhere, the air above that place will rise farther and faster than the surrounding air. This will cause a local area of low atmospheric pressure to develop. The difference in temperature is quite small. This is because the Tropics receive so much solar energy that air moves very quickly and temperatures and pressures are much the same throughout the region. Consequently, the effect of small, local differences tends to be much greater than it would be in middle latitudes, where there is much more variation. Nevertheless, local areas of slightly warmer water and lower pressure are common. Some are caused where the southern trade winds are stronger

57

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Hurricanes than elsewhere and push a section of the equatorial trough northward, forming a wave or “kink” in it. Others begin as tropical thunderstorms over land and then move westward. Over the ocean, the remains of the thunderstorm survive as an isolated, high-altitude tongue, or trough, of low pressure. The remains of a vigorous weather system in middle latitudes can also survive as a high-level trough “pointing” toward the equator. The resulting “kinks” are called easterly waves or tropical waves and, as the diagram on page 61 shows, they cause a slight distortion in the pattern made by the trade winds. Notice that the distortion makes the air follow a path that starts to curve in a counterclockwise direction. The diagram does not show an easterly wave in the Southern Hemisphere, but these curve in a clockwise direction. This direction of flow is called cyclonic, because it is the way air circulates around a cyclone, or area of low pressure (see sidebar on opposite page). The air temperature in the center of the trough is lower than that of the surrounding air, because it consists of air carried toward the equator from a higher latitude. This makes the environmental lapse rate steeper and increases the instability of the air below the trough (see sidebar, page 6). Air is accelerated upward, producing low pressure at the surface. Air is removed at high level, because of divergence—an outward flow of air—from the eastern side of the high-level trough. At first the drop in pressure is strong enough to cause only thunderstorms behind the advancing wave. If the low pressure persists for several days, it is known as a tropical disturbance. Most tropical disturbances are harmless, but a few strengthen as they move west, growing into tropical depressions, with winds up to 38 MPH (60 km/h), then into tropical storms, with winds up to 73 MPH (117 km/h). Now and again one grows further, into a full-scale hurricane.

Rising air For a tropical disturbance to grow, air within it must be rising very vigorously. In the Tropics, warm air is continuously rising high into the atmosphere, cooling, sinking, and returning toward the equator as the trade winds. It begins to sink, however, as soon as it reaches its maximum height and starts to cool. As it sinks, it warms adiabatically. This produces a layer of high-level air that is warmer than the air below it. This is an inversion— a layer of warm air overlying cooler air. Because it is associated with the trade winds, this one is known as the trade wind inversion, and it is present for most of the time. Ordinarily, rising warm air is trapped beneath it, and this limits the height to which storm clouds can grow. If they are growing very vigorously, however, driven by unusually strong upcurrents, they may penetrate the high-level inversion. When this happens, the clouds become towering giants up to 40,000 feet (12 km) tall, producing the fierce storms of a tropical depression.

How a hurricane begins

59

Christoph Buys Ballot and his law In 1857, the Dutch meteorologist Christoph Buys Ballot (1817–90) published a summary of his observations on the relationship between atmospheric pressure and wind. He had concluded that in the Northern Hemisphere winds flow counterclockwise around areas of low pressure and clockwise around areas of high pressure. In the Southern Hemisphere these directions are reversed. Unknown to Buys Ballot, a few months earlier the American meteorologist William Ferrel (1817–91) had applied the laws of physics and calculated that this would be the case. As soon as he learned of it, Buys Ballot acknowledged Ferrel’s prior claim to the discovery, but despite this, the phenomenon is now known as Buys Ballot’s law. This states that, in the Northern Hemisphere, if you stand with your back to the wind, the area of low pressure is to your left and the area of high pressure to your right. In the Southern Hemisphere, if you stand with your back to the wind, the area of low pressure is to your right and the area of high pressure to your left. (The law does

not apply very close to the equator.) The diagram below illustrates this. The law is a consequence of the combined effect of the pressure-gradient force (PGF) and the Coriolis effect, sometimes incorrectly (because no force is involved) called the Coriolis force, and always abbreviated as CorF. Air flows from an area of high pressure to one of low pressure, like water flowing downhill. Just as the speed of flowing water depends on the steepness of the slope (the gradient), so the speed of flowing air depends on the difference in pressure between high and low—the pressure gradient. Gravity is the force that makes water flow downhill. The force making air flow across a pressure gradient is the pressure-gradient force. As the air flows at right angles to the pressure gradient, the CorF, acting at right angles to the direction of flow, swings it to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. As it starts to swing to the right, the CorF decreases, until the CorF and PGF produce a resultant force that accelerates the moving air. (continues)

HIGH

W

wind direction

Cyclonic and anticyclonic flow. In the Northern Hemisphere, winds flow in a clockwise direction around a center of high pressure. This is anticyclonic flow. The winds flow counterclockwise around a center of low pressure. This is cyclonic flow. These directions are reversed in the Southern Hemisphere.

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Hurricanes

(continued) CorF is proportional to the speed of the moving air, so it increases, swinging the air still more to the right. This continues until the air is flowing parallel to the isobars (at right angles to the pressure gradient). At this point, the PGF and CorF are acting in opposite directions, but with equal magnitude, so they are in balance. If the PGF were the stronger force, the air would swing to the left and accelerate. This would increase the CorF, swinging it back to the right again. If the CorF were the stronger, the air would swing farther to the right, the PGF acting in the opposite direction would slow it, the CorF would decrease, and the air would swing to the left again.

The eventual result is to make the air flow parallel to the isobars (pressure gradient) rather than across them. The diagram below illustrates this. Near the ground, friction with the surface and objects on it slows the air. This reduces the magnitude of the CorF (which is proportional to wind speed), altering the balance in favor of the PGF and deflecting the air so it flows at an angle to the isobars, rather than parallel to them. Over land, where the surface is uneven and so friction is greatest, the wind usually blows across the isobars at an angle of about 45°. Over the ocean it crosses at about 30°. Clear of the surface, the air does flow parallel to the isobars. This is called the geostrophic wind.

LOW PRESSURE

PGF

PGF isobar PGF w in d d

i r e c ti o n isobar

The geostrophic wind. The balance between the pressure-gradient force (PGF) and Coriolis effect (CorF) causes the wind to flow parallel to the isobars.

CorF

CorF isobar

isobar HIGH PRESSURE

If a tropical storm is to grow still further, into a hurricane, the pressure in the high-level trough must continue to fall until there is a clearly defined cyclone at an altitude of about 56,000 feet (17 km), near the tropopause. The deepening low pressure draws more air upward, accelerating the winds feeding air into the column of rising air. Air flows from an area of high pressure to one of low pressure, but not in a straight line. Influenced by the Coriolis effect (see sidebar, page 69), the air swings to the right in the Northern Hemisphere until it flows around the high- or low-pressure center. Air circulates clockwise around

How a hurricane begins areas of high pressure and counterclockwise around areas of low pressure (these directions are reversed in the Southern Hemisphere). This was discovered in 1857 by the Dutch meteorologist Christoph Buys Ballot and is known as Buys Ballot’s law (see sidebar, page 57). Air drawn into the low-pressure area near the surface flows around it counterclockwise (in the Northern Hemisphere), strengthening the cyclonic circulation. The wind speed increases until the difference in pressure between the high- and low-pressure areas can accelerate it no further. As it approaches the center, it rises, spiraling upward to fill the high-level cyclone. Air flows into the low-pressure core at every level up to a height of 33,000 feet (10 km) or more. The wind speed decreases at higher levels, and the spiral spreads outward. When the diameter of the spiral is greater than about 125 miles (200 km), the air is rotating more slowly than the surface of the Earth beneath it. Its circulation then becomes anticyclonic, as the widening spiral carries air away from the core. The diagram on page 62 compares the wind and cloud patterns at low and high level. Alternatively, air may be swept away at high level if the surface lowpressure area forms near the subtropical jet stream, but not directly beneath it (see sidebar, page 00). The subtropical jet stream is usually at about latitude 30°, and it can blow at 100 MPH (160 km/h) or more, the wind speed being proportional to the difference in temperature of the air to either side. It is farther from the equator in summer than in winter, but waves develop in it, some of which may take it deep into the Tropics. Should a tropical depression intensify close to the jet stream, the high-level wind will carry away rising air, adding to the upper-level divergence.

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Development of a tropical storm. An easterly wave in the trade-wind pattern produces low surface pressure. This deepens, forming a tropical depression and then a tropical storm.

NE trade winds

easterly

tropica

SE trade winds

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Hurricanes

surface wind high-level wind

hurricane path

Wind directions around a hurricane. The surface winds spiral inward and the high-level winds spiral outward, removing rising air and thereby sustaining the vertical flow.

How a hurricane begins By removing the rising air, upper-level divergence draws more air upward. This makes the surface atmospheric pressure fall still further, although air directly below the jet stream is removed so quickly that a large, organized, upward spiral pattern is unable to form and a hurricane cannot develop. The pressure drop is not great. Ordinarily, the average sea-level pressure is 1,016 millibars (mb). At the center of a hurricane it may be no lower than 980 mb, and it rarely falls below 920 mb, although there have been hurricanes with core pressures lower than 900 mb. The lowest pressure ever recorded anywhere on Earth was 870 mb, in the eye of Typhoon Tip on October 12, 1979. This is a drop of only 4–10 percent, but it is enough.

The full-grown cyclone Now there is an area of low surface pressure, 400 miles (645 km) or more across, with a pressure of about 950 mb at the center. This is a very low pressure, and its effect is greater than it would be in middle latitudes because of another important difference between the middle-latitude system and the tropical one—their relative sizes. Big though it sounds, in fact the tropical system is much smaller than a middle-latitude cyclone—which can be more than 1,000 miles (1,600 km) across. Its smaller size means the low-pressure core is also much smaller. Consequently, the pressure gradient is steeper, and so inflowing air is accelerated much more than it would be if it were to flow across a shallower gradient. It is also accelerated more than air flowing into a middle-latitude cyclone because it is being drawn into a smaller area (see sidebar, page 71). Friction between the inflowing air and the ocean surface slows the wind and increases the angle at which it crosses the isobars. Because it crosses a warm ocean, the air feeds warmth and moisture into the core, intensifying convection. At the same time, it pushes the storm clouds closer together around the core, gathering scattered convection cells into an organized cluster. Inside the core, air that has not been swept away at high level is sinking and warming adiabatically as it does so. Consequently, the center of the hurricane—the eye—is markedly warmer than the air surrounding it. Air spiraling inward is already warm, but its temperature rises still further as it nears the warm eye. This increases its buoyancy, intensifying the convection still more. By now, huge storm clouds have developed around the eye. Warm, moist air, drawn toward the center, circulates cyclonically and in an upward spiral all the way to the cloud tops, then disperses outward, drawing more air from below. While it remains a tropical depression, there is cold air at the center, with warmer air circulating around it. There are rain showers at the center and the sky is mainly overcast. When the air in the center becomes warmer than the air flowing around it, the sky clears until only a few small clouds remain, there is no rain, and the wind speed falls to about 10 MPH (16 km/h). This calm center, often 20

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Hurricanes to 30 miles (32 to 48 km) across, is surrounded by what looks like a solid wall of cloud. It is the eye of what has now grown into a hurricane.

Vortices When you pull out the plug from a full bathtub and the water starts flowing away, usually it will soon begin to swirl in a spiral, forming a little whirlpool. If you watch the water each time you do this and note the direction in which it turns, you will find that sometimes it turns to the right and sometimes to the left. If you churn up the water while it is swirling and break up the whirlpool, then leave it to start swirling again, it may turn in the opposite direction to the one in which it turned earlier. The direction it turns is a matter of chance, and occasionally the water does not swirl at all. Some people will tell you that in the Northern Hemisphere the water always flows out of the bathtub counterclockwise and that it invariably flows clockwise in the Southern Hemisphere. They say that if you take a bath on one side of the equator, then cross to the other hemisphere and take a second bath there, the water will turn in different directions on either side. They will also tell you that if you place the bath precisely on the equator the water will not turn at all. What is more, they will usually tell you that they know someone who has tried it personally and can confirm it. This is a popular belief, similar to an urban myth, and very difficult to dislodge, but it is quite wrong—and you can easily show that it is wrong without going to all the trouble and expense of carrying your bathtub to the equator. The confusion is one of scale. A rule that applies to an ocean current or a mass of air hundreds of miles across does not necessarily apply to something as small as a bathtub. To convince yourself that the myth is just that, all you need do is watch what happens to the water every time you empty the bath.

Whirlpools On the small scale of a bathtub, moving water is affected by vorticity. This is the tendency of any moving fluid—a liquid or gas—to turn about an axis. It is what produces the whirlpool, or vortex. The axis may be horizontal or vertical. In the case of the bathtub, where the water is flowing downward, the axis is at right angles to the Earth’s surface. Air is also a fluid, and when it moves it also develops vorticity, and its vorticity about a vertical axis contributes greatly to the development of a hurricane. Vorticity often begins when two streams, of water or air, flow side by side in the same direction, but at different speeds. This produces a shearing force. The faster stream is slowed on one side by friction with the slower, and this makes it start to curve toward the slower. You use something similar to this effect to help you steer a rowboat. To make the boat turn, you row more strongly with one oar than with the other, and the boat follows a path that curves toward the side where you are rowing more slowly.

Vortices You can also watch it happen if you live near a river with a bridge across it. Stand on the bridge and drop a small stick into the river, somewhere near the middle. The stick will float away, of course, but as it does so, it may (or may not) also start to turn around. The stick’s rotational movement is its vorticity. If the stick turns, it possesses vorticity, which is positive or negative depending on the direction it turns; if it does not turn at all it is said to possess zero vorticity. This is just putting a name to the movement of the stick, though. It does not explain why the stick possesses vorticity. In order to understand that, you need to think about the water all around the stick. As you can see from the ripples and little waves on the surface, the water in the river is not really a single mass that all moves together. It is more like a vast number of tiny “particles” of water, all of them moving in their own way, but all carried along in the general movement. Flocks of birds and swarms of locusts move like this. If you film them and slow the film down, you find that although the flock or swarm moves as a whole, like the river, the individual birds or locusts are moving in every direction, including the direction opposite to the overall movement. The stick floats in the water and is carried by it, but the “particles” of water may be moving faster on one side of it than on the other. Water close to the bank moves more slowly than water in the middle of the river, for example, because it is retarded by friction, and islands, big rocks, and other obstructions will also cause friction. These local differences in speed represent the shear in the river current, and they will start the stick turning. Or the river itself may curve. In that case the water has to follow the curve, and water on the outside of the curve usually moves faster than water on the inside of the curve because it has farther to travel. This also produces a shear that will set the stick turning. Often, both of these operate together in imparting vorticity. The diagram on page 66 shows how shearing forces occur. Your twisting stick is caught in a tiny vortex, or whirlpool. Whirlpools in water can also be big and dangerous to small ships. In Greek mythology, the whirlpool in the Strait of Medina, between Sicily and Italy, was called Charybdis. It was described as a monster that lurked below the waves close to the Sicilian shore. Three times a day Charybdis sucks down water and then spews it out again. Sailors who try to avoid falling into her clutches by sailing close to the Italian shore run the risk of being caught by Scylla, another monster. She has 12 feet, six heads each with three rows of sharp teeth, and the barking heads of dogs all around her hips. She will devour any ship that ventures too close. In fact, Scylla was either a submerged rock or reef. There are other famous whirlpools, each with its own legends and stories. Off the north coast of Norway, just to the west of the Lofoten Islands, there is the Lofoten Maelstrom, known in Norwegian as the Moskstraumen. Jules Verne and Edgar Allan Poe wrote stories about the Maelstrom. Another whirlpool, also with its own legend, is located off the western coast of Scotland, in the Gulf of Corryvreckan between the islands of Scarba and Jura. Whirlpools such as these are produced by vorticity generated by obstructions to the flow of strong tidal currents. In the case of

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66 Vorticity. In the top example, the upper stream (a) is moving faster than the lower stream (b), because it must cover a greater distance in the same length of time. In the bottom example, suppose that stream (a) is moving faster than (b). In both A and B the difference in speed produces a shearing force.

Hurricanes

a b

a b

the Corryvreckan, for example, the tide flows northward very strongly through the narrow Sound of Jura. As part of it turns to the left and enters the Gulf of Corryvreckan, it encounters first a deep hole in the seabed and then a tall pinnacle of rock. Together, these produce the shearing force that sets the current turning. Whirlpools form only in water, of course, and they stay in the same place. There are atmospheric equivalents, however, and these can form anywhere the necessary conditions occur. We know these atmospheric “whirlpools” as depressions or cyclones—and as hurricanes and tornadoes.

Vorticity in moving air Air also moves and it, too, can be pictured as a vast number of minute “particles,” all twisting and turning and moving in all directions like birds in a flock or locusts in a swarm. Each “particle” possesses vorticity and vorticity itself has three components: magnitude, direction, and sense. The magnitude is equal to twice the angular velocity (conventionally symbolized by the Greek letter omega, ω), the direction refers to the horizontal or vertical orientation of the axis about which the particles rotate, and the sense is whether the rotation is clockwise or counterclockwise when viewed from above. The rotation of a large mass of air is the sum of the rotations of all its component “particles.” It is like the movement of the flock or swarm. In the Northern Hemisphere, air that turns counterclockwise is turning in the same direction as the Earth’s rotation. This is termed positive vorticity, and the rotation of the air is said to be cyclonic, because it is the direction in which air turns around a center of low atmospheric pressure, or cyclone. Clockwise rotation is termed negative vorticity, and the rotation is anticyclonic—the direction air circulates around an anticyclone, or center of high pressure. These are reversed in the Southern Hemisphere. The diagram below illustrates this. Buys Ballot’s law describes this kind of air movement and the reason for it (see sidebar, page 59).

Vortices

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There are also types of vorticity. Shear causes air to rotate about its own axis—giving it its shear vorticity. It also rotates about a center of high or low atmospheric pressure—its curvature vorticity. Together, these the relative vorticity of the air (symbolized by the Greek letter zeta, ζ). Air also rotates because the entire atmosphere is moving due to the rotation of the Earth. This is its Earth vorticity, and it varies according to the latitude. Its value is zero at the equator, where the local vertical axis is at right angles to the Earth’s rotational axis, and attains a maximum at the North and South Poles, where the local vertical and Earth axes are aligned. The sum of the relative and Earth vorticities is known as the absolute vorticity.

Vorticity and bad weather Curvature vorticity is the movement of air about a center of high or low pressure (see sidebar, page 73). The air spirals inward toward a center of low pressure and outward away from a center of high pressure. The speed of the moving air—the wind speed—is proportional to the difference in pressure between the center and outside of the system—the pressure gradient. This is another way of saying that the relative vorticity changes according to the pressure gradient. When pressure at a center is low and the vorticity of air circulating around it is high, the weather is often foul. When air spirals inward, its convergence near the center causes it to rise. This is due to another fundamental physical property: the conservation of mass in a fluid that is free to move in three dimensions. Convergence

Northern Hemisphere

negative anticyclonic

positive cyclonic

Southern Hemisphere

positive cyclonic

negative anticyclonic

Directions of positive, negative, cyclonic, and anticyclonic vorticity in the Northern and Southern Hemispheres

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Hurricanes reduces the horizontal dimensions of a volume of air. This would increase the mass of the converging air, but in fact the mass and volume remain constant because the air expands vertically. Divergence, when air flows away from an area, has the opposite effect of making air contract vertically and subside, but for the same reason. Rising air cools, often producing cloud and precipitation. If vorticity increases in the air high in the troposphere, the result is often a decrease in the surface pressure directly below, with consequent convergence. Weather forecasters pay close attention to the movement of air near the tropopause, for this reason. If they detect an increase in upper-level vorticity, they anticipate a drop in surface pressure and bad weather.

The Coriolis effect Buys Ballot’s law explains why, in the Northern Hemisphere, air moves counterclockwise around a center of low pressure and clockwise around a center of high pressure. This is what leads people to suppose that in the Northern Hemisphere water leaving a bathtub will invariably spiral clockwise around the low pressure at the center of the whirlpool. This does not happen, but not because the effect is unreal. It is real enough, but it applies only to masses of air or water that are very much bigger than the contents of a bathtub and that are moving across the surface of the Earth. When we observe air movements, we do so from a fixed position, standing (or sitting) in a particular place. It is easy to forget that the Earth itself is rotating and we are traveling with it. When you lie on your back in the grass, gazing at the peaceful summer sky, there is nothing to indicate that you are traveling eastward at around 800 MPH (1,287 km/h)—and far less that the Earth is carrying you around the Sun at about 48,000 MPH (72,000 km/h)! Air also travels with the Earth, but it is not attached firmly to the surface, and so it may be traveling at a different speed. Indeed, if a mass of air moves from one latitude to another it will start by traveling at approximately the same speed as the surface in its first latitude. This speed will be different from the speed the surface is moving in its new latitude because the Earth is a sphere. It takes 24 hours for the Earth to complete one turn about its axis, but places in different latitudes move at different speeds because they have different distances to cover. Consequently, air that changes latitude is bound to be traveling at a different speed from the surface. People realized long ago that the rotation of the Earth affects the way air moves over its surface, but it was not until 1835 that the French physicist and engineer Gaspard Gustave de Coriolis (1792–1843) found out why. Today this is known as the Coriolis effect (see sidebar, page 69). Its strength ranges from zero at the equator to a maximum at the poles.

Angular momentum When air moves, its relative vorticity causes it to turn more and more, until it moves in a circle. It then possesses angular momentum, which is the

Vortices

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The Coriolis effect Any object moving toward or away from the equator and not firmly attached to the surface does not travel in a straight line. As the diagram illustrates, it is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Moving air and water tend to follow a clockwise path in the Northern Hemisphere and a counterclockwise path in the Southern Hemisphere. The French physicist Gaspard Gustave de Coriolis (1792–1843) discovered the reason for this in 1835, and it is called the Coriolis effect. It happens because the Earth is a rotating sphere, and as an object moves above the surface, the Earth below is also moving. The effect used to be called the Coriolis “force,” and it

is still abbreviated as CorF, but it is not a force. It simply results from the fact that we observe motion in relation to fixed points on the surface. The Earth makes one complete turn on its axis every 24 hours. This means every point on the surface is constantly moving and returns to its original position (relative to the Sun) every 24 hours, but because the Earth is a sphere, different points on the surface travel different distances to do so. If you find it difficult to imagine that New York and Bogotá—or any other two places in different latitudes—are moving through space at different speeds, consider what would happen if this were not so: the world would tear itself apart. (continues)

direction of Earth’s rotation N

initial direction actual path S

Coriolis deflection. Moving air masses, winds, and ocean currents are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The Coriolis effect is at a maximum at the poles, and it does not exist at the equator.

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(continued) New York

Madrid

The Coriolis effect. The track of an aircraft flying from the equator to a destination in a higher latitude appears to be deflected to the east.

Consider two points on the surface, one at the equator and the other at 40° N, which is the approximate latitude of New York and Madrid. The equator, latitude 0°, is about 24,881 miles (40,033 km) long. That is how far a point on the equator must travel in 24 hours, which means it moves at about 1,037 MPH (1,668 km/h). At 40° N, the circumference parallel to the equator is about 19,057 miles (30,663 km). The point there has less distance to travel and so it moves at about 794 MPH (1,277 km/h). Suppose you planned to fly an aircraft to New York from the point on the equator due south of New York (and could ignore the winds). If you headed due north, you would not reach New York. At the equator you are already traveling eastward at 1,037 MPH (1,668 km/h). As you fly north, the surface beneath you is also traveling east, but at a slower speed the farther you travel. If the journey from 0° to 40° N took you 6 hours, in that time you would also move about 6,000 miles (9,654 km) to the east,

relative to the position of the surface beneath you, but the surface itself would also move, at New York by about 4,700 miles (7,562 km). Consequently, you would end not at New York, but (6,000 – 4,700 =) 1,300 miles (2,092 km) to the east of New York, way out over the Atlantic, somewhere due south of Greenland. The diagram illustrates this. The size of the Coriolis effect is directly proportional to the speed at which the body moves and the sine of its latitude. For each unit of its mass, a body is deflected by –2ω V sin ϕ, where ω is the angular velocity, V is the velocity of the mass, and ϕ is the latitude. The angular velocity of the Earth is 15° hr–1, which can also be expressed as 2π/24 rad hr–1 or 7.29 × 10–5 rad s–1. The effect on a body moving at 100 MPH (160 km/h) is 10 times greater than that on one moving at 10 MPH (16 km/h). Sin 0° = 0 (the equator) and sin 90° = 1 (the poles), so the Coriolis effect is greatest at the poles and zero at the equator.

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product of the mass of the air, its angular velocity, and the radius of the circle that its path describes. Angular momentum is another physical property that is conserved (see sidebar, page 78). If the circle it describes becomes smaller, the conservation of its angular momentum will make the air move faster. Around a hurricane, where air is spiraling inward, the wind speed increases the closer it comes to the central eye.

Conservation of angular momentum gram, her fingertips begin by describing the outer circle, and when she has withdrawn her arms she describes the inner circle. She has reduced one of the three variables and so one or both of the others must increase in order to compensate. Her mass cannot change (she cannot suddenly become heavier), and so the remaining variable, her angular velocity, has to change. It increases as her radius of spin decreases. In other words, she spins faster—but without making any additional effort beyond withdrawing her arms.

us

1

di

ra

radius 2

dir

ec n tio

Imagine a body that is spinning about its own axis. You can measure the mass of the body, the radius of the circle it describes, and the speed of its rotation. Its speed of rotation is known as its angular velocity and is measured as the number of degrees through which it turns in a given time. The Earth, for example, completes one revolution in 24 hours. One full turn takes it through 360°, so the Earth’s angular velocity is (360 ÷ 24 =) 15° per hour. Angular velocity is usually expressed in radians per hour or per second (rad hr–1 or rad s–1). A radian is the angle between two radii of a circle that marks out on the circumference an arc that is equal in length to the radius. Therefore the circumference of a circle is 2 π radians and 1 rad = 57.296°. Multiply these three values together and the product, called angular momentum, is a constant. Call the mass M, the radius R, and the angular velocity V, and M × R × V = a constant. M, R, and V are variables. They can be altered, but the constant must remain the same. This is called the conservation of angular momentum, and it means that if one of the variables changes, one or two of the others must also change in order that the constant remains the same. No one needs to do anything to make this happen: it is entirely automatic. Dancers and ice skaters make use of the conservation of angular momentum when they perform pirouettes. The dancer starts spinning with her arms fully outstretched. The distance from the center of her body (the axis of her rotation) to her fingertips is the diameter of the circle her body describes; the radius is half of this. Then she slowly draws her arms inward to her body. This reduces her radius of spin. In the dia-

of

ro

tat

io n

Conservation of angular momentum. If the radius of turn (radius 1) is reduced (radius 2), the mass of the rotating body and/or its angular velocity must increase in proportion.

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Hurricanes A tropical cyclone, with a diameter of about 400 miles (644 km), is only about half the size of a middle-latitude depression, but the pressure at its center is very much lower. The wild ferocity of the storm, compared with the merely drab wetness of a depression, results from its much steeper pressure gradient. This accelerates the air spiraling into the eyewall, so it is already moving fast when it joins the system. Its radius of rotation then shortens rapidly as the air approaches the center. The conservation of its angular momentum then increases the angular velocity—the wind speed. This is why the wind speed around a hurricane is greatest around the eyewall and decreases with distance from the eye. Vorticity and the conservation of angular momentum account for the ferocity of the winds around the eye of a hurricane, and the Coriolis effect explains the direction in which those winds blow. The Coriolis effect also explains why hurricanes do not form very close to the equator, and it contributes to the path they follow once they have formed.

What happens inside a hurricane As air flows in from surrounding regions of higher atmospheric pressure to fill the deepening tropical depression and the system starts rotating, a distinct structure begins to develop. By the time the tropical depression has grown into a tropical storm and then into a hurricane, it is this structure that sustains its force. Its energy is immense. An average hurricane possesses 10 times more energy than was released in the 1883 eruption of Krakatau (Krakatoa) and about 10,000 times more than the daily output of the Hoover Dam. If you wished to release a hurricane’s worth of energy by burning coal, you would need to shovel approximately 70 million tons (63.6 million tonnes) of it into your furnace. Alternatively, you could detonate around 210 thermonuclear weapons each with a one-megaton yield. A one-megaton bomb explodes with a force equal to that produced by detonating 1 million tons of TNT (conventional high explosive). An average hurricane packs enough energy to work all the streetlights in New York City for more than 27,000 years. Unfortunately, no one has yet figured a way to make hurricanes do useful work.

The structure of a hurricane The Coriolis effect (see sidebar, page 69) swings moving air along a circular path around the low-pressure area, but this movement is countered by friction between the wind and the sea. Friction slows the wind, reducing the Coriolis effect (which is proportional to wind speed), so the pressure-gradient force is slightly greater than the Coriolis effect and the

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Why the wind blows If you have ever sat on a cold winter’s day in a room heated only by a coal fire, you will know that coal fires look great, but they cause drafts. Hot air from the fire goes up the chimney—taking most of the heat with it—and this draws in air at floor level to replace it. The air at floor level is cold. That is the draft. It is also a wind. This is easier to picture if you imagine that the fire is outdoors, like the one in the drawing. Anyone standing a little way from the fire will feel the wind blowing toward the fire. It is why a fire warms your face and the front of your body, but makes your back colder. Wind blows toward places where air is rising. When air rises, it produces a region of low atmospheric pressure at the surface, so you can also think of wind as air moving toward a region of low pressure from an area where the pressure is higher. Air moves from areas of high pressure to areas of low pressure at a speed that is proportional to the difference in pressure. This difference produces a pressure gradient. On a weather map, where lines called isobars join places where the pressure is the

same, the distance between the isobars indicates the steepness of the gradient, just like the contour lines on an ordinary map. The “slope” down the pressure gradient from high to low exerts a pressure-gradient force (PGF). If this were all that happened, pressure gradients would disappear almost the moment they appeared and winds would amount to nothing more than brief gusts as air rushed to even out pressure differences. It is not what happens, however. The rotation of the Earth produces the Coriolis effect (CorF). This deflects the moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, acting at right angles to the PGF. CorF is proportional to the speed of the moving air. If the air accelerates, the CorF increases. In the absence of friction, the PGF and CorF strike a balance that results in the wind blowing parallel to the isobars. Friction with the land or sea surface makes a difference, however. It slows the moving air, but without altering the PGF. The frictional force acts in (continues)

rising air

What makes the wind blow

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(continued) low pressure isobar

Surface wind. The pressure-gradient force moves at right angles to the isobars. The moving air is deflected by the Coriolis effect (CorF) and is slowed by friction. The result is that the wind blows across the isobars.

pressure-gradien force

isobar

isobar

isobar rF isobar high pressure

the opposite direction to that of the wind. As the wind slows due to friction, the CorF decreases, so that the PGF is greater than the CorF. The wind then moves at an angle to the isobars, as shown in the diagram, and flows toward the low-pressure center. Depending on the magnitude of the PGF and the roughness of the surface, the wind crosses the isobars at between 10° and 30°. Air is now spiraling toward the center of low pressure. Because it follows a circular path, it possesses angular momentum, a property that is con-

served. With each turn of the spiral, the radius of the circle decreases and therefore, to conserve angular momentum, the speed of the wind increases. Wind blows because air is driven by the pressure-gradient force to move from an area of high atmospheric pressure to an area where the pressure is lower. The Coriolis effect deflects the moving air, and friction retards it. The balance of these forces produces a wind that spirals inward. The conservation of angular momentum then accelerates the wind as it approaches the center.

wind spirals inward to the low-pressure area (see sidebar, page 73). Friction with the sea also produces vast quantities of spray. Droplets of seawater evaporate in the warm air, adding to the water evaporated from the sea surface. Spiraling inward, the air is warmed by contact with the sea. This makes it expand and rise. The air is moist to a considerable height, because of the large amounts of water that evaporate into it from the tropical ocean and through the addition of spray. The rising air cools adiabatically (see sidebar, page 41), and its water vapor starts to condense. Condensation releases latent heat (see sidebar, page 50), warming the air again and making it continue to rise. Still spiraling inward and moving with increasing speed as it approaches the center because of the partial conservation of its angular momentum (see sidebar, page 71), the air continues to rise and its water vapor continues to condense. This vigorous convection, with upcurrents rising at up to 30 MPH (48 km/h), produces cumulonimbus (storm) clouds that sometimes tower to a

What happens inside a hurricane height of 50,000 feet (15.25 km). Violent thunderstorms develop and beneath the clouds the rainfall is torrential and often accompanied by hail. Wind speeds fall off rapidly near the top, until the air is moving more slowly than the rotating surface of the Earth. When this happens, again because of the Coriolis effect, the winds begin to move anticyclonically (clockwise in the Northern Hemisphere), producing the wide, curving trails of cloud that make a hurricane photographed from space look very like a spiral galaxy. This anticyclonic circulation carries away and disperses much of the rising air, but not quite all of it. Some sinks again, from the high-level region of high pressure. It descends at the center of the vortex, warming adiabatically as it does so. Its rising temperature increases its waterholding capacity. If fragments of the surrounding cloud are swept into it, most of the water droplets in them evaporate. The warm air produces almost clear skies and warm air at the eye. The warmth is very noticeable, and after the raging wind and rain have passed, the comparatively still air at the eye, several degrees warmer than the air outside the hurricane, can seem oppressive.

The eye and the eyewall In the middle and upper regions of the storm there is a large difference in temperature between the air inside the eye and that outside it. This difference in temperature produces a correspondingly large difference in air pressure, adding to the energy of the hurricane. Despite the sinking air, it is in the eye that the surface atmospheric pressure is at its lowest. It is often more than 50 millibars lower than the pressure outside the hurricane. In the most severe category of hurricanes the pressure in the eye is 920 mb or less. Taking the average sea-level pressure as 1,013 mb, this is a drop of 96 mb, and as a hurricane approaches, atmospheric pressure may fall at a rate of 3 mb per mile (1.9 mb km–1). If the storm travels at, say, 20 MPH (32 km/h), this means the pressure will fall about 1 mb every minute. Reduced pressure in the eye means a smaller weight of air is pressing down on the surface of the sea. This allows the sea surface to rise up to 3 feet (1 m) higher than the surrounding sea level. The power of a hurricane is often measured by comparing the temperature and pressure in the eye with those beyond the influence of the system. The outward flow of air, called divergence, at the top of the storm increases the inward flow, or convergence, near the surface. This intensifies the winds and, therefore, strengthens the upward spiral, which leads in turn to the condensation of water vapor and release of latent heat, and the rising air adds to that already at the top, sustaining the high pressure. At this stage the storm is feeding on itself, and it will continue to do so for as long as it has an ample supply of water and is being warmed from below. Towering cumulonimbus clouds form a circle surrounding the eye and extending almost from sea level all the way to the tropopause. This is the

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Height (feet) 65,000

direction of air movement/wind cirrus

cumulonimbus

cumulonimbus 30,000 towering cumulus

scattered small cumulus

towering cumulus

eye

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Cross section of a hurricane

0 Distance (miles)

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dark eyewall of the hurricane, often with tattered fragments of cloud falling down its face, and it is where conditions are most severe. As the drawing of a cross section through a hurricane shows, at the top of the eyewall, the remaining water vapor in the air that is carried away by divergence freezes to form cirrus and cirrostratus clouds, made from ice crystals. In satellite photographs of hurricanes, these thin, high-level clouds often show clearly, spiraling out from the edge of the main cloud mass.

Bands of cloud In all, there may be up to 200 towers of cumulonimbus. These are storm clouds, and they produce storms. There is lightning and thunder, with hail and torrential rain. Some of the storms are so violent that they trigger tornadoes—the vicious companions of hurricanes. The cloud forms in bands. Outside the eyewall cloud there is a cloudfree band. This is surrounded in its turn by a second circular band of cloud, then another cloud-free band, and further alternating bands of cloud and clear air, with the clouds becoming smaller and more scattered the farther they are from the center. At any one time, it will be raining or hailing in about 15 percent of the area covered by bands of cloud. The weather in the cloud-free bands is dry. The sky in the cloud-free bands is not blue, however. Divergence at the top of the storm spreads enough cloud to cover the sky. Within the area of the hurricane, often more than 400 miles (644 km) across, the “cloud towers” cover no more than about 1 percent of the sky, yet it is they that form the heart of the storm. They are its “engine,” releasing latent heat and producing the warm center without which a hurricane

What happens inside a hurricane cannot develop. In the cloud-free bands between the cloud bands, some of the air that was carried aloft is descending, warming adiabatically, and feeding back into the system at the surface.

Hurricane winds Rain, hail, and thunderstorms are common enough. Think of a hurricane, though, and the image that springs to mind is of the wind. That, after all, is its most important and obvious characteristic. Wind force is still measured by a scale devised at the beginning of the 19th century by a British naval officer (see sidebar). In the days of sailing ships, the Royal Navy thought it advisable to supply British naval commanders with a simple method for estimating the strength of the wind so they could know how much sail to set. A sailing ship could be severely damaged or even sunk if it carried too much sail for the wind conditions, and if it carried too little it might fail in its pursuit of the enemy. In those days ships did not carry instruments (called anemometers) for measuring wind speed. Even if they had, it is doubtful whether an ordinary anemometer could have withstood winds of hurricane force. The wind scale also provided a way to check on the performance of the captain, because everything had to be recorded in his daily log, which was handed in when the ship returned to port. By checking the sail that was set against the reported state of the wind, an experienced sailor back at the Admiralty could tell whether the warship was being operated correctly. The Beaufort scale is straightforward and easy for anyone to use. You need only look from your window to estimate the Beaufort wind force. That is its great advantage. Its disadvantage is that it classes as a hurricane, rated as Force 12, any wind stronger than 75 MPH (121 km/h), and this is the minimum wind speed for a hurricane. If the wind speed is less than 75 MPH, it is not a hurricane at all, and in the most severe hurricanes it is more than 155 MPH (249 km/h). For this reason, the U.S. Weather Bureau has added a further five categories to describe hurricanes with winds from 75 MPH, called Category 1, to Category 5, with winds of more than 155 MPH (see sidebar, page 114).

Storms at sea At sea, the winds raise huge waves then whip away their tops, so it becomes impossible to see through the spray where the sea ends and the sky begins. Spray that is blown across the sea surface is called spindrift. The wind speed in a hurricane is not constant. Like most winds, a hurricane produces gusts much stronger than the main wind. A sustained wind of 110 MPH (177 km/h) will raise 30-foot (9-m) waves, and most hurricanes can produce gusts much stronger than this. Not even the largest, most powerful ships sail through such a storm deliberately—but occasionally they do so by accident. In 1944, a U.S. battle fleet in the Philippine Sea was misinformed about the location and movement of a small but extremely violent typhoon

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Wind force and Admiral Beaufort In 1806, the Royal Navy issued a scale by which the commanders of its warships could estimate the strength of the wind by observing its effects. In its early versions (it was revised several times) no mention was made of the actual speed of the wind. Instead, the wind was defined in terms of the speed at which it would propel a man-of-war carrying a specified amount of sail. In a gentle breeze, for example, a ship under full sail and in smooth water would move at 3–4 knots (3.45–4.6 MPH; 5.5–7.4 km/h). Wind speeds were agreed in 1926 and finally added to the scale in 1939. The scale had been devised by Commander Francis Beaufort (1774–1857), and is still known as the Beaufort scale. It was adopted internationally at the International Meteorological Conference, held in Brussels in 1874. The Beaufort scale classifies winds into 13 named “forces” (in 1955 meteorologists at the U.S. Weather Bureau added five more to describe hurricane-force winds). Wind speeds were originally given in knots, the unit that is still often used by ships and aircraft. In the scale given here, knots have been converted to miles per hour and rounded to the nearest whole number. (1 knot = 1 nautical mile per hour = 1.15 MPH = 1.85 km/h.) Force 0. 1 MPH (1.6 km/h) or less. Calm. The air feels still and smoke rises vertically. Force 1. 1–3 MPH (1.6–4.8 km/h). Light air. Wind vanes and flags do not move, but rising smoke drifts.

Force 2. 4–7 MPH (6.4–11.3 km/h). Light breeze. Drifting smoke indicates the wind direction. Force 3. 8–12 MPH (12.9–19.3 km/h). Gentle breeze. Leaves rustle, small twigs move, and flags made from lightweight material stir gently. Force 4. 13–18 MPH (20.9–29.0 km/h). Moderate breeze. Loose leaves and pieces of paper blow about. Force 5. 19–24 MPH (30.6–38.6 km/h). Fresh breeze. Small trees that are in full leaf wave in the wind. Force 6. 25–31 MPH (40.2–49.9 km/h). Strong breeze. It becomes difficult to use an open umbrella. Force 7. 32–38 MPH (51.5–61.1 km/h). Moderate gale. The wind exerts strong pressure on people walking into it. Force 8. 39–46 MPH (62.7–74.0 km/h). Fresh gale. Small twigs are torn from trees. Force 9. 47–54 MPH (75.6–86.9 km/h). Strong gale. Chimneys blown down, slates and tiles torn from roofs. Force 10. 55–63 MPH (88.5–101.4 km/h). Whole gale. Trees are broken or uprooted. Force 11. 64–75 MPH (103.0–120.7 km/h). Storm. Trees are uprooted and blown some distance. Cars are overturned. Force 12. More than 75 MPH (120.7 km/h). Hurricane. Devastation is widespread. Buildings are destroyed, many trees uprooted. In the original instruction: “Or that which no canvas could withstand.”

(the name given to a hurricane in that part of the world) and sailed directly toward its center. Sailors know, from Buys Ballot’s law, that in the Northern Hemisphere, if they stand with their backs to the wind, the storm center will be to the left. In this case, however, the meteorological officer was confident he knew exactly where the storm was and that it was moving away from the fleet. By the time the commanding admiral realized a mistake had been made it was too late. His ships were sailing into the storm rather than away from it and already they were struggling through 70-foot

What happens inside a hurricane (21-m) waves and winds of more than 115 MPH (185 km/h). By the time the fleet was clear, three destroyers had sunk, 146 aircraft were destroyed on board carriers, the remaining ships were too severely damaged to continue with their mission, and 790 sailors had lost their lives. If a ship has no choice but to sail close to a hurricane, it is safer to pass on the side of its path nearest to the equator (in either hemisphere). That is the side where the winds—circling the eye counterclockwise in the Northern Hemisphere, clockwise in the Southern—will blow the ship behind the storm. It is known as the navigable semicircle. The winds on the other side, which in any case are stronger because they blow in the same direction as that in which the storm is moving, will carry it in front of the storm, with a serious risk of being drawn into it. This is the dangerous semicircle. Waves generated by the wind move outward from the center of the storm. As the storm moves over the ocean, it continues to produce waves. These mix with earlier waves, but the movement of the hurricane means the direction of the waves is changing constantly, so waves may cross one another. The sea looks as though it is boiling, especially behind the eye of the hurricane, and the disturbance affects the sea over vast distances.

Death of the storm With so much energy being released so violently, a hurricane cannot survive for long. Most last no more than two or three days before their wind speeds decrease. A dying hurricane may continue as a deep depression that brings gales strong enough to cause considerable damage, and may even keep the name it was given during its life as a hurricane, but by this time it is not really a hurricane anymore. Its structure, of concentric spirals of cloud around a very warm center, allows it to sustain itself during its brief life, but that structure results from three conditions which the storm does not produce for itself and over which it has no control. The sea-surface temperature must be higher than 80°F (27°C). Should it fall below 76°F (24°C), there will be insufficient heat to maintain the convective currents that generate the towering cumulonimbus clouds. As the hurricane moves into a higher latitude, it will cross cooler water and lose its source of heat. There must be abundant water vapor to release latent heat as it condenses. Should the storm move over land, it will lose its supply of water. Finally, there must be a high-level anticyclone to disperse the rising air and thereby draw more air from below. The anticyclone may weaken, or drift away, or the hurricane may move from beneath it. It is pure coincidence if all three of these necessary conditions are present at the same time and in the same place while a tropical storm is strengthening, and the coincidence cannot last. After a few days one or other condition will be lost. Then, quite quickly, the hurricane will weaken and disappear.

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HURRICANES, TYPHOONS, AND CYCLONES Hurricanes in the United States and the Caribbean In an average year, 10 tropical storms develop over the North Atlantic, of which six grow into hurricanes. Two of those hurricanes are likely to be major—category 3 or higher on the Saffir/Simpson scale (see sidebar, page 114). The year 2001 was not average. It produced 15 tropical storms, of which nine became hurricanes and four of those hurricanes were major. Most of the hurricanes occurred between September and November, toward the end of the hurricane season. For the first time ever recorded, there were three hurricanes in November. None of the 2001 hurricanes crossed the coast of the United States, although Juliette came very close, but two of them—called Iris and Michelle—struck islands in the Caribbean. Allison, a tropical storm that was almost a hurricane, did cross into the United States, where it caused devastating flooding.

Allison Allison was unusual in that it formed over the eastern North Pacific early in June and then crossed Central America and entered the Gulf of Mexico. Its winds increased to 60 MPH (96 km/h) before it crossed into Texas on June 5 with winds of about 50 MPH (80 km/h). It weakened over eastern Texas, crossed back over the Gulf and strengthened once more, and then crossed Louisiana on June 11. It entered North Carolina on June 14 and remained stationary there for three days. Then it crossed back over the Atlantic and headed northward, finally dissipating off Nova Scotia. Allison generated fairly weak winds, but it caused the worst flooding ever associated with a tropical storm. More than 30 inches (762 mm) of rain fell in several places around Houston, Texas. Rain fell very heavily everywhere from eastern Texas, across the Gulf States, and along the Atlantic coast. The damage was estimated to cost at least $5 billion. Tropical storms rarely kill people, but floods are the most dangerous of all weather disasters. The floods that Allison caused cost 41 lives—23 in Texas, eight in Florida, seven in Pennsylvania, and one each in Louisiana, Mississippi, and Virginia.

Hurricanes in the United States and the Caribbean

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Juliette Juliette formed on September 21 and crossed over Baja California, Mexico, on September 30. It then entered the northern end of the Gulf of California. The storm finally disappeared on October 3. At its peak, Juliette had one of the lowest eye pressures ever recorded, of 923 mb. Like most hurricanes, Juliette began as an easterly wave that grew into a tropical depression. It became a hurricane on September 23. At that time it was intensifying rapidly. By the following day its winds had reached 132 MPH (218 km/h) and on the 25th they reached 144 MPH (231 km/h). Juliette was then a category 4 hurricane, but it was already weakening. When the hurricane passed to the west of the tourist resort of Cabo San Lucas, at the southern tip of Baja California, on September 28, its winds had dropped to about 90 MPH (145 km/h). Juliette moved parallel to the coast at about 10 MPH (16 km/h), then moved inland on the 30th, passing close to the town of San Carlos. Its winds had then fallen to 40 MPH (65 km/h). The map shows the track the storm followed. Heavy rains flooded more than 200 homes on the mainland of Mexico and the storm cost two lives. One victim was a vacationer from Denver, Colorado, who drowned while surfing at Cabo San Lucas. The other was a fisherman whose boat capsized near Acapulco.

Hurricane Juliette September 21 to October 3, 2001 hurricane tropical storm

2 3 1 22

midnight position/date

30 29

27 26 923 mb

25 24

23 22 21

Track of Hurricane Juliette, September – October 2001

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Iris Iris was a small hurricane, but a fierce one, that struck Belize in October. It began as an easterly wave that grew into a tropical depression on October 4, when it was close to the Windward Islands. It became a hurricane on October 7 as it passed to the south of the Dominican Republic, where three people were killed. It passed to the south of Jamaica, uprooting trees, ripping the roofs from buildings and still intensifying. Iris reached Monkey River Town, Belize, on October 8. By then it had become a category 4 hurricane, with sustained winds of 145 MPH (233 km/h). The pressure in the eye was 948 mb; it produced a storm surge of 13–18 feet (4–5.5 m). Its fiercest winds were felt in an area that was no more than about 15 miles (24 km) across and all of the damage was contained in an area only 60 miles (96 km) across. Winds of up to 70 MPH (113 km/h) affected some places as much as 145 miles (233 km) from the eye, however, and people were evacuated from their homes in parts of Belize City, where streets were flooded. A boat, the Wave Dancer, capsized in the Monkey River, near Belize City, killing 18 people. They were all members of the Richmond Dive Club, from Richmond, Virginia, and Wave Dancer was one of two boats they had hired for a weeklong fishing trip. Including the victims of the Wave Dancer, Iris killed a total of 31 people.

Michelle On October 29 another monster appeared in the form of a tropical depression near the eastern coast of Nicaragua. It remained there for two days, dumping enough rain to cause flooding in Nicaragua and Honduras. Late on October 31 it moved away over the Caribbean and began to intensify. By November 3 it had become Hurricane Michelle and it was heading for Cuba. Later that day its winds increased to 140 MPH (225 km/h), and Michelle was classified as category 4. It weakened fairly quickly and was downgraded to category 3 by the time it struck Cuba on the afternoon of Sunday November 4—although this still exposed Cubans to winds of up to 135 MPH (217 km/h). As it crossed the island, its winds decreased to 110 MPH (175 km/h), and by Sunday evening it was a category 2 hurricane and still weakening. Michelle left Cuba on Sunday night. It crossed the Bahamas and then moved out over the Atlantic. It was the fiercest hurricane to strike Cuba for 50 years and caused widespread damage. In Soplillar, a town of 500 people close to where Michelle crossed the coast, 100 of the 156 homes were destroyed. Five people died in Cuba, but Cuba was not the only country to suffer. Homes were destroyed, roads and bridges wrecked, and six people lost their lives in Honduras, four in Nicaragua, and two in Jamaica.

Hurricanes in the United States and the Caribbean

Opal The United States escaped serious hurricane damage in both 2000 and 2001. This was the result of chance. There were plenty of hurricanes, but they failed to cross the coast. In other years the nation has not been so lucky. Opal touched the mainland in 1995, but a few years earlier, in 1989, Hugo moved overland from South Carolina to Labrador. The encounter with Opal began at around 6 P.M. on the evening of October 5, when screaming wind and torrential, lashing rains crossed the coast near Panama City, Florida, between Pensacola and Tallahassee. Opal, the second hurricane to assault the area in two months, was moving north from the Gulf of Mexico. Anticipating its arrival, the governors of Florida, Alabama, and Mississippi had declared states of emergency. People were compulsorily evacuated from coastal areas and offshore islands close to the hurricane’s predicted path. All public buildings were closed in Pensacola and Mobile, Alabama. Staff controlling the emergency services moved into an underground bunker, and the U.S. Navy removed all the aircraft from its Pensacola airbase. By the time Opal reached Florida, its wind speed was falling. At its worst, when it was over the Yucatán Peninsula, in Mexico, the wind speed had risen to 150 MPH (241 km/h), with gusts stronger than that, but then they fell to below 130 MPH (209 km/h). Even so, it was ferocious enough. Five inches (127 mm) of rain fell during the night of October 5. That is considerably more than the 3.5 inches (89 mm) of rain Pensacola and Tallahassee ordinarily expect to receive in the whole of October. It was not so much the wind or rain people feared, however, but the sea. The hurricane brought huge breakers, and as the wind drove water toward the coast, the sea rose 12 feet (3.7 m) above its usual high-tide level. Homes near beaches were washed away and small boats were plucked from their moorings and hurled ashore. The storm-driven sea and wind killed 13 people in the United States, having already killed 50 in Mexico and Guatemala. The damage it caused in Florida and Alabama was estimated to have cost about $4 billion. Opal continued to move north at about 25 MPH (40 km/h), but weakening all the time. Some nine hours after it crossed the Florida coast it was about 55 miles (88 km) east of Huntsville, Alabama, and had been reclassified as a tropical storm, although it was still bringing heavy rain and galeforce winds. This hurricane was the 15th storm of the 1995 season (not all were hurricanes) and the ninth to strike the Gulf coast. Several storms faded harmlessly far from land, but had Opal not weakened so quickly it might well have proved one of the most violent of the century. In other respects it was fairly typical. It formed over Yucatán on September 27 and by the end of the month had intensified sufficiently to be classed as a tropical storm. It continued to grow fiercer, and on October 2 it was reclassified as a hurricane. Four

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days later it was already dying. Four days is the average life expectancy for an Atlantic hurricane. During its brief life, Opal moved almost directly north. Had it lasted longer, by then reduced to a tropical storm rather than being a full hurricane, probably its track would have curved to the east, much like Hurricane Hugo, which crossed the United States and Canada in 1989. Hugo lasted much longer and followed a more complete track, shown on the map.

Hugo Hugo began to form on September 11 in the eastern North Atlantic, not far from the African coast (the numbers on the map are the dates in September and the location of the storm on those dates). By the next day it had become a tropical depression. It was a tropical storm by the 14th and a hurricane by the 19th. Typically, its track had carried it westward and as it developed into a hurricane it swung northwest, crossing the coast at Charleston, South Carolina, at about 10 P.M. on the night of the 21st. Moving at an average speed of 29 MPH (47 km/h), by the 23rd its track

Hurricanes in the United States and the Caribbean was curving toward the east, and on that day and the next it crossed the northeastern United States. Its passage over land had isolated it from the warm water needed to sustain it, so by the time it reached North Carolina it had weakened to a tropical storm. It moved through Québec and Labrador, Canada, on the 24th, and on the 25th it crossed the coast again, moving out into the North Atlantic and heading northeast, now as an extratropical storm. Throughout its journey across eastern North America, the eye of Hugo had a diameter of 30 miles (48 km), which is large. Some are no more than five miles (8 km) across. A large eye means the entire storm is large, because the hurricane forms as circles around the eye, and the bigger the storm the fiercer are its winds (see sidebar, page 71). Hugo covered a vast area within which it caused a great deal of damage, estimated to have cost about $10.5 billion in the United States. This made it possibly the most destructive hurricane in American history in financial terms, though fortunately few lives were lost. Hurricanes develop quickly, but nowadays satellite monitoring gives the U.S. authorities several days to prepare for their arrival. As Hugo approached, about 12,000 people in Charleston were evacuated into shelters, although some who sought safety in Charlotte, North Carolina, found they had placed themselves directly in the hurricane’s path. There were only four deaths in the continental United States: one in Charleston, one in Charlotte, and two in Virginia. In fact, Charleston was lucky. The eye of Hugo passed to the east, exposing the city to the less violent winds on the left side of the storm (where the winds blow in the opposite direction to that in which the hurricane is traveling, reducing their speed). Had the eye crossed the coast just 20 miles (32 km) farther south, Charleston would have experienced the full force of the much more dangerous right side of the storm (where the 29 MPH (47 km/h) at which the storm was moving would have added to the speed of the spiraling wind). The Caribbean islands fared worse. They are more exposed and have less time to prepare, because the storm is already close to them when it first forms. Hugo reached first Guadeloupe and then Dominica, in the Leeward Islands, on September 17, and the U.S. Virgin Islands and Puerto Rico on the 19th. There was time in Puerto Rico to warn shipping to clear the area, close the airport, and organize street patrols to prevent the looting that took place in the wake of the storm in St. Thomas and St. Croix, in the Virgin Islands, but the death toll was high. Eleven people were killed in Guadeloupe, 10 in Montserrat, six in the Virgin Islands, and 12 in Puerto Rico.

Luis Atlantic hurricanes travel west, then north, and finally east, but not all of them reach the United States. In 1988, for example, 11 tropical storms developed in the Atlantic, including five hurricanes, three of which generated winds of more than 131 MPH (211 km/h). Only four of them

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Hurricanes crossed the United States coast, although one of the hurricanes, Gilbert, came close enough for its outer margins to cause some damage in Texas. If a hurricane develops far enough to the east, its curving track may carry it parallel to the coast of Florida and the Carolinas, but it will remain over the ocean. Communities on land will then escape its full force. This is partly because they are well clear of the eye and partly because at all times they are to the left of the eye, in the quarter where the wind speed is lower (see the figure on page ix). That is how the United States escaped Luis, which struck the U.S. Virgin Islands and Puerto Rico on September 6, 1995. With an eye 60 miles (96 km) across and a total diameter of about 700 miles (1,126 km), it covered a surface area of about 385,000 square miles (997,150 km2), Luis was even bigger than Hugo in 1989. It was not quite so fierce, though, with winds gusting up to 140 MPH (225 km/h). Traveling west at about 12 MPH (19 km/h), it struck Guadeloupe, the U.S. Virgin Islands, and Puerto Rico. As is usually the case, the sea did as much damage as the wind and torrential rain, rising up to 9 feet (2.7 m) above its usual high-tide level. In Guadeloupe, a huge wave swept a French tourist to his death as he was trying to photograph the sea. The Caribbean islands are popular tourist destinations, and airlines, tour operators, and foreign governments all issued warnings to their customers or citizens. In this case, Thomas Cook, the British tour operator, flew its customers to Barbados or Miami and the British Foreign Office advised travelers to avoid the area or, if they were there already, to remain in secure accommodations and follow the instructions of the local authorities. Puerto Rico, the U.S. Virgin Islands, and the countries of Central America suffer more from hurricanes than most countries. In September 1995, for example, hurricane Marilyn destroyed four-fifths of the houses on St. Thomas, in the Virgin Islands, although the only casualties occurred among people on boats when the storm struck. Of those, three died and 100 were injured or missing. The team arriving by air with emergency supplies said none of the buildings on St. Thomas appeared to have roofs. Hurricane Roxanne, which brought 115-MPH (185-km/h) winds to Mexico in October 1995, killed at least 14 people and forced tens of thousands to leave their homes.

Gilbert From September 12–17, 1988, Gilbert, one of the worst of all hurricanes in recent years, ravaged Jamaica and Mexico. Then it moved into Texas and then into Oklahoma. It triggered more than 29 tornadoes, which caused property damage costing an estimated $40–$50 million, although by the time it reached Texas its force had largely abated and farmers in Texas welcomed the rain it brought. At its peak it was a category 5 hurricane. It killed a total of 318 people. A minor deflection in the trade winds making up the easterly wave that would develop into Gilbert was first noticed on September 3 as a

Hurricanes in the United States and the Caribbean group of clouds moving westward over the ocean, away from the coast of West Africa. It was designated a tropical storm on September 9, when its wind speeds exceeded 39 MPH (63 km/h). That was the day it moved through the Lesser Antilles. A day later, with wind speeds exceeding 74 MPH (119 km/h), it officially became a hurricane, and within about 12 hours its winds had increased to more than 96 MPH (154 km/h). On the 11th the National Hurricane Center in the United States warned the Jamaican authorities to expect a hurricane with winds in excess of 100 MPH (160 km/h) in the next 12 to 24 hours. When it reached Jamaica, passing over Kingston around noon on September 12, the winds were more than 111 MPH (178 km/h) and the hurricane was still intensifying. When its center passed about 20 miles (32 km) to the south of Grand Cayman Islands, at 9 A.M. on September 13, its winds (on the dangerous, northern side) were blowing at more than 131 MPH (211 km/h), and two hours later they exceeded 155 MPH (249 km/h). It had reached category 5. By 6 P.M. on September 13 the atmospheric pressure in the eye was 888 mb. That is the lowest surface pressure ever recorded in the Western Hemisphere. The storm reached the Mexican coast on September 14. By then it had weakened to category 3, but nevertheless a Cuban ship several miles out at sea was thrown onto the shore as the sea rose by 20 feet (6 m). Gilbert’s eyewall began weakening as the hurricane moved inland, but away from the center the winds retained their force for some time longer. Once the eyewall disappeared, however, it did not form again.

Lives and property When a hurricane does reach the United States coast, adequate early warning and efficient, experienced emergency services minimize casualties. Property is not so easily protected, however, and over the years in which they have killed and injured ever fewer people, the cost of hurricane damage has increased. This is due to the popularity of vulnerable areas as places to live and to the increase in the amount of property that is insured. The cost that is quoted for hurricane damage—as well as damage from any other cause— is based on the insurance cost. Hurricanes are likely to cross the coast of the United States anywhere along a stretch of about 2,000 miles (3,200 km) from Texas on the Gulf of Mexico, around Florida, to Virginia on the Atlantic. The number of people living along this coastline has roughly doubled since the 1930s, and there are now more people living in the Miami and Fort Lauderdale areas of Florida than lived along the entire stretch of coast in the first half of the 20th century. To make matters worse, many of them live in mobile homes. Not only can a hurricane demolish such dwellings entirely, but in doing so it can hurl debris from them against other buildings to cause further damage. According to Lloyds of London, the biggest insurance organization in the world, between 1966 and 1987 there was no single natural disaster costing more than $1 billion

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Hurricanes in insurance claims, but between 1987 and 1992 there were 10 wind storms with a combined cost of more than $15 billion. Camille, a hurricane that struck Mississippi and Louisiana in 1969, illustrates this. Of comparable strength to Gilbert, in 1988, but with a higher eye pressure (905 mb), it killed some 250 people living along the coast and a further 125 died in the flooding it caused. The cost of the damage it did to property came to $1.42 billion. In 1989, however, Hugo caused 43 deaths and damage in the U.S. costing $10.5 billion, while in 1995 Opal caused 13 U.S. (and 50 Central American) deaths and property damage costing $4 billion. Hurricanes are becoming less deadly, but more costly. Hurricane Camille illustrates the further point that hurricanes may interact with other weather systems. Camille was very strong, but it had weakened to a tropical storm when it turned east and crossed the Blue Ridge Mountains, Virginia. There, its winds gathered moist air that was moving inland from the Atlantic and funneled it through two narrow valleys, of the Rockfish and Tye Rivers. Then this moist air met an advancing cold front that was already producing thunderstorms. The moist, low-pressure air rose up the front (see sidebar, page 19) and produced 18 inches (457 mm) of rain in a matter of hours. Most of the damage Camille caused was due to the resulting floods. Ordinarily, in recent years there have been an average of nine or 10 named storms and six hurricanes. There were 11 Atlantic hurricanes in 1995, compared with only three in 1994. When tropical storms that failed to develop into full hurricanes are included, the 1995 season was more active than any for more than 60 years, although this is not the case when only named hurricanes are counted. As the graph shows, there were 12 hurricanes in 1969 and 11 in 1950. All this illustrates is the extent to which the number of hurricanes can vary from one year to another. Some scientists believe there have been more hurricanes in recent years and that they are becoming more frequent. In fact, though, as the graph illustrates, the number each year has changed little since 1945, although there have been particular years with more—for example, 1950, 1969, 1995, and 1998. It is also true that hurricanes were more frequent during the first half of the 20th century than they are now. Indeed, it is the long period during which hurricanes were fairly uncommon that lulled so many people into moving into hurricane-prone areas believing they were safe. Some scientists suspect that hurricanes may be becoming more frequent, but there is no clear evidence for this. There was a sharp increase of about 50 percent in hurricane frequency from 1994 to 1999, but it appears to have leveled off, and five years is far too short a period to reveal a long-term trend. The trend from 1945 to the present day shows no increase in frequency. Certainly there is no evidence that hurricanes are becoming more violent. In fact, the average sustained wind speed in hurricanes has been decreasing slowly but steadily since 1940. On the present evidence, there is no reason to fear that hurricanes in the Atlantic and Caribbean are becoming more frequent or more violent.

Hurricanes that reach Europe

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Hurricanes that reach Europe Western Europe enjoys an equable climate. True, it rains a lot, but extremes are fairly uncommon. It is not a part of the world where people expect to be scorched or frozen, or to watch windstorms demolish their homes. These are not what Europeans expect. But occasionally it is what happens. At least 17 people died in January 2002 in storms that lashed northern Europe from Scotland to Poland. Five of them were drivers of trucks that were blown over by winds gusting to 100 MPH (160 km/h). A state of emergency was declared in Kaliningrad, a piece of Russia bordering Lithuania and Poland, when the winds reached 74 MPH (120 km/h). The sea level rose more than 13 feet (4 m) above its usual level along the Danish North Sea coast and residents were evacuated. It happened around Christmastime in 1999—and the weather struck not once, but three times. By the time the storms had passed, nearly 200 persons had lost their lives and the number of trees that were blown down over three days was equal to six months’ European timber harvest. France was the country that suffered most. The streets of Paris were littered with fallen trees, and in the country as a whole about 100 square miles (259 km2) of forest was flattened—more than 2.5 times the amount of timber harvested annually. More than 400,000 French homes lost their telephone service, more than 2 million were without power, and in some rural areas the water supply failed. Insurance companies estimated the cost of the damage at about $385 million. When it was over, the French government declared a national emergency covering two-thirds of the country and

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Number of Atlantic hurricanes each year 1944 –2000 and the fiveyear running mean

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Hurricanes mobilized 6,000 troops to clear debris and help restore supplies of drinking water. The first storm occurred on December 11. It affected Denmark and Sweden, bringing the fiercest gales for more than a century to Denmark and causing severe damage to forests. This storm was but a prelude to the monsters that would soon assault the lands farther south. A very deep depression formed on December 23 over the western North Atlantic, a little to the south of the latitude of Cape Hatteras, then moved along a track that carried it northeastward to the latitude of Newfoundland and then eastward. It passed to the south of Ireland and reached the French coast in the early hours of December 26. The storm generated winds gusting to 107 MPH (173 km/h) at Orly Airport, Paris, and in places reaching 136 MPH (219 km/h). By noon the storm had reached Germany. It reached Poland by evening, but by then it was weakening. People were still reeling from this storm when the second arrived. Its life began on Christmas Day, as a depression that formed at about 38° N 65° W—over the North Atlantic due south of Nova Scotia. It crossed the ocean, moving northward as it did so, and reached the Britanny coast of France on the afternoon of December 27. This storm produced winds just as violent as those of the first, but the storm track lay farther to the south. It affected northern Spain and after crossing central Germany, the storm moved through Switzerland and into Italy. It killed 122 people, 70 of them in France. As well as the wind, the second storm caused flooding in parts of Belgium, Germany, Switzerland, and Spain, and heavy snowfalls in the mountains, leading to avalanche alerts being issued for the French Alps. A group of nine German tourists celebrating the millennium in a rented mountain hut were among the 12 people killed by avalanches in Austria. The combination of wind, rain, and snow brought down trees and power lines in Transylvania, about 280 miles (450 km) from Bucharest, Romania, cutting the power supply to more than 100 villages. Both storms generated winds of hurricane strength, but the storms themselves were not hurricanes. They formed outside the Tropics, and so failed to qualify as “tropical cyclones,” and they lacked the warm center that is a diagnostic feature of true hurricanes. Charley, on the other hand, was a hurricane.

Gentle Charley As hurricanes go, Charley was nothing special. It reached the coast of North Carolina on August 17, 1986, moved northward along the coast of Maryland, then turned east and headed out over the Atlantic. It caused little damage. Indeed, it brought welcome rain to farmers in the Carolinas, Virginia, and Maryland. There were only three Atlantic hurricanes in that year, and none was particularly strong. Charley, the gentle hurricane, might have been forgotten, only it persisted. Its track carried it right across the North Atlantic and a few days later it reached Britain. By this time it was classified as only a storm. Its warm eye had cooled and its winds weakened. Nevertheless, it retained some of its old

Hurricanes that reach Europe vigor, and people living in southwestern Wales were unprepared for its arrival. It took them by surprise and caused a great deal of damage. Although officially it was no longer a hurricane, it retained the essential structure of one. It still had the remains of an eyewall of cloud bringing gales and torrential rain and, behind that, the eye in which the sky was clear and the air still. As the storm passed overhead, the eye moved away to be followed by the second side of the eyewall, and even fiercer winds. It took two days and the night between to pass. The combination of rain and high seas caused widespread flooding. The Tenby inshore lifeboat had to rescue holidaymakers from one inland trailer park. Dyfed, the northern part of which took the full force of the storm, is the county occupying the peninsula of southwest Wales. It is mainly rural, a sparsely populated place of farms, hills, and small villages. This limited the damage to property—there was not much property for the storm to wreck. The next hurricane to strike Britain did so in England, and the consequences were much worse.

Floyd Hurricanes can cross the Atlantic and occasionally they do, arriving with enough power to wreak considerable havoc. Floyd, the last hurricane of the 1987 season, formed on October 9, and on the 12th its winds reached more than 75 MPH (121 km/h), the lowest speed to qualify it as a hurricane. It passed through the Florida Keys, but within 12 hours it had weakened to less than hurricane force. Then it headed out over the ocean. European meteorologists saw it coming, but miscalculated its track. This is easily done, because a degree or two can make a great deal of difference. The meteorologists believed Floyd would pass through the English Channel, well south of the British coast, then head into the North Sea, weakening all the time. Obviously, it would endanger shipping, and the English Channel is one of the busiest sea-lanes in the world, but they said people on shore had nothing to fear. It arrived on the night of October 15, traveling a few degrees to the left of its predicted path, and caused havoc throughout the densely populated towns and villages of southern England. Though barely a hurricane in the strict sense, its winds gusted to more than 80 MPH (129 km/h) and by dawn on the 16th, 19 million trees had been uprooted. In Kent, to the southeast of London, there is a small town called Sevenoaks. Its name refers to seven oak trees growing there. What came to be called “The October Storm” reached Sevenoaks in the early hours of Friday, October 16, and it brought down six of the seven trees. People joked about the need to rename the town, but it was unnecessary, because the trees were soon replaced. The storm killed 19 people, and the damage cost about $2.25 billion (about £1.5 billion) in 1987 money.

Burns Night For Scots people everywhere, the evening of January 25 is Burns Night, when the celebration meal is paraded into the dining room to the sound of

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Hurricanes the bagpipes, haggis is eaten and a certain amount of whisky is drunk, and the national poet, Robert Burns, is honored by reading aloud from his works. On that day in 1990, Britain, along with much of northwestern Europe, suffered the fiercest storm in many years. This time, although it was technically a storm rather than a hurricane, winds reached more than 100 MPH (160 km/h). Its track was predicted accurately, but there was nothing anyone could do to protect property. Roofs were torn from buildings, trees uprooted, power lines brought down, and transport and communications seriously disrupted. Most of Britain was affected and about 47 people died. The storm then moved into continental Europe, killing 19 people in the Netherlands, 10 in Belgium, eight in France, seven in Germany, and four in Denmark. Soon after that, on February 3, 29 people died when winds of hurricane force struck France and Germany, and a month later, on February 26, another windstorm killed at least 51 people in Britain, the Netherlands, Germany, France, Belgium, Switzerland, Ireland, and Italy. Winds strong enough to qualify a storm as a hurricane occur in Europe every few years. There was one in December 1993, for example, which killed 12 people in Britain, and on January 25 and 26, 1989 hurricane-force winds killed at least 12 people in Spain. The middle 1980s was a time of little hurricane activity. The Atlantic was quiet. Few tropical storms developed, few of those grew into hurricanes, and those that did were generally weak. Weak storms are just as likely to cross the ocean as strong ones, however, because what matters is not the size or power of the storm, but the distance it travels over land. Hurricanes are sustained by a limitless supply of water. It is evaporating seawater and sea-spray that releases the latent heat to feed the vigorous convection needed to build the warm, towering clouds of the eyewall. If, like Charley and Floyd, a hurricane turns north without penetrating the American mainland, there is a chance it will turn east before it begins to weaken significantly. It may then encounter weather systems that reinvigorate it, so what seemed to be a dying hurricane strikes Europe with formidable strength.

Frontal storms When steady wind speeds exceed 75 MPH (121 km/h), they are rated as “hurricane force” on the Beaufort scale (see sidebar, page 78). Winds of this force in Europe are not really hurricanes, however, even if that is how they began, far away in the tropical Atlantic. They lack the extreme violence of a tropical cyclone, because they are not fed energy by a very warm sea, and most European “hurricanes” are associated with frontal systems. Tropical cyclones are not frontal. Over the Tropics, the air temperature and pressure is fairly constant throughout vast areas, and a quite small disturbance is enough to trigger

Air masses and the weather they bring As air moves slowly across the surface, it is sometimes warmed, sometimes cooled, in some places water evaporates into it, and in others it loses moisture. Its characteristics change. When it crosses a very large region, such as a continent or ocean, its principal characteristics are evened out, and all the air is at much the same temperature and pressure over a vast area and is equally moist or dry. Such a body of air is called an air mass. Air masses are warm, cool, moist, or dry according to the region over which they formed and they are named accordingly. The names and their abbreviations are straightforward. Continental (c) air masses form over continents, maritime (m) ones over oceans. Depending on the latitude in which they form, air masses may be arctic (A), polar (P), tropical (T), or equatorial (E). Except in the case of equatorial air, these categories are then combined to give continental arctic (cA), maritime arctic (mA), continental polar (cP), maritime polar (mP), conti-

nental tropical (cT), and maritime tropical (mT). Equatorial air is always maritime (mE), because oceans cover most of the equatorial region. North America is affected by mP, cP, cT, and mT air, the maritime air masses originating over the Pacific, Atlantic, or Gulf. These are shown on the map. As they move from where they formed (called their source regions) air masses change, but they do so slowly and at first they bring with them the weather conditions that produced them. As their names suggest, maritime air is moist, continental air is dry, polar air is cool, and tropical air is warm. At the surface there is little difference between polar and arctic air, but there are differences in the upper atmosphere. It is cP air spilling south when the cT and mT move toward the equator in the fall that brings cold, dry winters to the central United States. It is the meeting of mT air from the Gulf and cT air from inland that produces fierce storms in the southeast of the country.

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Hurricanes the development of a tropical cyclone. In middle latitudes the situation is very different. Air masses with quite different characteristics cross constantly, mainly from west to east (see sidebar). Where two air masses meet while moving at different speeds a front develops (see sidebar, page 19). Where a warm and cold front meet at surface level, there is often a region of low pressure, called a depression. Depressions also move, generally from west to east, dragged along by waves that move through the jet stream overhead (see sidebar below). Air flowing into a depression is deflected by the Coriolis effect into a spiraling path around it and the greater the difference in pressure between the center of the depression and the air surrounding it, the stronger the winds will be. As they approach the center, the conservation of their angular momentum (see sidebar, page 71), causes the winds to increase in speed. Depressions in middle latitudes can be deep and, therefore, the winds around them can be strong. Winds often reach gale force, and at sea they

Depressions and the jet stream A front is the boundary between two air masses. An air mass is a large volume of air with distinctive characteristics of temperature, pressure, and humidity. These characteristics are fairly constant throughout the air mass at any given height. Adjacent air masses do not mix readily, because the air in them is at different densities. The sharp difference in temperature to either side of a front produces the difference in pressure, a difference that increases with height. This is because cold air is more compressed than warm air is. Consequently, air pressure decreases with height more rapidly in cold air than it does in warm air, so that the difference in pressure on either side of the front increases with height, reaching a maximum at the tropopause. The pressure difference generates a wind, called a thermal wind because of its association with a temperature difference. The polar fronts between polar and tropical air are permanent in both hemispheres. They are located in the middle latitudes, although their positions shift with the seasons. These are the fronts that produce the strongest thermal winds. At the tropopause, where the thermal wind speed reaches

a maximum, they are known as the polar front jet streams. The jet stream is usually located in the cold air, and it flows with the cold air to its left in the Northern Hemisphere and to its right in the Southern Hemisphere—resulting in a west-to-east direction of flow in both hemispheres. There is also a subtropical jet stream. It lies at about 30° N and 30° S throughout the year; the pressure difference generating it occurs only in the upper troposphere. The polar front jet stream does not blow in a straight line. There are waves in it. Waves that carry the jet stream toward the pole are known as ridges, and those that carry it toward the equator are troughs. Downwind of each ridge, air is being drawn into the jet stream and air is subsiding. This produces a region of high surface pressure. Air is leaving the jet stream downwind of each trough. This draws more air upward, producing a region of low surface pressure. The diagram shows the relative positions of the ridges, troughs, and surface highs and lows. A center of low surface pressure is known as a depression. It appears on weather maps at the crest of a frontal wave, with a warm front on one side and a cold front on the other. Between the fronts there is a

Hurricanes that reach Europe

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can sometimes reach hurricane force, of more than 75 MPH (121 km/h). Winds this strong are uncommon over land, because their passage over uneven ground generates friction, which slows them, but now and then one crosses a coast and may even penetrate deep inland. To a meteorologist, tropical and midlatitude depressions are cyclones, regions of low pressure around which air flows cyclonically (counterclockwise in the Northern Hemisphere), and any cyclone can produce strong winds. A tropical cyclone forms in the absence of fronts, whereas most midlatitude cyclones are frontal, but the principal difference is found at the center. A tropical cyclone forms over very warm water and turns into a hurricane when it develops an eye and eyewall at a markedly higher temperature than the surrounding air. A midlatitude cyclone lacks this warm core. It is the warmth that drives the hurricane, and because they lack this huge source of energy, midlatitude depressions may produce fierce storms, but they can never grow into full-scale hurricanes.

wedge of warm air—this area is known as the warm sector. It is from the warm sector that air is rising. The waves in the jet stream move from west to east (in both hemispheres). As they do so, the surface pressure pattern moves with them, highs (anti-

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cyclones) always ahead of each ridge and lows (cyclones) ahead of each trough. This is why depressions travel from west to east in middle latitudes, and why the weather in this part of the world is so changeable.

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Asian typhoons and cyclones The people of southern China, Taiwan, and the Philippines are no strangers to tropical storms and typhoons, and 2001 was a fairly typical year. It is the wind speed that determines whether or not a storm is deemed violent enough to be called a typhoon, but to its victims this is often a distinction without a difference. It is not so much the wind that wrecks homes and destroys lives, but the rain. Typhoon Durian struck all three countries in late June 2001. It began in the Philippines, crossed Taiwan, and arrived in Guangdong Province, China, on Sunday, July 1. Its winds reached 104 MPH (167 km/h), and it delivered up to 12 inches (305 mm) of rain. No deaths were reported, but the Chinese authorities said that repairing the damage in their country would cost at least $446 million. Durian moved next to northern Vietnam, where up to 17 inches (432 mm) of rain fell over three days. Six people died when a landslide buried their homes, and three people were swept away in the floods and drowned. Yutu struck Guangdong in the last week of July with winds of up to 94 MPH (151 km/h). The typhoon destroyed about 2,340 houses around the city of Maoming and damaged thousands more, as well as destroying crops. The total cost of that storm was put at $76 million. In early July 2001 Typhoon Utor generated winds of 80 MPH (129 km/h) gusting to 97 MPH (156 km/h) and rain that triggered flash floods and mudslides. Utor killed 121 people in the Philippines and left more than 40 unaccounted for. Most of the victims died when they were caught in mudslides or landslides. The typhoon merely grazed southern Taiwan. Landslides and floods blocked some roads, and one person was swept into a river and drowned. From Taiwan, Utor crossed into Guangdong Province, in southern China, where it dissipated. Toraji arrived in Taiwan early on Monday, July 30. One survivor wept as he described how floodwater had rushed through the living room of his house, carrying away his wife and nine other members of his family so rapidly there was nothing he could do to save them. He was left after the storm, digging through mud and debris in the hope of finding some trace of them. By the time it left Taiwan later that day, Toraji had cost at least 100 lives and many more people were missing. Early on Tuesday morning, Toraji arrived in Fujian Province, in mainland China, but it was weakening and caused no serious damage or loss of life there. Danas was weak. Its wind speeds barely entitled it to be called a typhoon. Nevertheless, it caused havoc when it struck Tokyo in September 2001. Five people died. Four of them were buried by mudslides just to the north of Tokyo. Trees fell, rain lashed down, and Toyota had to close 12 of its factories because it was impossible to move vehicles and supplies. Even as Danas—the name is a Philippine word meaning “to experience”—was moving away, another storm was approaching. Nari—which

Asian typhoons and cyclones means “lily” in Korean—was another weak typhoon. Most of the time it was classed as a tropical storm. Its wind speeds were misleading, however. Between early Sunday and around noon on Monday, September 16 and 17, Nari dumped 32 inches (813 mm) of rain on Taipei, Taiwan, overwhelming the drainage system, filling the streets to the height of car roofs and flooding parts of the subway. Rain rushed down mountainsides. Some people drowned in their beds. Five people were buried in mudslides. Homes were demolished and bridges destroyed in the north of the island, railroads and rail bridges were wrecked, and a total of more than 90 people lost their lives. A week later, people in Taipei were building sandbag walls in preparation for the arrival of Lekima—the name of a Vietnamese fruit tree. Its winds reached only about 74 MPH (119 km/h), but it moved very slowly, dropping around 20 inches (508 mm) of rain over parts of northern Taiwan. Lekima passed close to the Philippines. Tropical storm Lingling struck the islands in early November, killing around 300 people. On the island of Camiguin, about 440 miles (708 km) southeast of Manila, four hours of torrential rain triggered floods that swept away whole houses and sent boulders from the volcano Hibok-Hibok crashing into villages. The storm was so fierce it generated a waterspout. After leaving the Philippines, Lingling moved to Vietnam, destroying houses, uprooting trees, and sinking fishing boats.

The “big wind” In Mandarin Chinese, ta feng means “big wind,” and in some Chinese dialects it is pronounced tai fung. It is the name people give to the ferocious tropical cyclones that form in the South China Sea. These can devastate offshore islands and often cross the mainland coast, sometimes traveling a considerable distance inland and leaving a trail of destruction and human misery. Borrowing the Chinese phrase, we call these cyclones typhoons and extend the name to all tropical cyclones forming over the Pacific. The name is not used consistently, however. Tropical cyclones occurring near Indonesia and the Philippines are sometimes called baguios— Baguio is a town in the Philippines—and those near Australia are often called cyclones. “Cyclone,” in the sense of a tropical cyclone rather than the strict meteorological sense of a low-pressure region around which air moves cyclonically, is the name that used to be reserved for those occurring in the northern Indian Ocean. Nowadays tropical cyclones occurring in the southern Indian Ocean are often called “cyclones” as well. Of all the tropical cyclones that form, almost 90 percent are typhoons or cyclones. Eastern Asia, the Indian subcontinent, Indonesia, the Philippines, and the smaller islands of the tropical Pacific suffer nine such visitations for every one that occurs in the Atlantic and Caribbean, and Pacific typhoons are often fiercer than Atlantic hurricanes. The fiercest hurricane ever recorded was a Pacific typhoon called Tip, in October

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Hurricanes 1979. It had sustained winds of 190 MPH (305 km/h). Their ferocity is because the Pacific is much wider than the Atlantic. Developing storms have much farther to travel before reaching a continent and losing their source of warm water, and so they have more time in which to grow and intensify. For the same reason, a typhoon can cover a much larger area than a hurricane. “Supertyphoons” are rare, but when they occur they can cover 3 million square miles (7.8 million km2), an area equal to that of the mainland United States. Tropical cyclones leave their traces along the Great Barrier Reef, off the Australian coast, and along the eastern coast of the Gulf of Carpentaria, in northern Queensland. Scientists have used these to calculate the frequency of “supercyclones” in that region over the past 5,000 years. “Supercyclones” are those in categories 4 and 5 on the Saffir/Simpson scale (see sidebar, page 114). They have found that a supercyclone occurs every 200–300 years. No category-5 storm struck northeastern Australia during the 20th century, and the one that occurred in 1899 is the only one recorded since European colonization began in the middle of the 19th century, but there were two earlier in that century.

Wind and rain Think of a typhoon, and it is the wind that comes to mind. This is the most important feature of a tropical cyclone, especially while it remains at sea. Typhoons are huge. Winds of more than 75 MPH (121 km/h)—hurricane force—may extend around the eye to a diameter of 300 miles (483 km). They affect an area of more than 70,000 square miles (181,300 km2). These are the hurricane-force winds, but winds of gale force (more than 40 MPH; 64 km/h) can also cause considerable damage in the area surrounding the fiercest winds. In a tropical cyclone with a circle of hurricane-force winds that is 300 miles (483 km) in diameter, there are gales up to a further 240 miles (386 km) from the center. This adds more than 150,000 square miles (388,500 km2) to the total area affected. If a storm this size were centered over Kansas City, Missouri, its gales would be blowing hard as far away as Duluth, Minnesota; Amarillo, Texas; and Jackson, Mississippi. Wind is only one of the hazards, however. Torrential rain is at least as dangerous, and it is not unusual for a tropical cyclone to deliver 20 inches (508 mm) of rain in the space of 48 hours. The record is held by a typhoon that brought 38.5 inches (979 mm) of rain to Manila, in the Philippines, in 24 hours on October 17, 1967. Rainfall in the Philippines is heaviest in summer, between July and October. In the north this is due to the influence of the Asian monsoon (see sidebar), but in the south it is due mainly to typhoons. Even so, the wettest places expect no more than 20 inches (508 mm) of rain in the rainiest month, so the typhoon delivered almost double what is usually the heaviest monthly rainfall.

Monsoon The English word monsoon comes from an Arabic word, mawsim, which means “season.” Monsoons are strongly marked seasons. In Asia the winter monsoon is dry and the summer monsoon brings heavy rain. Monsoons resemble land and sea breezes, but occur on a huge scale. They are due to the different rates at which the land and sea warm and cool. During summer, the land heats much faster than the sea. Warm air rises over the land, forming a large area in which the atmospheric pressure is lower than it is over the cooler sea. This draws in air that is moist from its contact with the ocean surface. In winter, the reverse happens. The land cools rapidly and pressure over it rises. Air flows from land to sea, and, because the air originates deep inside a continent, it is very dry. So there is a dry winter monsoon and a wet summer monsoon.

WINTER

HIGH pressure

Monsoon climates occur over most of the Tropics. They affect much of tropical Africa, the northern part of Madagascar, the southern part of the Arabian peninsula, the Indian subcontinent, southern Asia, eastern Asia as far north as Japan, and northern Australia. Parts of North America also have a monsoon climate. West of the Rockies it brings a dry summer and rainy winter. On the eastern side of the continent the summer is wet and the winter dry. Monsoons are strongest over southern Asia, however, because of other changes in the seasonal distribution of air masses. During the summer, the equatorial trough moves north, intensifying the low pressure over land. At the same time, the Himalayan Mountains are so high they divide the air circulating over Asia into two distinct masses. In winter, the jet stream lies over the mountains and the polar high-pressure area covers most of northern Asia. Air flowing outward from it 60° intensifies the offshore winds. In summer, the jet stream weakens as 50° the continent warms, reducing the difference in air temperature to the 40° north and south of the mountains. The map shows how the changing 30° distribution of pressure alters the 20° direction of the wind. 10°

LOW pressure

0° 10°

SUMMER

60°

50°

LOW pressure 40° 30° 20° 10°

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one (ITCZ)

Winter and summer Asian monsoons. In winter, high pressure over central Asia generates dry winds blowing from land to sea. In summer, pressure is higher over the sea and moist winds blow over the continent.

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Cyclones Tropical cyclones in the Pacific and Indian Oceans are often severe. One that struck the Indian state of Orissa on October 29, 1999, destroyed several villages and caused widespread flooding. After it passed, the government confirmed that 9,463 people had lost their lives and around 8,000 were still missing. The cyclone season in the southern Indian Ocean (in the Southern Hemisphere) lasts from January to March. The season in the Bay of Bengal and Arabian Sea lasts from May to September. There is an average of 9.68 cyclones a year in the southern Indian Ocean and 8.75 in the Bay of Bengal and Arabian Sea. The actual number varies from year to year in both regions, but between 1945 and 2000 the average number decreased slightly in the southern Indian Ocean and very steeply in the Bay of Bengal, where cyclones are now uncommon. Ando was fairly typical of the cyclones that develop in the southern Indian Ocean. It formed on January 2, 2001, about 745 miles (1,200 km) east of the northernmost tip of Madagascar. It moved toward Madagascar and intensified, but on January 5 it turned to the south and passed between Madagascar and Réunion. It was traveling at 8–10 knots (9–11.5 MPH; 15–18.5 kmh) and came within 150 miles (240 km) of Réunion, but without crossing any of the islands in that area. Had it done so it would have caused severe damage, because at its peak intensity, reached on January 6, it was rated category 5. The pressure in its eye fell to 930 mb, its winds reached 140 MPH (225 km/h) with gusts up to 168 MPH (270 km/h), and it generated waves 20 feet (6 m) high. Its eye was well developed and 25 miles (40 km) in diameter. Geralda, a typhoon (or cyclone) that struck Madagascar on February 2–4, 1994, was even worse. It brought torrential rains driven by winds of up to 220 MPH (354 km/h). It left at least 70 people dead and half a million homeless, almost totally destroyed the country’s principal port, and flooded more than two-thirds of its farmland. Geralda was described as the “cyclone of the century,” but there have been others almost as fierce. On May 2, 1994, a cyclone with winds of up to 180 MPH (290 km/h) moving north across the Bay of Bengal crossed islands at the mouth of the Ganges, in Bangladesh. It killed more than 200 people. The death toll would have been even higher but for an early warning system that gave time for a major evacuation. A cyclone that reached the coastal islands of Bangladesh on April 30, 1991, before the early warning system was established, killed at least 131,000 people. Like all tropical cyclones, those of the Eastern Hemisphere form between latitudes 5° and 20°, but unlike the Atlantic, where no tropical cyclones form south of the equator, in the Pacific they develop in both Northern and Southern Hemispheres. They travel west in both hemispheres, then follow tracks carrying them away from the equator, heading north in the Northern Hemisphere and south in the Southern. In the Southern Hemisphere, of course, the Coriolis effect makes their winds spiral clockwise around the eye.

Asian typhoons and cyclones Except for those few that develop off the eastern coast of Central America and usually move out to sea, Pacific storm tracks carry typhoons toward densely populated lands. Cyclones cross the coasts of India, Pakistan, and Bangladesh and some, moving north through the Arabian Sea, reach Oman. South of the equator, they move toward Madagascar, some passing to the west of the island into the Mozambique Channel between Madagascar and Mozambique. In March 1994 a typhoon left 1.5 million people homeless in Mozambique.

Vulnerability of Bangladesh The Ganges, India’s greatest river, flows sluggishly and its level varies greatly. During the dry winter it carries little water and the level is low, but in spring it rises as meltwater from the Himalayas pours into it. It reaches its highest levels during the heavy monsoon rains of summer. This is also the time of year when cyclones are most likely to form in the Bay of Bengal to the south and move northward. Although the Asian monsoon is strongest in India, it also affects all of southern Asia and, on a smaller scale, there is also a monsoon season in tropical Africa. Most of Bangladesh is low-lying, with fertile plains nourished by silt that is deposited when the rivers flood. It is in Bangladesh that the Ganges, known as the Padma at this point, joins India’s other great river, the Brahmaputra, known locally as the Jamuna, and the combined rivers, known as the Meghna, flow toward the sea, swelled further by the many smaller tributaries that join them. Where the river enters the sea it forms a large delta. The Meghna has no single “mouth,” but many “mouths,” the individual branches of the river weaving their tortuous courses through a maze of islands and, in the south, an almost completely uninhabited region of mangrove swamp called the Sundarbans. The northern part of the delta is inhabited by people who build their homes on earth platforms or embankments to keep them clear of the seasonal floods, when all the river channels merge into a single stream. Bangladesh is one of the most densely populated countries in the world, with an average of nearly 2,000 people to every square mile (772 km–2), and most of them live in the countryside. Fish and other aquatic animals, such as prawns, are their most important source of dietary protein. Most are freshwater species, caught by fishermen who live in the countless villages beside rivers and in the northern part of the delta. When a cyclone strikes, they have little protection. After tropical storms on April 17, 1994, 200 fishermen from the town of Cox’s Bazar were missing and feared drowned, and 5,000 fishermen were missing from coastal islands after a cyclone in 1991, which caused at least 131,000 deaths. Even this was not the most severe storm recorded. A cyclone in November 1970 left some 500,000 Bangladeshi people dead. It was one of the worst natural disasters of the 20th century.

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Pacific typhoons As typhoons move westward across the Pacific, their tracks are likely to bring them close to many small islands and then to the Malay Archipelago, the largest archipelago in the world. Indonesia, occupying the larger part of the archipelago and with an area of about 741,000 square miles (1.92 million km2), is a string of about 3,000 islands straddling the equator and stretching for more than 3,000 miles (4,800 km) from the Malay Peninsula to New Guinea. Geographically, the Indonesian islands are joined to the Philippines. These cover a smaller area, of about 116,000 square miles (300,440 km2), but comprise 1,000 inhabited islands and 6,107 uninhabited ones. Most of Indonesia lies to the west of the usual typhoon tracks, which are turning north by the time they reach the islands, although Indonesia does not escape entirely. Turning north, however, the storms head directly for the islands lying between Indonesia and China—the Philippines.

The Christmas Day storm Much of the northern coast of Australia north of about 20° S is exposed to Pacific typhoons, although severe ones are rare so far south. On Christmas Day in 1974, however, the country experienced its worst natural disaster ever when Cyclone Tracy devastated the northern city of Darwin. The cyclone started to form on December 21 over the Arafura Sea between Australia and New Guinea. In the following days it intensified and moved southwestward on a track carrying it toward the Timor Sea, south of Timor, and from there it was expected to move into the Indian Ocean. Forecasters predicted it would remain at least 60 miles (96.5 km) from the Australian coast. Then, on Christmas Eve, Tracy intensified still further and changed course, now heading southeast and directly toward Australia. It reached Darwin at 4 A.M. on Christmas morning, with winds of up to 150 MPH (241 km/h). The storm lashed the city for about four hours. By the time it moved away, more than 50 people had lost their lives, 48,000 had been evacuated, and 90 percent of the buildings had been destroyed, including 8,000 homes.

Southern and eastern Asia Perhaps the most vulnerable regions of all are in southern and eastern Asia. By the time they reach eastern Indonesia, many typhoons are turning north, toward the Philippines, Vietnam, China, and, beyond them, Korea and Japan. Ten people died in August 1994 in Taiwan in a fairly weak typhoon, with 85-MPH (137-km/h) winds, but typhoons in that part of the East China Sea can be much worse. On August 20–21 of the same year, Typhoon Fred battered Zhejiang Province, China, for 43 hours without a break. It killed about 1,000 people and caused damage costing an

Asian typhoons and cyclones estimated $1.1 billion. The same region suffered twice in August 1990, when Typhoon Yancy killed 216 people in Fujian and Zhejiang Provinces and Typhoon Abe killed 48 people in Zhejiang. The following year Typhoon Amy killed at least 35 people in southern China. It is appropriate that we derive our name for such storms from the Chinese. Taiwan lies at the southern end of the East China Sea, where tropical cyclones formed in the South China Sea are turning north. Still farther north there lie the islands of Japan. Japan, in the East China Sea but between about 30° N and 45° N, might be thought safe from all but occasional typhoons that have weakened to tropical storms by the time they arrive, but typhoons often retain their power long enough for them to travel considerable distances. In 1953 about one-third of the Japanese city of Nagoya, just north of latitude 35° N on the island of Honshu, was destroyed by a typhoon that left 1 million people homeless. This was far from being the most devastating tropical cyclone the Japanese have had to endure. Hokkaido, the northernmost island, was struck in 1954 by a tropical storm that left 1,600 people dead, and in September 1959 Honshu suffered the worst typhoon in modern Japanese history. Called Vera, it killed more than 4,000 people and left 1.5 million homeless.

The Fujiwara effect Occasionally, two typhoons approach within about 900 miles (1,448 km) of each other. This is close enough for them to interact, and they start to turn around their common center, much like two stars orbiting their common center of gravity that lies somewhere between them. If the typhoons are of approximately equal size, they will turn around a point more or less halfway between their two centers. If one is much larger than the other, they will turn about a point closer to the larger one, and in this case the larger one usually absorbs the smaller. This can also happen in the Atlantic. On August 23, 1995, Tropical Storm Iris was approaching the Windward Islands, with Hurricane Humberto close behind. Iris slowed and turned a little to the south, while Humberto moved to the north. As the two began to circle each other, both of them weakened and then separated. About a week later, Iris was to the east of Bermuda and heading northward, now a full hurricane. Tropical Storm Karen moved in behind it, and again they began to circle. This time, though, Karen was much the weaker of the two, and Iris absorbed it. It happened most recently on September 6–7, 2001. Gil and Henriette, two Pacific storms, circled each other. Fortunately, neither of them crossed over land. This circling is called the Fujiwara (or Fujiwhara) effect, after Sakuhei Fujiwara (or Fujiwhara), the meteorologist who first described it. In the years following World War I, Fujiwara was a professor at the University of Tokyo and director of the Japanese Meteorological Society.

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Arctic and Antarctic hurricanes JOIDES Resolution is a research ship that is used to study the ocean floor. On each cruise she usually carries a crew of 52 to operate the ship, as well as about 20 engineers and technicians and 30 scientists. “JOIDES,” pronounced “joy-deez,” stands for Joint Oceanographic Institutions for Deep Earth Sampling, which is the former, rather unwieldy name for what is now known as the Ocean Drilling Program (ODP). The ODP is an international scientific project that involves examining rock and sediment collected in the form of cores that are drilled from the seabed. The work done on JOIDES Resolution forms part of the ODP. She can work in seas up to 27,000 feet (8.2 km) deep and has gathered cores from all of the world’s oceans. Her drilling rig, the pipes carried on her deck, and the 23-feet- (7-m) wide hole—called the “moon pool”—through her hull down which the drill string passes make the ship look a little unwieldy, but JOIDES Resolution is big and tough. Originally built in Halifax, Nova Scotia, for oil exploration, she went into service in 1978, with the name Sedco/BP 471. She was later converted to equip her for scientific research and went into service with the ODP in 1985. JOIDES Resolution measures 469 feet (143 m) from stem to stern, 68.9 feet (21 m) from side to side, and the top of her drilling derrick is 202 feet (61.5 m) above the water. She is owned by a company called Ocean Drilling Ltd. Both oil exploration and scientific exploration demand working in bad weather and JOIDES Resolution is built to withstand anything the sea can throw at her. In the fall of 1995, however, she came close to sinking in an Atlantic storm. The voyage began uneventfully. JOIDES Resolution set sail from Iceland in late September with 120 people on board, a few more than usual, and headed for the Greenland Sea, deep inside the Arctic Circle to the east of Greenland. The weather was not good, but JOIDES Resolution sailed unperturbed through conditions that would have caused difficulties for a smaller vessel. Edwin G. Oonk, the captain, had to maneuver the ship repeatedly to avoid icebergs drifting out from the glaciers of Greenland, to the west. Then the air pressure began falling sharply. There was a severe storm to the east and another to the south. The ship might have sheltered somewhere along the coast of Greenland, but there were icebergs between JOIDES Resolution and the coast, making it impossible to move closer inshore. Captain Oonk decided to sail forward into the weather. His tactic might have worked, but the two storms merged, pressure dropped still further, and the wind speed increased. For two days the storm raged. The dial on the windspeed indicator had a maximum reading of 115 MPH (185 km/h), and that was what it read in the gusts. Breasting waves 70 feet (21 m) high, like solid walls of water, the ship repeatedly rose high into the air and plunged into the troughs. At times the main propellers were lifted clear of the water. Lookouts, securely lashed down, had to be stationed in the stern to watch for

Arctic and Antarctic hurricanes icebergs, because at times the ship was being carried backward at more than 4 MPH (6.4 km/h). After a day and a half of this, Resolution was in danger of sinking. The storm abated at last, and the vessel made its slow way to port for repairs.

An extratropical hurricane This was not a tropical cyclone, but it was as fierce as one. Its winds of more than 115 MPH (185 km/h) were classified as storm force 12+ (12 is hurricane force on the Beaufort scale of wind strength). In lower latitudes it would have been rated a category 3 hurricane, strong enough to uproot large trees and wreck mobile homes. In 1954, the Swedish meteorologist Tor Bergeron (1891–1977) called storms of this force extratropical hurricanes. Unfortunately, this name can cause confusion, because it is used to describe more than one kind of storm. A tropical cyclone (or hurricane) that moves outside the Tropics soon loses some of its most distinctive characteristics. Its thunderstorms die down, its diameter increases considerably, and its strongest winds cease to occur close to the central eye. This formerly tropical storm is then known as an “extratropical hurricane.” The winds in a storm of this kind may be fairly light, but they can also reach hurricane force. They are especially likely to do so if two tropical hurricanes move away from the Tropics independently, both change their character, and then merge because they are traveling at different speeds and the second storm catches up with the first. A merger between two storms usually produces a huge and extremely dangerous monster of a storm. That is what produced the storm that nearly sank JOIDES Resolution. Extratropical hurricanes of this type most often form off the coast of New England or Canada. These are the ones that can cross the Atlantic to cause havoc in Europe. Tropical cyclones that move away from the Tropics can sometimes be revitalized. This is another way extratropical hurricanes develop. When a tropical cyclone moves out of the Tropics, it crosses cooler water. Water evaporates into it more slowly, and this reduces the vigor of its convective clouds, weakening it. Eventually, it will die completely, unless it encounters a cold front. When that happens, the warm air of the cyclone rides up the sloping boundary of the cooler air, and this forced lifting triggers a new bout of convection and convective warming as the rising air cools adiabatically and its water vapor condenses, releasing latent heat. The dying cyclone comes back to life and continues its journey, once more with the power of a hurricane. None of these is what Bergeron meant by an “extratropical hurricane.” He was describing a storm that originates a long way outside the Tropics. It is a storm that has never been tropical. Extratropical hurricanes of this type are very similar in some ways to tropical cyclones, however. They occur in both north and south polar regions. Around Cape Horn, at the southernmost tip of South America, the frequent gales associated with deep cyclonic storms led sailors to name

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Hurricanes the latitudes the roaring forties, furious fifties, and shrieking sixties. This region of frequent gales extends all the way around the world and is not confined to the area south of Cape Horn. It is associated with Cape Horn only because South America projects into the belt, and before the Panama Canal was built ships were compelled to pass around it in order to cross between the Atlantic and Pacific. At Byrd Station, Antarctica, the wind is strong enough to produce severe gales for about two-thirds of the time. Winds over the Southern Ocean are much stronger than those in the Arctic, because there is no large landmass between the southern tip of South America and the northern tip of the Antarctic Peninsula, whereas North America, Greenland, Scandinavia, and Siberia extend into the Arctic. Land slows and deflects the wind. With no land to moderate the wind, it blows more strongly and more frequently. It also blows over a longer uninterrupted distance. This means that it generates much bigger waves than those found in the northern oceans. Occasionally the weather systems that cause extratropical hurricanes can bring severe weather to places in lower latitudes. Around Christmas, 1995, for example, one extended south and brought extreme cold and heavy snow to Scotland and northern England, with snow drifts 30 feet (9 m) deep in the Shetlands, the northernmost group of Scottish islands. It reduced December temperatures over Britain to 4°F (2°C) below the monthly average.

Polar lows Extratropical hurricanes, of the type Bergeron described, usually develop from polar lows. These are areas of relatively low atmospheric pressure that form near the edge of the sea ice. There, the air temperature over the ice can fall locally as low as –40°F (–40°C), while the temperature over the sea is close to freezing (+32°F; 0°C). This means there is a temperature difference of 72°F (40°C) or more between air over the water and air over the ice. When there is also a region of low pressure along the edge of the ice, cold air is drawn into the system from over the ice and warm air from over the ocean. The merging of air at such widely different temperatures generates and then maintains the storm. This is a polar low. Such a sharp temperature difference is quite common in the Arctic. Warm air and ocean currents bring nearly twice as much warmth into the Arctic Ocean as is absorbed from solar radiation, and the seawater never cools below about 29°F (–5.4°C), so there is a constant source of warmth. Polar lows form in winter in the Arctic and throughout the year in the seas around Antarctica. There is also a sharp contrast in temperature between tropical air moving toward the pole and polar air moving away from it. These airflows meet at the polar front, where the warmer air rises as part of the system of convection cells that form the basis of the global circulation of the atmosphere (see sidebar, page 26). The rising air produces a belt of generally low surface air pressure. Winds in both hemispheres are from the east on the poleward side of the polar front and from the west on the side nearest the equator.

Arctic and Antarctic hurricanes The temperature difference to either side of the polar front can cause frontal systems to develop (see sidebar, page 19) with associated areas of low pressure (depressions, or extratropical cyclones as meteorologists call them). If the polar front is drawn as a line on a map, these fronts appear as waves along the main front, with the depressions at the wave crests. Such depressions form repeatedly along the polar front, often in “families,” as frontal systems in which warmer, less dense air rises over cooler, denser air. When all the warm air has been raised clear of the surface, the fronts are said to be occluded. It is at this stage that an intense polar low may develop behind the occluded fronts if an extreme temperature difference triggers a local disturbance. Where there is a large temperature difference in the air over the ice and air over the sea close to the polar front, a deeper depression develops over the water. Air is drawn in from the adjacent region of higher pressure and the converging air rises. As it does so, the Coriolis effect starts it rotating (see sidebar, page 69). This increases the temperature differences at the surface, because the rotation carries cold air from the poleward side of the polar front into a warmer region and warm air from the opposite side into a cooler one. It, too, is then a polar low.

Polar low into extratropical hurricane Compared with most depressions, a polar low is small. When it starts to form, it may be no more than 600 miles (965 km) across. Air flows into the low near the surface, rises, and flows away from the low at high altitude. This vertical movement intensifies the flow, as it does with a tropical cyclone, and intensification causes the low to shrink in size until, as a fully developed extratropical hurricane, it may be no more than 200 miles (322 km) in diameter. For comparison, depressions in middle latitudes can be anything from 100 miles (160 km) to 2,000 miles (3,200 km) in diameter, but on average they are approximately 1,000 miles (1,600 km) across. At the center of a polar low the surface atmospheric pressure may be less than 970 mb (the average sea-level pressure is 1,013 mb). This is comparable to the pressure at the center of a fairly gentle hurricane— gentle as hurricanes go, that is. This pressure will generate sustained winds of about 45 MPH (72 km/h) with gusts to 70 MPH (113 km/h). This is less than hurricane force, but the pressure may be lower and wind speeds much higher. Once it has formed, an extratropical hurricane is much like a tropical one. It is circular in shape, with a clearly defined eye that is relatively free from cloud and in which the air is calm. Spiraling walls of cumulus and cumulonimbus cloud surround the eye and extend all the way to the tropopause. Above the hurricane, the outflow of air produces a thick tail of high-level cirrus clouds. Seen from space, it has the same “spiral galaxy” shape as a tropical cyclone. There are important differences, however. An extratropical hurricane has an even shorter life than a tropical one. It develops from a polar low to

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Hurricanes a full hurricane in 12 to 24 hours and, once formed, it moves at up to 35 MPH (56 km/h). This is about twice the speed of a tropical cyclone, and it moves in the opposite direction. Carried with the prevailing winds, tropical cyclones travel from east to west, but high-latitude extratropical cyclones travel from west to east. In the Northern Hemisphere, their speed soon brings them to a large land mass, where they weaken and die, so they last no more than 36 to 48 hours. There is much less land at these latitudes in the Southern Hemisphere, so storms there travel farther and last longer. When an extratropical hurricane does cross a coast, it brings heavy snow or sleet driven by ferocious winds. The winds are seldom strong enough to cause severe damage to buildings, but they can bring down power lines and uproot trees, and when they deliver drifting snow they can seriously disrupt transport and communication systems. While they remain at sea, these storms present a serious danger to ships.

Hurricane damage

WHAT A HURRICANE CAN DO Hurricane damage By the time Cyclone Tracy moved away from Darwin, Australia, on Christmas Day 1974, it had demolished about 8,000 homes. The city was almost completely destroyed. After devastating the Bahamas, in late August 1992 Hurricane Andrew crossed southern Florida and Louisiana. With winds gusting to 164 MPH (264 km/h), it demolished some 63,000 homes in Florida. The towns of Florida City and Homestead were almost totally devastated. The hurricane also left 44,000 people homeless in Louisiana. In terms of the insurance claims, Andrew was the costliest hurricane in American history. It caused damage in those two states estimated at around $25 billion. A few days later, on the other side of the world, Tropical Storm Polly formed in the China Sea, then traveled westward to the coast of China. It killed 165 people and left 5 million without homes. In 1998, Mitch caused appalling damage in Central America (see “What happened when Mitch struck?,” page 1). Typhoon Olga made 80,000 people homeless in the Philippines in August 1999, and in September of the same year rain from Hurricane Floyd flooded 30,000 homes in North Carolina. Worse still, between January and March 2000, first Cyclone Eline and then Tropical Storm Gloria struck Madagascar, leaving 500,000 people homeless. Typhoon Maria caused $175 million in damage in China in September of that year. Tropical Storm Allison caused up to $5 billion in damage, mainly in the Houston area, in June 2001.

Destroying livelihoods and amenities Wherever they occur, tropical storms and cyclones cause immense destruction when they cross over land. Usually we think of the harm they do in terms of buildings demolished, but it is not only homes, offices, and factories that suffer. Hurricane Andrew destroyed at least half the Louisiana sugarcane crop. The world is not short of sugar, but farmers had been relying on that crop. It was their livelihoods that Andrew threatened and a great deal of their work that it reduced to piles of useless, sodden vegetation. Crop destruction is especially serious in the less industrialized countries, where the lost food may be difficult to replace and where a much larger proportion of the population work on farms than they do in North America, Europe, or Australia. Typhoon Cecil devastated crops in central Vietnam in May 1989. It also demolished around 36,000 homes, but houses

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Hurricanes can be rebuilt more easily than food can be found by a country that can ill afford to import it. In September of the same year, 1989, Hurricane Hugo swept over islands of the Caribbean and the eastern United States, destroying crops of corn and soybeans. Like most tropical cyclones, Hugo also uprooted and killed trees. Orchards were ruined and forests laid waste. The Caribbean National Forest in Puerto Rico was badly damaged, and the Francis Marion National Forest in South Carolina lost more than two-thirds of its trees and three-quarters of its population of the endangered red cockade woodpecker. In Charleston nearly all the trees were destroyed, and in North Carolina the city of Charlotte lost 20,000 of the trees that had once lined its streets and stood in its parks. The severe storm that crossed southern England in October 1987, with winds gusting to more than 80 MPH (129 km/h), uprooted and destroyed 19 million trees. These included many rare specimens in the Royal Botanic Gardens, at Kew, near London, which has one of the world’s largest and most important plant collections. The tree-lined streets gracing most of our cities and city parks, where we can relax, walk, or play, are important to us. If their trees are destroyed, we feel the loss keenly. The trees will be replaced, of course, but it will be many years before the new saplings grow into the large, mature trees that soften the hard lines of streets and buildings and provide us with shade in summer. Not all hurricane damage can be repaired quickly. The long-term effects on wildlife are usually less severe. Tropical cyclones and windstorms are natural events. They have occurred at intervals throughout history, and forests in the Tropics have survived many. Strong winds blow down trees, but in natural forests this creates gaps that are soon filled. In tropical rain forest, trees blown down by the wind often sprout new growth, so a downed tree can recover by regrowing from itself, from its own ruins, as it were. Few areas of rain forest, perhaps none, have stood for more than about 200 years without being disturbed by winds of hurricane force or by fire. Oddly, it seems that the tallest rain forest trees, with crowns that project above those of the trees around them, withstand hurricane-force winds better than the smaller trees.

Weighing air It was Evangelista Torricelli (1608–47), an assistant and secretary to Galileo, who first confirmed that air can be weighed. In 1644 he invented the mercury barometer. We use barometers to measure the pressure the atmosphere exerts, but it exerts that pressure because of its weight. In effect, a barometer is weighing the air above itself, in a column reaching all the way to the top of the atmosphere. Weight is the word we use for the force gravity exerts on a given mass. Mass is a property of all physical objects. They possess mass even if they are beyond the reach of gravity and weigh nothing.

Hurricane damage

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If we say that an object weighs one pound, for example, we mean by this that the mass of the Earth, which is very large, and the mass of the object, which is negligible by comparison, attract one another with a force we measure as one pound. The weight of an object, then, is its mass multiplied by the gravitational force. We all live on the Earth, however, and for most practical purposes the Earth’s gravitational attraction is everywhere the same, so it is usually the mass of the object that interests us. The obvious way to obtain the information we need, therefore, is to give the gravitational force a value of 1. Then, when the two masses are multiplied together, the product is the mass of the object we are weighing (because x × 1 = x). (In fact, the Earth’s gravitational force varies from place to place, but the differences are extremely small and of importance only to scientists who measure them to learn more about the composition of the rocks beneath the ground surface.) This useful trick, of declaring mass and weight to be equal, makes it easy to measure the mass of objects, but it works only so long as we remain on Earth. Move away from Earth, say to Mars, where the gravitational force at the surface is only 38 percent of that on Earth, and an object will weigh less. If it weighs one pound (0.454 kg) on Earth it will weigh a little more than six ounces (0.170 kg) on Mars. Its weight changes, but only because the mass of Mars is 38 percent of the mass of Earth (x × 0.38 = 0.38x). The mass of the object remains the same no matter where it is. If you set off for Mars in your spaceship with a one-pound rock from Earth and threw it when you arrived, the rock would fly higher and farther than it does on Earth. That is because Mars would pull it less strongly than Earth does. But if it hit someone, it would hurt just as much as if you threw it in your own backyard. Its mass remains the same.

Energy of motion When a mass moves, it possesses energy of motion, called kinetic energy (see sidebar). If a moving object hits something, part or all of its kinetic energy is transferred to whatever it strikes. That is what can hurt, and it is why air, which has mass like anything else, can cause damage if it moves fast enough.

Kinetic energy and wind force Kinetic energy (KE), which is the energy of motion, is equal to half the mass (m) of a moving body multiplied by the square of its velocity (or speed, v). Expressed algebraically, KE = 1/2 mv2. This formula gives a result in joules if m is in kilograms and v is in meters per second. If you need

to calculate the force in pounds that is exerted by a mass measured in pounds moving in miles per hour, the formula must be modified slightly to: KE = mv2÷2g, where v is converted to feet per second (ft. per second = MPH × 5,280 ÷ 3,600) and g is 32 (the acceleration due to gravity in feet per second).

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Hurricanes Air can be compressed, so the mass of any given volume is very variable, but on average, at sea level, one cubic foot of air has a mass of about 1.2 ounces (which is 0.075 pounds, and equal to 1.2 kg m–3.) When it moves, as wind, its kinetic energy is equal to 0.075 multiplied by the square of its speed and divided by twice the acceleration due to gravity (or 0.6 multiplied by the square of its speed to give a result in joules). It is this energy that exerts a pressure force against anything in its path. You can feel it when you walk in a strong wind, and if the wind is powerful enough, walking can be very difficult or even impossible. Consider a gentle breeze blowing at 10 MPH (16 km/h). The force with which it presses against objects in its path amounts to about four ounces per square foot (12 J m–2). This is sufficient to set a flag moving and leaves rustling, but you would barely feel it. See what happens, though, when the wind speed doubles. At 20 MPH (32 km/h) the wind presses against objects with a force of one pound per square foot (47 J m–2). Wind speed has doubled, but the force it exerts has increased fourfold. Now it can make small trees sway, and you will certainly feel it. Increase the wind to 75 MPH (121 km/h), which is the minimum speed needed to qualify it as a hurricane, and it exerts a pressure of 14 pounds per square foot (678 J m–2), and a 100-MPH (160-km/h) wind exerts a pressure of 25 pounds per square foot (1,185 J m–2). This may not sound like a very large pressure, but it is being felt on every square foot (or meter) of a surface exposed to it. Suppose a mobile home is side-on to a hurricane wind. If, say, the trailer is 30 feet long and 9 feet high (9 × 2.7 m), the side facing the wind has an area of 270 square feet (24.3 m2). In a 75-MPH (121-km/h) wind the force pressing against the whole side will be about 1.9 tons (1.7 t) and a 100-MPH (161-km/h) hurricane will exert a force of 3.4 tons (3 t) against it.

Using leverage Winds press hard against surfaces, but they also use leverage. Where there is a gap or a projecting surface the wind may press from the side or beneath it and this can tear structures apart. Roofs made from sheeting can be loosened along one edge then torn away as a single piece. Ground-level winds do not blow at constant speed. The moving air is slowed by friction, due to its contact with the ground and structures on it, and deflected by hills, trees, and buildings. As it twists and turns, the resulting eddies create gusts. These are brief, but often large, increases in wind speed, and they have an important effect. If a steady wind pushes against a structure strongly enough to weaken it, a sudden gust may be enough to demolish it. A 75-MPH (121-km/h) wind gusting to 90 MPH (145 km/h) will press against our imagined mobile home with a force of 1.9 tons (1.7 t) that now and then increases to about 2.8 tons (2.5 t) and drops again. This will set the trailer rocking with increasing violence until it is overturned.

Hurricane damage Much wind damage is caused indirectly. Once a large fragment has been torn free it may be thrown against another structure with enough force to damage it. This weakens the second structure, and perhaps fractures some of its surfaces, increasing the likelihood that the wind will complete its total destruction. Trees are more often uprooted by gusts than by a steady wind. Rocking shakes their roots. Smaller roots snap, soil is loosened around the larger roots, and the trees are less firmly anchored in the ground. Once they are in this state, a final gust is enough to tear the tree free and make it fall. Power pylons, radio masts, and telephone poles are felled in the same way. In a forest, a falling tree may well bring down others that strike still more and bring them down as well.

Eddies and tornadoes Eddies due to friction are not the only cause of variations in wind direction and speed. The eyewall of a tropical cyclone consists of towering cumulonimbus clouds. These are the clouds that produce thunderstorms, and there is a great deal of thunder and lightning around the eye of the storm. Intense convection associated with the storms generates local disturbances where rising air rotates, as small cyclones surrounding the main one, and when they pass, the wind changes in both speed and direction, so structures are buffeted not only by strong gusts, but by gusts from different directions. Just imagine being inside a mobile home that is being hit hard first from one side and then from the other. As though the gusting, hurricane-force winds were not hazard enough, tropical cyclones often trigger tornadoes around the eyewall. Like all tornadoes, these develop from the small cyclones in and beneath the storm clouds. Each tornado lasts for only a few minutes, and they are not ferocious compared with many tornadoes, but while they last the wind speed increases greatly and there may be many of them appearing and vanishing unpredictably.

Classifying winds When Admiral Beaufort devised the first version of his scale for wind force (see sidebar, page 78) he included no descriptions of the effect of the wind. It was meant to guide naval officers regarding the amount of sail their ships should carry under the various wind conditions. Once values for wind speeds had been added (not until 1939), his scale ended with 75-MPH (121-km/h) winds. In the original, handwritten memorandum in which Beaufort outlined his classification in 1806, there were 14 categories, from 0 to 13, and a force-13 wind was designated “storm.” The scheme was later revised to make 13 forces (0–12), and force 12 was then designated “hurricane.” The scale was good enough for mariners who needed to know how much sail a ship should carry. A wind of hurricane force would quickly

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Hurricanes reduce any sail to tatters—and probably carry away the spar to which it was attached and possibly the mast as well. No sail should be carried. In the version that was issued to naval commanders, force 12, “hurricane,” carried the instruction “Or that which no canvas could withstand.” The Beaufort scale is not much use to people on land who may find themselves in the path of an approaching hurricane and who need to know whether it will destroy their homes. It is not even of much value to modern sailors, because they do not use sails—although in its most recent version it includes descriptions of sea conditions. An additional scale is needed, to start where the Beaufort scale ends. Several have been compiled, but the one most widely used is called the Saffir/Simpson Hurricane Scale (see sidebar below). When hurricane forecasters at the National

Saffir/Simpson Hurricane Scale Number Pressure in eye

Wind

Storm surge

(mb) (in of mercury) (cm of mercury)

(MPH) (km/h)

(feet) (meters)

Damage

1

980 28.94 73.51

74–95 119–153

4–5 1.2–1.5

Trees and shrubs lose leaves and twigs; mobile homes damaged

2

965–979 28.5–28.91 72.39–73.43

96–110 154.4–177

6–8 1.8–2.4

Small trees blown down; exposed mobile homes severely damaged; chimneys and tiles blown from roofs

3

945–964 27.91–28.47 70.89–72.31 920–944 27.17–27.88 69.01–70.81

111–130 178.5–209

9–12 2.7–3.6

131–155 210.8–249.4

13–18 3.9–5.4

920 or less below 17.17 below 43.61

155 or more 250 or more

18 or more 5.4 or more

Leaves stripped from trees; large trees blown down; mobile homes demolished; small buildings damaged structurally Extensive damage to windows, roofs, doors; mobile homes destroyed completely; flooding to six miles inland; severe damage to lower parts of buildings near exposed coasts Catastrophic; all buildings damaged severely; small buildings destroyed; major damage to lower parts of all buildings less than 15 feet (4.6 m) above sea level to 0.3 mile (0.5 km) inland

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5

Hurricane damage Oceanic and Atmospheric Administration issue warnings, it is the Saffir/Simpson scale they use to assign categories to hurricanes. Unlike the Beaufort scale, the Saffir/Simpson scale relates the damage a hurricane is likely to cause only partly due to its wind speed. Despite the ferocity of the wind, hurricanes also cause flooding, and in most cases water does at least as much damage as the wind itself (see “Storm surges,” page 122). The scale also reports the atmospheric pressure in the eye, which is a useful indicator of the intensity of a tropical cyclone, because wind speed is directly related to the difference in pressure inside and outside the system.

Storms at sea We live on land and it is easy to forget what a tropical cyclone can do to ships. On September 11, 1989, Typhoon Sarah swept across the China Sea toward Taiwan. On its way it met a Panamanian freighter and broke it in two. In early November of the same year, 93 workers died in the Gulf of Thailand when Typhoon Gay capsized an American ship that was drilling for gas. An oil tanker was broken in two off the Philippines in October 1994 by Typhoon Teresa. Fishermen, working in much smaller boats, are especially vulnerable to tropical cyclones. Tropical storms killed 200 fishermen at Cox’s Bazar, Bangladesh, in April 1994. Far from land, a Force 12 wind on the Beaufort scale makes the sea completely white, and driving spray—called spindrift—reduces visibility almost to zero. Beneath the spray, the waves themselves are huge. Their size depends on the wind speed, the distance over which the wind is blowing (called the fetch), and the length of time for which the wind blows. Waves are generated by the transfer of energy from wind to water, and the energy that waves possess is proportional to their height, so the bigger the wave the more energy it has. As wind speed increases, so more wind energy is transferred to the sea and the bigger the waves grow. A wind of 110 MPH (177 km/h) can produce 30-foot (9-m) waves. It is only when the wind blows over a large fetch for many hours that the waves reach their maximum size, and the stronger the wind the greater the fetch and time that is needed for the waves to reach their maximum. This sounds contradictory, but energy can be transferred from air to water at only a certain rate. It is like pouring water into a container. The more water there is to pour, the longer it will take. In the same way, the greater the amount of energy that is to be transferred from wind to waves, the longer that will take. In a tropical or extratropical cyclone, the winds approximately encircle the eye, so they blow from different directions at each point and are not constant enough to allow waves to develop fully. Waves travel in different directions and interfere with one another. It is the interference that makes the sea look as though it is boiling, but when several waves combine the result can be an extremely large wave. When two waves merge it may happen that the crests of one coincide with the troughs of the other. The two

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Hurricanes will then cancel one another and the water will be calm. Should the crests of both coincide, however, they will add to one another, and the height of the combined wave will be roughly equal to the sum of the heights of the two individual waves. It is difficult to predict the size and power of waves, but under any sea condition it is common for some waves to be much bigger than average. If 100 waves pass a particular point, for example, one of them can be expected to be nearly 6.5 times higher than the average, and in every 1,000 waves there will be one that is nearly eight times higher. There is a limit to the size of waves, however, because beyond a certain height a wave breaks as its own weight makes the upper part of the wave fall forward. The research ship JOIDES Resolution reported 70-feet (21-m) waves (measured from trough to crest) when it sailed through an extratropical hurricane in the Greenland Sea in 1994. Typhoon Cobra, which caused havoc with U.S. Navy Task Force 38 in December 1944 in the Philippine Sea, also produced waves 60–70 feet (18–21 m) high. These are probably close to being the largest waves possible. It is big enough. A 70–feet (21–m) wave is about three times the height of a three-story house. Waves travel and those from a tropical cyclone can be felt at great distances. Larger than usual ocean waves in Alaska were once traced to storms near Australia, and waves formed near the Aleutian Islands are known to have reached California in about five days, having traveled at about 35 MPH (56 km/h).

Daniel Bernoulli and how hurricanes can lift roofs Hurricane Marilyn raged through the U.S. Virgin Islands and Puerto Rico in September 1995. After it had passed, Wilfred Barry, a U.S. marshal who flew over the island of St. Thomas in a military jet to inspect the damage, was reported to have said that there were no roofs at all left on the island. Marilyn was rated as only a category 1 hurricane on the Saffir/Simpson scale (see sidebar, page 114). Nevertheless, every building had been stripped of its roof. Hurricanes commonly tear roofs from buildings, especially roofs with a shallow slope up to a ridge that are made as a few large sections. Corrugated iron roofs are especially prone to flying away. So are the canopies over the fuel pumps at gas filling stations. It is hardly surprising. Filling stations are located in exposed sites beside roads, and there are only ties to the tops of pillars to hold their canopies down. Slate or tile roofs are also destroyed in strong winds, but they disintegrate piecemeal as the slates or tiles are loosened, then dislodged, one at a time.

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You can imagine what happens. As the wind crosses the rough ground surface, friction slows the air closest to the ground. There are then layers of air moving at different speeds. The airflow becomes turbulent, with many eddies. If the roof is made from sheeting, eddies that are gusting upward push against a projecting edge, striking it repeatedly, like a series of hammer blows, until they succeed in detaching that edge. Then the wind can bend the section sufficiently to provide a bigger surface on which to push, increasing the force. Slates and tiles overlap one another, so the roof surface is uneven. The wind works on this unevenness, penetrating the tiny air space between each slate or tile and the one overlapping it, and pushing up and back until the securing nails are withdrawn or broken. Once its securing nails are gone, the slate or tile is easy to shift, and its departure leaves a gap through which the wind can exert even more force on the surrounding part of the roof. The diagram shows what happens. Having devastated St. Thomas, the most seriously affected island in the group, Marilyn moved on to Puerto Rico. There it was Culebra Airport that felt the full force of the storm. One newspaper photograph showed a small airplane turned on its back, totally wrecked, with its fuselage broken and its fin and rudder twisted to one side. When you see pictures of hurricane damage it is not unusual to find some that depict light airplanes thrown upside down. These make dramatic photographs, of course, and one reason for the vulnerability of light airplanes is fairly obvious. Airfields are large areas of level, open ground where there is little or no shelter from the wind. If the airplanes are not moved into hangars or firmly anchored to the ground with strong cables, it is easy to imagine a strong gust catching the underside of a wing and flipping an unsecured plane onto its back.

laminar flow

turbulent flow

gas station

Why hurricanes lift roofs. Friction slows the air at ground level, so layers of air are moving at different speeds close to the ground. This makes the flow turbulent, so in some places the wind is blowing upward and in others downward. Above this level the flow is laminar.

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Pushed or pulled? In both cases, the picture is of the wind pushing upward, lifting the airplane or roof by exerting a force from below. No doubt this happens sometimes, but it is only part of the explanation. In the majority of such incidents the wind also lifts wings and roofs from above, as though pulling them upward, and only then, when they are tilted sufficiently for a large surface to be exposed facing the wind, does the serious pushing begin from below. In 1738 the Swiss scientist Daniel Bernoulli (1700–82) discovered the reason why this happens. A physicist, mathematician, doctor, and botanist, Bernoulli was a truly remarkable member of an extremely remarkable family. His discovery, now known as the Bernoulli principle (or Bernoulli’s law), states that the internal pressure in a moving fluid (liquid or gas) is lower than it would be were the fluid at rest. His principle sounds wrong, because you would expect moving air to exert more pressure than still air, not less, yet its truth is very simply demonstrated. Try placing a small coin—a penny is ideal—on a table, about 0.4 inch (1 cm) from the edge. Bend down so your mouth is level with the tabletop, and blow hard across the top of the coin. The coin will jump, because by blowing you made the air pressure above it drop suddenly. Alternatively, hold two sheets of typing paper (the size is not important) side by side and about 1 inch (2.5 cm) apart. Blow gently between them. You might expect them to move apart, but in fact they will move toward each other. Again, blowing makes the pressure fall. The principle sounds wrong because we think of the wind, or water in a fast-flowing river, pushing against obstacles with considerable force and therefore increasing the pressure on them. In fact, however, pressure is reduced. But the pressure that interested Bernoulli was the internal pressure: the pressure inside the fluid, not the force with which the fluid pushes against external objects. “Force” and “pressure” are not the same thing. We exploit his principle in several everyday devices. A spray gun blows a stream of air across the end of a tube, the other end of which is immersed in a liquid. Reduced pressure in the moving air causes the ordinary (higher) air pressure to force liquid up the tube. The diagram shows how this works. A carburetor mixes droplets of fuel with flowing air in much the same way. Indeed, you are probably familiar with the Bernoulli principle already. If you ride a bike, you know that when a car passes close while it is overtaking you the bike tends to be pulled toward the car, so you have to steer to avoid colliding or falling. This happens because the moving car drags air with it, so the air pressure between you and the passing car is lower than the air on the side of you away from the car. This difference in pressure exerts a force pushing you and your bike in the direction of the car. What is more, the faster the car is traveling the greater will be the force pushing you toward it. The same thing happens when two express trains pass each other, both traveling at full speed in opposite directions. Passengers in both trains hear a dull “thump” as the air being pushed ahead of the approaching train

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low pressure air pump high pressure

high pressure

liquid

liquid

strikes their own train and then travels along it as a shock wave. Once the shock wave has passed, the reduced pressure between them pushes the two trains toward each other. Railroad engineers know this will happen, of course, and the trains are designed to withstand it. Even if you don’t ride a bike, you may have experienced a similar effect with shower curtains. Provided the water pressure is fairly strong so the water flows fast, when the shower is running the curtain is drawn inward, toward the shower. If the cubicle is small, the curtain may even try to wrap itself around you. As it falls, each shower droplet is surrounded by a tiny envelope of air that clings to the water. The droplets drag their envelopes of air down with them, so there is a wind blowing in the same direction as the water. This produces a region around the spray where the air pressure is just a little lower than it is outside the cubicle, on the other side of the curtain. The higher air pressure on the outside of the curtain pushes the curtain inward. This is the Bernoulli principle at work, and the relationship between pressure and the speed of the moving fluid is what makes airplanes and roofs fly.

Why planes fly When a stream of air flows smoothly over a bulging surface, such as a ridged roof or the upper side of an airplane wing, it has farther to travel than surrounding air that is also moving, but that does not cross the surface. As the diagram shows, however, it is not allowed any more time to complete its longer journey. All the air must rejoin on the far side of the surface. This means the air flowing the greater distance must move faster and, therefore, the pressure inside it—its internal pressure—will be lower.

How a spray gun works. The lever pushes a cylinder, drawing air across the top of the tube. This produces a region of low pressure above the tube. Air pressure inside the reservoir pushes liquid up the tube. The airflow then drives it through the nozzle, breaking it into fine droplets.

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Hurricanes Mathematically, Bernoulli showed that the pressure in a flowing fluid is proportional to the square of its velocity by the equation: p + 1/2rV2 = a constant, where p is the pressure, r is the density, and V is the velocity of the air (or liquid). While it sits motionless on the ground, the air pressure is the same above and below the wings of an airplane. Once the plane starts moving forward, air flowing over the shaped upper surface of its wings travels farther and so air pressure falls over those surfaces. The faster the airplane moves the more the pressure above its wings falls until the difference in pressure above and beneath the wings is greater than the weight of the airplane. At that point the plane leaves the ground. Because the force supporting the airplane is called “lift,” it sounds as though the plane is being pulled upward from above, the way you might lift an object from the ground. This is not what happens. The force acts from below, pushing the plane upward, but the force exists because of the difference in pressure above and below the wings. It is not only the wings that experience this force, of course. It acts across all the convex (curving

Force acting upward (lift) Air travels further; p ressure f alls

Direction of airflow

Force acting upward (lift) Direction of airflow

Air tra vels f urthe r; pr essu re

The Bernoulli principle

fall s

Daniel Bernoulli and how hurricanes can lift roofs upward) surfaces, including the fuselage and tail-plane. A helicopter’s rotor blades are wings that move, and they also generate lift, as does the propeller of a propeller-driven airplane, although in this case the lifting force acts horizontally rather than vertically. The airflow must be smooth (the technical term is laminar). If it is turbulent, countless small eddies form in it. These slow down the flow of air, thus reducing the amount of lift. As turbulence spreads, the pressure over the surface increases until it equals the surrounding pressure and there is no longer a lifting force. This is what happens when the airspeed of an airplane (its speed in relation to the airflow) falls so low that the plane stalls. If you “stall” a car, the engine stops. “Stalling” in an airplane has nothing to do with the engine. It refers to the loss of lift that causes the plane to start falling; it is the wings that stall, not the engine. Suppose, though, that the airplane remains in one place on the ground, but is facing more or less directly into the wind. Air is now flowing over its wings just as if the air were still and the plane were moving forward. Lift will exert a force on the wings. If the wind speed increases sufficiently, the lifting force may exceed the weight of the plane. The plane will not take off, of course, but it may rise from the ground. It cannot sustain the rise, because it has no forward motion, and it rises unevenly. One wing may experience more lift than the other, the nose or tail may rise, and the plane immediately loses its lift and stalls. Completely unstable, the plane is at the mercy of the wind, which may well throw it onto its back. Once it is upside down, the plane is much more stable, because any lift generated by the wind flowing over its wings will act downward, pressing it to the ground.

Why roofs fly A building is not designed to fly, so it is less aerodynamically efficient than a plane, but the Bernoulli principle allows no exceptions. Wind, which is moving air, accelerates as it crosses the greater distance imposed by the shaped roof. Because it accelerates, the air pressure above the roof falls by an amount that is proportional to the square of the wind speed. Inside the building, beneath the roof, the air pressure does not change, so the result is a difference in pressure that exerts a force. Just like the lift experienced by an aircraft wing, this force acts upward on the roof. The roof is pushed upward from inside the building, by the greater air pressure. It is not true, however, that this pressure difference can ever be so large as to make the building explode outward. Some people will tell you that during a severe storm—one that may produce tornadoes—you should leave some of the windows open to equalize the air pressure inside and outside the house. Unless you do, they say, there is a risk that the difference between the normal air pressure inside and the very low pressure outside will make the house burst. It is nonsense. Hurricanes often generate tornadoes, but the pressure even at the center of the fiercest storm imaginable

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Hurricanes never falls anywhere near low enough to smash walls, burst open closed doors, or shatter windows in this way. What the pressure difference can do, however, is snap the ties holding the roof to the walls and lift the roof, or a section of it, clear of the building. Once free, the roof is even less stable than an uncontrolled airplane. It may be that one side will drop, tilting the roof in relation to the airflow. If the side facing into the wind drops, the lift over the section will increase and the roof will climb. If the opposite side drops, the full force of the wind will press against the exposed underside and the roof will fly horizontally. If the roof tilts to left or right in relation to the airflow, it may follow a curving path. In extreme cases the roof will tilt first one way then another as it tumbles, twists, and turns unpredictably through a short, chaotic flight before crashing to the ground. Flipping airplanes usually harm no one, because no one is around, but flying roofs can do a great deal of damage. They may crash into other buildings, onto vehicles, or land on people. Airborne slates and tiles are less dramatic, but they are even more dangerous. They can also generate enough lift to travel some distance and, because they are relatively small and there is so much else happening, people may not see them coming. A roof slate weighs several pounds and has sharp edges or, if it is broken, jagged points. Many people have been seriously injured, and some killed, by flying slates.

Storm surges Hurricane Opal crossed Mexico and Florida in October 1995, causing damage that cost around $4 billion in 1995 dollars. It was not wind that caused the greatest devastation, however, but water. In this, Opal was a typical hurricane. Tropical cyclones always bring torrential rain. That alone is enough to cause flooding, because the amount of rain falling in a very short period far exceeds the usual rate of precipitation and it is the usual rate that natural drainage systems are able to remove. Ordinarily, when rain reaches the ground, most of the water soaks down into the soil. Much of this water remains fairly close to the surface. Plant roots absorb some of it and return it to the air through transpiration, and some rises through the soil and evaporates from the ground surface. The remainder continues to sink downward until it reaches an impermeable layer of rock or compacted clay. It collects above this layer, as groundwater, flowing slowly downhill, and eventually enters streams and rivers that carry it away, eventually to a large lake or the sea. Together, the flow of groundwater and surface streams and rivers link together to form a drainage network that removes surplus water. If the rain falls very heavily, however, some of the water may flow across the surface as surface runoff,

Storm surges because the impact of the raindrops batters soil particles, forming them into a thin, compacted layer across the ground surface that makes it more difficult for water to drain vertically.

When the water has nowhere to go This is what happens with normal rainfall, but when 20 inches (508 mm) of rain, or sometimes more, falls within a 48-hour period, the natural drainage network is overwhelmed. Tropical Storm Alberto delivered 24 inches (610 mm) of rain in some parts of the state of Georgia in July 1994, and caused so much flooding in Georgia, Alabama, and Florida that all three states were declared national disaster areas. In October 1993, rain brought by Tropical Storm Flo caused mudflows that buried 200 homes in Luzon, Philippines. Mitch, in 1998, was the fiercest hurricane to strike the Caribbean and Central America in recent years, and it was the rain that caused the high death toll. Mitch dumped water at the astounding rate of 12–24 inches (305–610 mm) a day in some places and up to 75 inches (1,905 mm) over the 6–8 days the storm lasted (see “What happened when Mitch struck?” page 00). When this amount of rain falls, water reaches the ground much faster than it can soak downward. Most of it flows over the surface, moving rapidly downhill and accumulating on low-lying ground, but some finds it way into rivers. It makes their levels rise until the rivers overflow their banks. Rain need not fall as intensively as this to cause floods, of course. Far less violent storms can cause flash floods, and any prolonged period of heavy rain may deliver more water than natural drainage is able to remove.

When the sea rises If that were all a tropical cyclone can do with water it would be serious enough. People would expect hurricanes to cause at least some flooding. Unfortunately, it is by no means all they can do. They also cause the sea to rise 10 feet (3 m) or more above its usual level, with huge waves that sweep inland, destroying everything in their paths. These sudden rises in sea level are called storm surges. Mitch generated only minor storm surges, but in September 2001 Tropical Storm Gordon sent 6-foot (1.8-m) waves crashing over the Florida coast. Opal hit the U.S. coast with 12-foot (3.7-m) waves, and it was those and the flooding they produced that caused most of the damage to property. Storm surges can be even larger than these, and when they are their consequences are much more serious. In September 1961, Typhoon Muroto II produced a 13-foot (4-m) storm surge that sent sea waves rushing through the Japanese city of Osaka, and in late August 1992, Tropical Storm Polly caused a 20-foot (6-m) surge at the Chinese port of Tianjin. Hurricane Frederic generated a 15-ft (4.6-m) storm surge at the mouth of Mobile Bay, Alabama, in 1979.

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Hurricanes Big though they are, these are trifling compared with the biggest storm surge ever recorded. That one happened in March 1899, when a typhoon struck Bathurst Bay, near Cape Melville in northern Queensland, Australia. The storm that came to be known as the Bathurst Bay Hurricane produced a storm surge of 42 feet (13 m)—a figure that was estimated after the event by measuring the height of the debris it left behind. The waves destroyed a fleet of pearling boats and killed the crews of 100 vessels and around 100 people on the shore. Bathurst Bay suffered the biggest storm surge ever recorded, but not the highest number of casualties. The most deadly storm surge happened in 1900 and completely covered the densely populated island of Galveston, Texas (see “Historic hurricanes,” page 130).

The force of wind and water Wind damage occurs because air has mass, and when it moves it exerts a force pushing against objects in its path. This is familiar, because all of us experience it. On a windy day you can feel the pressure of the wind. We do not usually feel the pressure of moving water, but it can be much greater than that of wind, because water is much denser than air. The mass of 1 cubic foot of air is about 1.2 ounces (0.075 pounds; 1.2 kg m–3). The mass of 1 cubic foot of water is about 62 pounds (1 tonne m–3)—more than 800 times greater. Consequently, flowing water exerts a force more than 800 times greater than that exerted by a wind blowing at the same speed. That is not all. Gusts add greatly to the force of the wind. They strike repeatedly with varying force and from slightly different directions. The sea behaves differently. It arrives as a series of waves that crash into structures, flow over, around, and past them, and then return, flowing in the opposite direction, so they exert a push–pull force that is much more destructive even than the repeated hammer blows of the gusting wind.

Bulging sea surface Four factors combine to produce a storm surge. The first is the fall in atmospheric pressure inside a tropical cyclone. This causes the sea level to rise beneath the eye of the storm. We are used to the idea that “water finds its own level.” If one open-topped container full of water is connected by a pipe near the bottom to another standing at the same level that is half full, water will flow from the full container to the half-full one until the level of water is the same in both. The two levels are the same because the air pressure is the same above the water in each container. The weight of the overlying atmosphere presses equally on each square inch of both water surfaces. That is what makes it possible to siphon water from one container to another. Over the ocean, the atmospheric pressure varies from place to place, so the weight of overlying air is not everywhere the same. Where pressure

Storm surges is high, the sea surface is depressed, and where it is low the sea surface rises. Surprisingly perhaps, the sea level is not the same everywhere. A drop in pressure of 0.07 pounds per square inch (1 millibar, mb) allows the sea to rise by about half an inch (13 mm). Scientists make use of this fact. Instruments carried on satellites are able to measure the height of sea level very accurately. Meteorologists can convert these measurements into reliable calculations of sea-surface air pressure. Beneath the eye of a category 1 tropical cyclone (see sidebar, page 114), the atmospheric pressure is 0.5 pounds per square inch (36 mb) lower than the average of 15 pounds per square inch 1,013 mb. In the deepest cyclone (category 5) it may be 1.5 pounds per square inch (100 mb) lower. These pressures raise the sea level by about 14 inches (35.5 cm) in a category 1 hurricane and by about 40 inches (1 m) in a really intense, category 5, tropical cyclone. As the eye approaches the coast, this is the amount by which the sea level will rise from this cause alone. It may seem a small rise, but if it coincides with a high tide it may be sufficient to flood low-lying coastal land.

Tides Ocean tides are caused by the combined effects of the Earth’s rotation and the gravitational attraction between the Earth, Moon, and Sun. As you know, if you fasten a weight to the end of a length of string and whirl it around in a circle, the weight pulls outward, making the string taut. This is due to what used to be called centrifugal force. In fact, it is the tendency of a moving body to continue moving in a straight line, counteracted by another force, called centripetal force, in this case exerted by the string, that prevents it from doing so. The tendency of a moving body to continue in a straight line is called inertia, and there is no such thing as centrifugal force. On the Earth, gravitational attraction plays the part of the string, providing the centripetal force that prevents us all from flying away into space, and the oceans behave rather like the weight. They are pulled outward, as a bulge around the Earth. Gravitational attraction, mainly from the Moon, modifies this effect. The Moon pulls the oceans toward it, producing two bulges on directly opposite sides of the Earth. One bulge is directly below the Moon, where the lunar gravitational force is strongest, and the other is on the opposite side of the Earth, where lunar gravity reduces the Earth’s gravitational force and allows the oceans to rise by inertia. The Sun also contributes, but its influence is much smaller than that of the Moon. The gravitational force decreases in proportion to the square of the distance between bodies—this is the inverse square law discovered by Isaac Newton. Consequently, the huge difference in distance from Earth to the Moon and Earth to the Sun mean that the attraction between the Earth and Moon has about twice the force of that between the Earth and Sun. When the Earth, Moon, and Sun lie in a straight line, however, they add to the bulge due to the Earth’s rotation. This produces spring (high)

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tide due to Sun tide due to Moon third quarter

Spring and neap tides tides. When the Moon and Sun are at right angles to the plane of the Earth’s rotation, they produce neap (low) tides (see illustration). Spring tides occur when the Moon is new and full, neap tides when it is at its first and last quarter. Because the “bulge” is due to Earth-Moon attraction, it follows the Moon, circling the Earth every 12 hours and 25 minutes. The two crests are of equal size only when the Moon is directly above the equator. At all other times one bulge (one tide) is bigger than the other. The tidal bulges travel across the oceans as waves—true tidal waves and not to be confused with tsunamis. They are only about 2 feet (60 cm) high, but when they approach a coast or an enclosed area of the sea, such

Storm surges as a bay, they are reflected. If the crests and troughs of the reflected waves coincide with those of the waves generated by the tides, there will be a very large tidal movement. In the Bay of Fundy, Canada, this resonance generates spring tides of about 50 feet (15.25 m). Because of its size and shape, the Bay of Fundy has the largest tides in the world, but they are considerable along most coasts bordering oceans. At Boston, Massachusetts, for example, the average tide is 9 feet (2.7 m), ranging from 12 feet (3.7 m) at springs to 6 feet (1.8 m) at neaps. Add, say, a 2-foot (60-cm) sea-level rise due to low pressure to a 12-foot (3.7-m) spring tide, and the sea may flood low-lying coastal areas.

Storm waves Tropical cyclones also produce huge, wind-driven waves. These are the second factor contributing to storm surges. The waves move across the ocean, traveling outward from the center of the storm. To the right of the eye (in the Northern Hemisphere), where the wind is strongest because it is blowing in the same direction as that in which the storm itself is moving, the waves build into a “pile” of water markedly higher than the usual sea level. This “heaped-up” water is likely to cross the coast close to the “bulge” beneath the eye, or a little way ahead of it. The storm waves driven by the wind, together with the dense cloud of spray whipped from their breaking tops, ride on top of the raised water. As the storm crosses the coast, onshore winds on the right side of the eye hurl this mass of water directly at the land, with immense power. Low pressure that raises sea level and waves driven by the wind, both possibly adding to the sea-level rise due to high tides, clearly present a threat to coastal areas. Whether that threat translates into serious damage depends on the nature of the coast itself. This is the fourth factor contributing to storm surges.

Coastal shelving Continents do not end abruptly where they meet the sea. Coasts are, in effect, hillsides. As you approach the sea, you are moving downhill and the shoreline marks the elevation on the hillside that the sea reaches. The slope may be more or less steep, but on continental coasts it is always a slope, never a vertical drop. This is true even where there are high cliffs, with the sea crashing against their bases. The cliffs are made by the sea, as over many thousands of years it has washed rocks away, cutting into the hillside, but at the foot of the cliffs the sea is often quite shallow, and the submerged surface slopes seaward. The top drawing on page 128 illustrates how coastal erosion gradually transforms a gently shelving coast into one with high sea cliffs. Beyond the line reached by the tides, the continuing slope is called the continental shelf. It is part of the continent that just happens to lie below sea

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level, and it extends for a considerable distance as a very gentle slope. Its extent is very variable. Off the Canadian Atlantic coast, for example, it continues for up to 250 miles (400 km), but it extends for only a few miles off the North American Pacific coast. The edge of the continental shelf is the effective edge of the continent, where the gradient suddenly becomes much steeper down the continental slope. This ends at a depth of 1.2–3 miles (2–5 km) from where the ocean floor extends as the vast, flat, abyssal plain.

continental slope

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Storm surges The bottom illustration on page 128 shows this in a very condensed way, with the distances greatly shortened. It also shows that coasts can shelve steeply into the sea or very gently. The difference is important. Waves and storm surges simply crash against a steep coast (A), but can advance a long way up a gentle slope (B). All continents are shaped like this, but islands in the middle of the ocean are really the uppermost parts of high mountains—usually volcanoes—that rise steeply from the ocean floor.

When waves reach the coast Sea waves are disturbances that move through the water. The water itself moves vertically (strictly, it moves in small circles) but not forward. If you tie one end of a rope to a support and shake the other end, you can make waves travel along the rope, but the rope itself moves only up and down, not forward with the waves. The distance between one wave crest and the next is known as the length of the wave, or its wavelength. Its amplitude is the height of each crest and depth of each trough in relation to the level of the undisturbed water, and the height of the wave is the distance between a trough and a crest, equal to twice the amplitude. The steepness of a wave is its height divided by its wavelength. The time that elapses between two crests passing a fixed point is the wave period. Waves move in groups. As they cross the sea, waves move from behind, overtake those in front of them, and when they reach the front of the group they disappear. Groups move at half the speed of their individual waves and the energy of the waves moves forward at the speed of the group. As a group of waves nears a coast, it enters increasingly shallow water. When the depth of water is equal to half the wavelength, the vertical movement at the base of the wave is curtailed by contact with the seabed. This slows the forward speed of the wave, and as the water depth continues to decrease, the advancing wave slows more and more. The wave period (the time between the passage of individual wave crests) remains unchanged, however, because waves continue to arrive at the same rate from deeper water. If the forward speed of the waves decreases, but the same number pass a fixed point each minute, it follows that the distance between one wave and the next must have decreased. As the speed of the group decreases, so does the wavelength of its individual waves. The waves also continue to carry an undiminished amount of energy. As they slow, this is possible only if their height increases. As waves approach the shore, they slow down, but become higher and the distance decreases between each wave and the next. There is a limit to the height a wave can attain. Although the water appears to move only vertically, in fact small “particles” of water are moving in circles. As the waves grow higher they also become steeper and the “particles” move faster. A point is reached at which the “particles” at a wave crest are moving faster than the wave itself. When this happens the wave becomes unstable, its crest spills forward, and it becomes a breaker.

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Hurricanes Storm surges happen when these four factors combine. Low pressure raises the sea level, carrying water to above the ordinary high-tide mark. Fierce winds produce large waves on top of the raised water. As these waves approach the coast, the sea becomes increasingly shallow. That makes the waves grow higher and steeper. Storm waves will batter the shore, but the waves contribute to a major storm surge only if they arrive while the tide is high, and especially if it is a spring tide.

How big a surge? Every tropical cyclone is likely to cause a storm surge, but its size and seriousness in any particular place depends on the shape of the coast. If the shore shelves steeply into deep water, the waves are much closer inshore before they enter the shallow water that makes them grow bigger, so they never reach the size of waves approaching up a long, shallow slope. Many oceanic islands escape the worst effects of storm surges for this reason. They are the tops of submarine mountains or volcanoes that project above the sea surface, with sides that slope steeply. Along the east coast of the United States, on the other hand, the slope is shallow and large land areas are 10 feet (3 m) or less above the mean tide line. Storm surge waves can grow very large and travel quite a long way inland before they meet ground that is high enough to check them. Bays and other partly enclosed areas can increase the effect still further by reflecting waves that can add to the size of incoming waves. Storm surges also produce sea currents that flow with considerable force along coasts. These can cause severe coastal erosion. Sometimes they can wash away highways built close to the shore. Torrential rain and storm surges make water by far the most dangerous aspect of a hurricane.

Historic hurricanes Tropical cyclones are entirely natural events, and nowadays we know quite a lot about them, although scientists still have much more to learn. We have satellites to observe them from space, and when they strike land, television pictures of their effects are seen within a few hours in homes all over the world. People in distant lands give generously to help relieve the suffering they cause. We are keenly aware of the harm the weather can do. This is a recent development, made possible by technological advances achieved in the last 50 years. It allows us to know more about these storms and to observe those that dissipate over the oceans without ever reaching land. Consequently, we observe and record weather systems that never cross a coast or busy shipping lane. No one on the surface ever sees them, so years ago we would have known nothing about them. Obviously, the fact

Historic hurricanes that we are able to record more tropical cyclones than was possible in the past does not mean they are more frequent now than they were then. People have been at their mercy throughout history, but the winds and storm surges, and the human tragedies associated with them, often affected remote communities. Unrecorded, the damage they did was repaired and in time they were forgotten, even in the places where they occurred.

Jonathan Dickinson and Daniel Defoe Occasionally, though, a record has survived, sometimes because the storm marked the start of other events. In 1696, for example, a party of Quakers sailing from Jamaica to Philadelphia was caught in a hurricane and shipwrecked in the middle of the night on what is now Jupiter Island, to the north of Palm Beach, Florida. That event and the hardships the travelers endured during the remainder of their journey were described by one of them, Jonathan Dickinson, after whom a Florida state park is now named. We know of this hurricane only because it turned a fairly routine sea voyage into an epic overland journey. Other storms are remembered because of the scale of the damage they caused. In 1099, for example, a hurricane moving through the English Channel produced a storm surge that killed 100,000 people along the English and Dutch coasts. At that time the population was less than 10 percent of its present size, so the loss of 100,000 people then would be equivalent to the loss of 1 million today—approximately equivalent to the entire present-day population of Dallas, Texas. Quite apart from the innumerable personal tragedies such a disaster represents, it would have caused major economic disruption, producing a shortage of labor leading to wage rises over a large area. It is no wonder the catastrophe was remembered. Another storm entered history because of the awe-inspiring scale of its destruction. This happened on December 21, 1674, when the wind uprooted entire forests in Scotland. Only winds of hurricane force could cause such widespread devastation, but that is as much as we know. We know much more about the hurricane that crossed southern England in 1703, because Daniel Defoe (1660–1731), the author of Robinson Crusoe, wrote a description of it. The storm triggered tornadoes. Defoe saw one that “snapped the body of an oak.” It all began on November 24, after a fall of tremendous storms. That is when the hurricane winds began to blow, reaching their greatest intensity on November 26 and 27. According to Defoe, people were afraid to go outdoors or to go to bed. Perhaps they were wise to remain alert, but they were not entirely safe indoors. The Bishop of Bath and Wells died in his bed when a chimney fell on him. Along the south coast, Eddystone Lighthouse, off Plymouth, was washed away and 12 warships were sunk, as well as hundreds of other vessels. The Rivers Severn and Thames flooded, and as the storm moved eastward large areas were inundated along the coast of the Netherlands. In all, around 8,000 sailors lost their

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Hurricanes lives, untold numbers perished in the floods on shore, and 14,000 homes were destroyed. In Kent, in southeastern England, 110 houses and barns were destroyed, and in one place 16,000 sheep were drowned. In all, 100 churches, 800 houses, and 400 windmills were demolished in England. Some of the windmills burned down, because they turned so fast in the fierce winds that friction generated enough heat to set them on fire. The damage in London alone was estimated to have cost £2 million (at 18thcentury prices). This was thought to have been the most violent windstorm ever known in England, but another, on December 7 and 8, may have been stronger. There have also been hurricanes that affected the course of wars. In September 1854 the British army invaded the Crimea in the south of Ukraine (then part of the Russian Empire), at the start of the 1853–56 Crimean War, and when the brief campaign that the generals had planned failed, the troops were compelled to spend the winter there. The British fleet duly arrived with winter supplies, but the Russians had mined the harbor of Sevastopol, and the ships had to anchor outside, in the Black Sea. On November 14 the supply ships were destroyed by a hurricane. This event caused severe deprivation among the soldiers.

Kamikaze, the divine wind The Crimean hurricane seems of minor importance when compared with the most famous hurricane in the whole of military history. That hurricane did not affect the outcome of the war, but this one did. Indeed, it saved a nation and altered the course of history. Strictly speaking, it was not a hurricane but a typhoon from the China Sea, and for once its consequences were beneficial, at least for the Japanese. They called it a “divine wind,” or kamikaze. It happened in the year 1281. The Mongols, who at that time ruled China and Korea, had ordered the Japanese to submit to them. When the Japanese refused, a Mongol army set sail in Korean ships for the southernmost Japanese island of Ky ush ¯ u. ¯ As the map shows, the distance between the southern tip of Korea and Ky ush ¯ u¯ is not great. The would-be invaders overwhelmed Japanese defenders on the small islands of Tsushima and Iki and some Mongol forces managed to land at various places on Ky ush ¯ u, ¯ but in numbers that the Japanese warriors were able to contain. It was then that the typhoon destroyed most of the Mongol fleet, saving Japan from the main invasion force and probable defeat. People believed that the kamikaze had been sent by the gods to save the Japanese from conquest by foreigners. The event became the subject of epic stories and led to a religious revival. This must be the only occasion when a typhoon has inspired religious celebrations praising it. Almost certainly, the kamikaze is the only tropical cyclone that has ever brought any benefit to anyone at all, and even then the price was a heavy one, paid by untold numbers of Mongol soldiers and Korean sailors who died.

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Most such storms bring only death and destruction, sometimes on a vast scale. In 1876, the Bakarganj cyclone moving north from the Bay of Bengal and across the islands in the Meghna (Ganges) Delta during the monsoon, when the river was at its highest level, drowned 100,000 people in half an hour.

The Galveston tragedy Measured in terms of the loss of human life, the worst tropical cyclone in the history of the United States lasted from August 27 to September 15, 1900, but the devastation it caused took only a few hours. The storm formed in the Caribbean, crossed the Gulf of Mexico, and reached Galveston, Texas, on September 8. With winds of 77 MPH (124 km/h) gusting to 120 MPH (193 km/h) it does not seem the most ferocious of hurricanes, but like most hurricanes it brought a storm surge and it was the water that caused much of the destruction (see “Storm surges,” page 122).

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Hurricanes Galveston is a port, and these days a vacation resort, on Galveston Island, a barrier island separating Galveston Bay from the Gulf of Mexico. Nowhere is it more than 3 miles (5 km) wide and its average height above sea level is about 4.5 feet (1.4 m). The highest point on the island rises to no more than 8.7 feet (2.6 m) above sea level. In 1900 the city had a population of nearly 40,000. It was thriving and commercially important, its port handling more than two-thirds of the U.S. cotton crop and substantial amounts of grain. The U.S. Weather Bureau had warned of an approaching storm, but the citizens of Galveston took little notice. By dawn on Saturday, September 8 there was clear cause for anxiety. It was raining heavily, the wind was increasing, and the air pressure was falling rapidly. Some people left the island, and others sought shelter in buildings in the city center, but there were many who spent much of the morning gazing in wonder at the mighty surf crashing onto the beach. By late morning the wind was blowing at 50 MPH (80 km/h) and had begun to change direction. Until then it had been blowing from the north—from the land out to sea—but now it began blowing from the east. This allowed the storm surge to grow. As the eye of the storm drew closer, the sea level rose, and by noon the bridges linking the island to the mainland were submerged, cutting off the only escape routes. During the afternoon, huge, breaking waves destroyed buildings near the shore, and soon the entire city was flooded to a depth of about 4 feet (1.2 m). Houses, most of them made from wood, were torn from their foundations by the wind. Many simply disintegrated, sending planks and other debris flying through the air, some of it killing or injuring people who were trying to wade to safety. It was not until about 10 P.M. that the hurricane began moving away and the wind abated. First thing the following morning people began to list the casualties and assess the damage. More than 2,600 homes had been destroyed and around 10,000 people were homeless. At least 8,000 people had been killed and 5,000 injured. A few brick buildings remained standing, but the city of Galveston was largely reduced to piles of smashed wood and rubble. Galveston declined in economic importance after that, although the destruction wrought by the hurricane was only one reason. As a port, Galveston was unable to compete with others on the mainland, especially Houston. A sea wall was built after the 1900 hurricane, 17 feet (5.2 m) high and 10 miles (16 km) long. A wide boulevard runs parallel to the wall, contributing to the relaxed appearance of a resort, but the wall exists for protection and it is needed. It helped protect the city when the next hurricane struck. That was in August 1915, and, despite the sea wall, the 12-foot (3.7-m) storm surge flooded the city to a depth of 5 or 6 feet (1.5–1.8 m) and 275 people died. The most powerful hurricane ever to batter Galveston occurred in September 1961, and once again the sea wall held. On that occasion fewer than 50 people died, although there was extensive wind damage and flooding.

Historic hurricanes

Florida and the Labor Day storm Water is always the principal enemy. It was water that killed most of the 1,836 people who died in America’s second most serious hurricane of the last century. That one occurred in Florida in September 1928, when the winds drove the waters of Lake Okeechobee into populated areas. Levees were built to contain the lake after that disaster, and the next time a hurricane crossed directly over Okeechobee the water did not overflow and only two people died. That was in August 1949 and the hurricane was strong, with winds of 110 MPH gusting to 153 MPH (177–246 km/h). It was not as fierce as the Labor Day storm of 1935, in which 408 people died in southern Florida. Winds of 150–200 MPH (241–322 km/h) were estimated (no instrument could measure them) on some of the Florida Keys. The eye of that hurricane had an atmospheric pressure of 12.9 pounds per square inch (892.4 mb), the lowest ever recorded in the Western Hemisphere until Hurricane Gilbert in 1988—though neither was the lowest in the world. Many of the victims of the Labor Day storm were unemployed war veterans who had recently arrived in the area to help with the construction of U.S. Highway One. They were living in tents and shacks and, unlike the local residents, they had never experienced a hurricane and had no conception of what it could do. Around noon on September 2, with the storm approaching rapidly, they telegraphed for a train to come from Miami and evacuate them. There were delays and the train did not arrive until after 8 P.M. Just as the passengers had been hauled on board, a huge wave threw the train onto its side and pushed 10 of the cars for nearly 100 feet (33 m). That is when most of the veterans died, together with many residents and visitors who also had been hoping to escape.

Destruction by water Much of the land along the southeastern coast of the United States is lowlying and flat. The shallow slope of the seabed causes storm-surge waves to grow large, and occasionally they are able to inundate low ground for a considerable distance inland. During a hurricane in 1915, many people died because despite warnings they remained in their homes in low-lying areas of Louisiana. In June 1957 Hurricane Audrey produced a storm surge of more than 12 feet (3.7 m) along the Louisiana coast, a surge that caused flooding as much as 25 miles (40 km) inland. An even bigger storm surge, of 24.2 feet (7.4 m), crossed the Mississippi coast at Pass Christian when Hurricane Camille arrived on August 17, 1969. That time the sea did not inundate inland areas, but in Virginia Camille delivered 27 inches (686 mm) of rain in eight hours, driven by winds of 100 MPH (160 km/h) gusting to as much as 175 MPH (282 km/h). The rain caused flash floods in which 109 people died and 41 remained unaccounted for. The final toll of casualties from both states was

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Hurricanes 255 dead and 68 missing. It was flooding due to rain that caused 30 deaths when a hurricane crossed Georgia, the Carolinas, and Tennessee in August 1940, while the wind caused 20 more.

Storms that head north States bordering the southeastern and Gulf coasts of the United States are most at risk from hurricanes, but more northerly states are far from immune. In September 1938 a hurricane caused 600 deaths in Long Island, New York, and the southern part of New England. It traveled at 56 MPH (90 km/h) and winds of 121 MPH gusting to 183 MPH (195–294 km/h) were recorded in Massachusetts. It was 17 years before the United States suffered a storm fiercer than this. Other hurricanes caused damage in New England in 1944, 1954, 1955, 1960, 1972, 1976, 1979, 1996 (Hurricane Bertha), and 1999 (Hurricane Floyd). The table lists those northeastern states that have been struck by hurricanes and tropical storms since 1886 and the number of each. Indeed, in 1954 the region suffered three hurricanes. In August, Carol caused more damage to property than any storm recorded in the region up to that time, much of it due to a storm surge that flooded many low-lying areas. Just as people were recovering from that disaster, in September Edna arrived, with a 120-MPH (193-km/h) gust of wind recorded at Martha’s Vineyard, off the Massachusetts coast. Hazel, in October of that year, was the third to arrive. It was one of the strongest storms ever to reach North America and also one of the biggest, affecting an area of 9,000 square miles (23,310 km2). It destroyed three towns in Haiti on October 12, killing about 1,000 people, while at the same time drenching Puerto Rico, 500 miles (800 km) away, with 12 inches (305 mm) of rain. It crossed the Bahamas, its winds strengthening to more than 120 MPH (193 km/h), and reached the U.S. mainland near Myrtle Beach, South Carolina, on October 15. The storm surge, of 17 feet

NORTHEASTERN HURRICANES AND TROPICAL STORMS SINCE 1886 State New York Connecticut Maine New Jersey Massachusetts Rhode Island

Hurricanes

Tropical storms

7 5 3 2 2 1

6 5 3 2 1 0

Historic hurricanes (5.2 m) in some places, caused devastation along 170 miles (273 km) of coast. Then the hurricane turned north and, unusually for a storm once it was over land, it intensified. It produced a gust of 113 MPH (182 km/h) in New York City, where it arrived during the rush hour, and then continued north into Canada.

Ferocity of typhoons Atlantic hurricanes are often severe, but they cannot match Asian typhoons for sheer ferocity. The deepest cyclone ever recorded was Tip, a typhoon that developed in the northwestern Pacific in October 1979. On October 12, when the center of the storm was 520 miles (837 km) northwest of Guam, an aircraft monitoring its location and strength dropped an instrument package—called a dropsonde—into the eye. It measured the sea-level atmospheric pressure as 12.62 pounds per square inch (870 mb). That is the lowest sea-level pressure ever recorded on Earth. Tip produced sustained winds of 190 MPH (306 km/h) and galeforce winds over a radius of 675 miles (1,086 km). Fortunately, Tip did not cross land, but as it passed Japan on October 19, the edge of it caused widespread damage, killing 36 people including 12 U.S. marines at a training base near Mount Fuji. When Typhoon Vera crossed the Japanese island of Honshu¯ in September 1959, it destroyed 40,000 homes, left 1.5 million people homeless, and the death toll reached nearly 4,500. Japan, bordering the notoriously treacherous China Sea, is especially vulnerable. About one-third of the city of Nagoya was destroyed in 1953 by a typhoon that left 1 million people without homes, and the following year 1,600 people died when a typhoon struck Hokkaido. Fran, a typhoon that struck southern Japan in September 1976 with 100-MPH (160-km/h) winds and 60 inches (1,524 mm) of rain, left 325,000 people without homes, and in central and northern Japan Typhoon Tad left 20,000 homeless on August 23, 1981. All the countries bordering the China Sea are vulnerable, and in most years up to 20 typhoons form there. Three million people lost their homes on July 23, 1980, when Typhoon Joe crossed northern Vietnam, and flash floods and landslides caused by Typhoon Bess as it crossed South Korea were responsible for widespread damage in August 1982. Bess was a category 5 storm, with winds of 160 MPH (257 km/h). South Korea endured two typhoons that month. The second, called Cecil, was a category 4 storm with winds of 144 MPH (232 km/h). It caused damage costing more than $30 million. Not all typhoons originate in the China Sea, of course. Many begin over the Pacific then cross the China Sea as they move westward. In September 1984 Typhoon Ike killed more than 1,300 people and left more than 1 million homeless in the Philippines, and then moved on to cause widespread damage on the Chinese coast in the province of Guangxi Zhuang, bordering the Gulf of Tonkin. Storm surges along the Chinese

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Hurricanes coast are especially dangerous, because in many places, including the Gulf of Tonkin, spring tides rise 20 feet (6 m). The Philippines lie directly in the path of many typhoons, some of which generate very strong winds. Agnes, a storm that killed 300 people and left 100,000 homeless in November 1984, brought winds of 185 MPH (298 km/h). The extent of the damage caused by typhoons is not always due to flimsy construction. A typhoon is capable of demolishing substantial buildings. Nagoya is a major industrial city, and Tracy almost totally destroyed Darwin, Australia, on Christmas Day 1974. It was not a typhoon, but in November 1965 winds of 85 MPH (137 km/h) demolished a cooling tower, 375 feet (114 m) tall, at the Ferrybridge electricity generating station in England.

Cyclones in the Indian Ocean Cyclones from the Indian Ocean can also be extremely destructive. By far the deadliest storm of modern times was a cyclone. It struck Bangladesh in November 1970. The combination of winds and storm surge killed between 300,000 and 500,000 people. In June 1976 a cyclone destroyed almost all the buildings on the island of Masirah, Oman. On November 19 of the following year India suffered a much worse disaster when a cyclone struck Andhra Pradesh, in the east of the country bordering the Bay of Bengal. The cyclone produced a storm surge that completely washed away 21 villages and caused extensive damage in a further 44. That cyclone destroyed the homes of more than 2 million people and caused some 20,000 deaths. Another storm surge, of 10–15 feet (3–4.5 m), may have killed more than 10,000 people living on islands in the Meghna (Ganges) Delta of Bangladesh on May 25, 1985. About 45 villages were flooded, more than half a million buildings were destroyed, and 1,500 people were killed by a cyclone in Sri Lanka and southern India on November 23, 1978. In Madagascar, a cyclone with 150-MPH (240km/h) winds destroyed four-fifths of the town of Mahajanga on April 12, 1984, and in March 1994, a cyclone left 1.5 million people homeless in Nampula Province.

Storms and heroism at sea Tropical and extratropical cyclones form and intensify over the sea, and at sea they are even more terrifying and dangerous than they are over land. Once over dry land they usually weaken, but over the sea they may still be growing. Given adequate warning, ships make for the relative safety of harbors, but that is not always possible. There may be insufficient time and harbors may be full. Until recently, of course, the only warning mariners had came from their own observations and experience of what particular conditions of the sea and sky might herald. The kamikaze saved the Japan-

Historic hurricanes ese, but only by destroying the fleet exposed to it in the China Sea. The English hurricane of 1703 sank 12 ships in the English Channel. Task Force 38 suffered severe losses when it sailed through Typhoon Cobra during World War II (see “What happens inside a hurricane,” page 72). The following summer, of 1945, 33 ships were damaged and 76 airplanes destroyed in another U.S. fleet under the same commander, Admiral Halsey, when it sailed through Typhoon Viper. In 1979 only 75 yachts completed the Fastnet Race between England and Ireland, out of 302 that started, because of a sea storm that intensified unexpectedly. These were spectacular events, but others have entered history because they inspired acts of heroism. Grace Darling (1815–42) is one of the most famous of all heroines. She was the daughter of the keeper of the Longstone lighthouse, off the coast of northeastern England. When a storm drove the luxury liner Forfarshire onto rocks near the light on the morning of September 7, 1838, she saw there were survivors in a sea that was too dangerous for the lifeboat. Grace and her father rowed their own small boat a mile out to sea, rescued four men and a woman, and then Grace set out again with two of the men to help with the rowing and rescued the remaining four survivors. Deservedly, Grace became a national heroine. More recently, in December 1981, at the tiny village of Mousehole, Cornwall, in the far southwest of Britain, the entire crew of the Penlee lifeboat Solomon Browne was lost in a fierce storm while attempting to rescue the crew of a 1,400-ton coaster, the Union Star, that had been driven onto rocks. Each year since then the lights that decorate boats in Mousehole harbor and the buildings around it at Christmas are turned off for a time to commemorate the heroism of those lifeboatmen, and their loss. These were small incidents, though poignant. Others have involved tragedy on a larger scale. More than 500 casualties resulted when a number of ships were lost at sea during a hurricane in the Gulf of Mexico in September 1919. A cyclone in the Bay of Bengal capsized 200 or more Bangladeshi fishing boats on December 9, 1973. Most of the 1,000 people who died in that cyclone were drowned. More than 100 fishermen died when Typhoon Orchid struck South Korea in September 1980, and about the same number of fishermen died at sea during Hurricane Tico, in October 1983, off the coast of Mazatlán, Mexico. Today, early warning allows people to be evacuated, and skilled, wellequipped emergency services can reach disaster areas rapidly. Tropical cyclones cause fewer casualties than they did in the past, but they almost invariably cause some, and their capacity to destroy property has not diminished significantly. For the families affected, lost homes and possessions are tragedies, and the destruction of crops brings hardship to farmers. Severe storms destroy lives, even of those they fail to kill.

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LIVING WITH FIERCE STORMS How hurricanes are named and tracked If a hurricane is heading your way, you need to know how strong it is and when it will arrive. You do not need to know which hurricane it is, only that it is the one due to hit you tomorrow afternoon. Thinking back on the event in a few years’ time, however, you might want to compare one hurricane with another. Then you would need a way to identify each of them. Of course, you could simply label them by their arrival dates. That is the only way we have of describing storms of long ago, but it is not very satisfactory. The 1900 Galveston hurricane was just that, the 1900 Galveston hurricane, but did it affect any other part of the country? The name does not tell us. The Labor Day storm struck on Labor Day, but the name does not tell us where it struck (Florida) or the year (1935). Modern meteorologists need a better system, because they often find themselves monitoring several hurricanes at the same time, all of them moving. At first they listed them by the latitude and longitude where they were first reported. This was cumbersome and confusing. A name like 12.3:54.7—the latitude and longitude—is difficult to remember, and it would be easy to confuse this storm with another that was first seen two months later, but at almost the same location. Alternatively, they could have numbered them, perhaps with the year and the sequence for that year. Then you could have, for example, hurricanes 1:03, 2:03, and so on for the hurricanes occurring in 2003. That would work, but there was a problem. In the 1940s, when meteorologists began airborne studies of tropical cyclones, ships and aircraft communicated mainly in Morse code. This was satisfactory for letters of the alphabet, but it was not very good at dealing with numerals. With a dot (.) representing a short signal and a dash (-) representing a long one, . – – – – – – – – – … – –, the Morse code for 1:03, is slow and can cause confusion.

Why not use names as names? Morse was abandoned when ship and aircraft radios started using voice communications. American meteorologists then listed tropical cyclones alphabetically, using the international phonetic alphabet for radios: Able, Baker, Charlie, Dog, etc. That was in 1951, but in 1953 a new international alphabet was introduced (Alpha, Bravo, Cocoa, Delta, etc.). This caused

How hurricanes are named and tracked confusion, because one operator might report “Hurricane Dog,” another “Hurricane Delta,” and it would not be clear whether these were both the same hurricane or two separate ones. So that system also had to be abandoned, and in 1953 meteorologists began using women’s names instead. The idea of giving personal names to hurricanes was not new. In the West Indies people had long named hurricanes after the saint on whose day they struck, and the practice had been adopted in other Caribbean islands. The storm that swept across Puerto Rico on July 26, 1825, for example, was known locally as Hurricane Santa Ana. Personal names were also being used elsewhere. “Saxby’s Gale,” which occurred in Canada in 1869, was named after a naval officer who was thought to have predicted it, and some meteorologists had been giving tropical cyclones women’s names since the late 19th century. Women’s names remained in use until 1978, when some storm lists prepared for the eastern Pacific included men’s names. In 1979 both women’s and men’s names were used to compile lists for the Atlantic and Gulf of Mexico, and this still remains the practice, with male and female names alternating (Andrew, Bonnie, Charley, Danielle, for example). Also since 1979, the lists include names from non-English-speaking cultures. The names are a substitute for the international phonetic code, and so they are arranged alphabetically. In 2002, for example, the first Atlantic hurricane was called Arthur, the next Bertha, and so on from a list that continued down to Wilfred, although not all the names might be needed, of course. There are no names beginning with Q, U, X, Y, and Z because of the shortage of such names in English and Spanish, the languages used for Atlantic hurricane names, although these letters are used outside the Atlantic and Caribbean, where many names do begin with them. Since different names must be allotted to Atlantic hurricanes and Pacific typhoons and all the names must follow an alphabetical sequence so that each list must not contain two names beginning with the same letter, it will not take many years to use up all the names in the world. This difficulty is avoided quite simply. Lists are compiled in advance for six years, and in the seventh year the first list is used again. The 2001 Atlantic hurricane list was the same as the 1995 list, and in 2007 the 2001 list will be recycled. Lists for the eastern North Pacific are similarly recycled on a six-year rota. If this might cause confusion between two storms with the same name but in different years, adding the year solves the problem. Barry 2001 is not the same hurricane as Barry 1995. A slightly different method is used for typhoons in the central Pacific. There, four short lists are used, and although the names are arranged alphabetically, not all letters of the alphabet are used. Each typhoon is allotted the next name on the list. When all the names in List 1 have been used, the next typhoon takes the first name from List 2 and when the end of that is reached Lists 3 and 4 are used in the same way. The names continue from one year to the next, so the first typhoon of the year takes the name following that of the last typhoon of the previous year.

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Hurricanes Typhoons in the western North Pacific are named from five lists, used in the same way. The names themselves are proposed by the 28 nations of the region, with each nation being allocated five names, making 140 names in all. Storms around Australia, Fiji, and Papua New Guinea are named from lists that are also used in this way. Typhoons that develop in the oceans around the Philippines are allocated names by the Philippines weather service—the full name is the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA). Cyclones in the northern Indian Ocean are not given names, but those that form to the west of latitude 90° E have been named since 1960. All of the names for tropical cyclones are listed in the appendix at the end of this book.

Retired names Interest in most tropical cyclones usually diminishes rapidly once the season has ended, but there are exceptions. Andrew, in 1992, is still remembered as the most costly storm in the history of the United States. Camille, in 1969, was one of the most destructive of all storms, and Mitch, in 1998, was another devastating hurricane. Storms like these are not forgotten quickly. This means that their names are likely to remain in use for many years—in articles about meteorology and history, for example, and in negotiations over insurance claims. It would be confusing to use those names again. Consequently there is a procedure for removing names from the lists. All names that are used for tropical storms and cyclones must be approved by the World Meteorological Organization (WMO), an agency of the United Nations. The authorities in the country most severely affected by a particular storm can apply to the WMO to have that name removed. Once it has been removed, the name must not be used again for at least 10 years. A new name, chosen by member nations of the WMO in that part of the world, is substituted for the one removed. This must begin with the same letter and must be of the same gender and language as the name it replaces. Names can also be retired if they cause confusion, even though no formal application is made for their removal. “Carol” (1954 and 1965) and “Edna” (1968) have been retired in this way. The retired names for Atlantic and Caribbean hurricanes include Andrew (1992), Camille (1969), George (1998), Gilbert (1988), Hugo (1989), Mitch (1998), Opal (1995), and Roxanne (1995). A name is allocated as soon as the air around a disturbance starts rotating cyclonically (counterclockwise in the Northern Hemisphere) and its winds exceed 38 MPH (35 knots, or 61 km/h). At this stage it has become a tropical storm, and until it intensifies into a hurricane the name is prefixed with TS. Once it has grown into a hurricane the prefix is dropped and it is known simply by its name, but it keeps the same name.

How hurricanes are named and tracked

Reporting tropical cyclones Until the late 1940s, spotting a tropical cyclone over the ocean was very much a matter of chance. A passing ship might report it, but unless it was close to a shipping lane it was unlikely to be noticed. In those days there were few airlines flying intercontinental routes and the aircraft they used lacked the range to fly far over the open sea. The most developed North Atlantic route, for example, was between New York or Montreal and London with refueling stops in Labrador or Newfoundland, Iceland or Ireland, and sometimes at Prestwick, in Scotland. Aircraft were improving, however, and their numbers were increasing. More advanced instruments allowed pilots to fly safely through cloud and meteorologists made increasing use of aircraft, often asking pilots for reports on weather conditions, especially the height of cloud bases and tops. On military airbases, if it was uncertain whether conditions were suitable for taking off and landing, the day usually began with one pilot flying around the area of the field to check the weather. Pilots would not fly deliberately into a large cumulonimbus (storm) cloud, but even that was changing. Planes of the 1940s were stronger than those of the 1930s and had more powerful engines. Flying through a storm was not quite so dangerous as it had been. By 1945, U.S. Navy and Army aircrews were flying meteorologists through tropical cyclones fairly routinely, gathering instrument readings from which the scientists came to understand the structure of these weather systems. Aircraft still fly scientific missions into hurricanes and typhoons, and the National Oceanic and Atmospheric Administration (NOAA) routinely sends research aircraft into hurricanes and other severe weather systems. The United States is the only country that regularly sends aircraft into and through hurricanes in order to monitor conditions inside them. The NOAA uses two WP-3D aircraft of its own and several WC-130 aircraft belonging to the Air Force Reserve. The WP-3D is a modified version of the Lockheed P-3C used for anti-submarine patrols, and the WC130 is a version of the Lockheed C-130 transport. Both aircraft are powered by four turboprop engines. The WP-3Ds have a crew of eight and computer workstations for up to 10 scientists. They carry radar in the nose and on the bottom of the fuselage, and Doppler radar in the tail as well as instruments to measure temperature, air pressure, wind speed and direction, and humidity. Their radars measure the sizes and densities of water droplets and ice crystals. They send the data they collect, including radar images showing the structure of the storm, to the Hurricane Center in Miami. The aircraft are able to release dropsondes—instrument packages that descend by parachute, gathering measurements as they do so and transmitting them by radio to the Hurricane Center. The aircraft are also equipped to drop buoys into the ocean. Known as airborne expendable bathythermograph system (AXBT) buoys, these also transmit data by radio.

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Weather satellites These missions do not locate the storms, of course, but are directed toward storms that have already been identified. The early identification of atmospheric disturbances relies on satellites. The first weather satellite, TIROS (Television and Infrared Observation Satellite) was launched in April 1960 and within a few days had sent pictures of a typhoon no one had known existed, 800 miles (1,287 km) from Brisbane, Australia. TIROS satellites are now known as NOAA-class satellites and they are still used. Their instruments are sensitive to visible light and infrared radiation and scan a path 1,864 miles (3,000 km) wide up to a height of 1.2 miles (2 km). Today there are many weather satellites in orbit and new ones are launched at a rate of about two each year. The overlapping coverage of some forms a network, or “constellation,” providing constant monitoring of the entire Earth. Satellites can be placed in one of three types of orbit, called polar, Sunsynchronous, and geostationary. NOAA-class satellites are in polar orbits. A polar orbit carries the satellite over both poles and in a series of orbits over the entire world. At a height of about 530 miles (860 km), the satellite makes a complete orbit of the Earth every 102 minutes. While it is doing so, the Earth is rotating beneath it. In 102 minutes the Earth turns 25.5° to the east, so with each orbit the satellite flies over a region 25.5° to the west of its previous pass. Sun-synchronous orbits are similar, but the satellite remains in the same position relative to the Sun. The orbit is at an altitude of about 560 miles (900 km), equal to one-seventh of the radius of the Earth, and passes close to the poles, but at an angle to the lines of longitude. A satellite in this orbit takes about 100 minutes to complete one circuit of the Earth, and it passes over each point on the surface about 15 times a day, always at the same time of day. Satellites in geostationary orbit are directly above the equator, at a height of about 22,370 miles (36,000 km), and travel in the same direction as the Earth’s rotation. Their altitude is equal to 5.6 times the radius of the Earth, and their orbital speed is the same as that of the surface beneath them, so they remain permanently over a particular point on the equator. The diagram on page 145 illustrates this. GOES (Geostationary Operational Environmental Satellite) are U.S. weather satellites in geostationary orbit. Two of them, known as GOES-East and GOES-West, are operational at any time. At present, these are GOES-8 and GOES-10, orbiting at 75° W and at 135° W respectively. Between them, the two GOES monitor the whole of North and South America, the western Atlantic, and the eastern Pacific. They are augmented by a Meteosat satellite, owned by the European Space Agency, Himawari, owned by Japan, and Insat, owned by India. These five satellites, all in geostationary orbit, are able to observe almost all of the world (except for small areas around the North and South Poles that lie below their horizon). Despite their much greater altitude, satellites in geostationary orbit provide

How hurricanes are named and tracked

Ear radiu th s (R)

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Geostationary orbit images with nearly as much resolution as the images received from satellites in polar orbit. Information from orbiting satellites passes to the organization that owns them, and all meteorological services are coordinated by the WMO. U.S. weather satellites are operated by the National Oceanic and Atmospheric Administration (NOAA), and observations of tropical disturbances are sent to the NOAA National Hurricane Center.

Interpreting the information Satellite photographs are studied closely. The meteorologists watch for the development of cumulus clouds with a wide layer of cirrostratus (thin, high-level, featureless sheets of cloud made from ice crystals). This combination indicates a strong convective system. Cloud movements are monitored to reveal the direction and strength of winds. The scientists do not rely only on satellite images. Ships and aircraft also radio reports to them, with information on atmospheric pressure and ways it may be changing, winds, and rain. If rain showers merge into steady rain, atmospheric pressure is falling, and winds are strengthening, the weather conditions will be classified as a tropical depression. As data continue to arrive at the Hurricane Center, any further intensification of

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Hurricanes the depression will be noticed almost as soon as it happens and the track of the system will be plotted carefully. If it becomes a tropical storm, with a cyclonic circulation and winds of more than 38 MPH (61 km/h), the next name in the list will be assigned to it. When a tropical depression that has grown into a storm comes within a few hundred miles of the U.S. coast, aircraft join in the task of monitoring. The first to arrive are the “Hurricane Trackers”—the WC-130s of the U.S. Air Force Reserve. Their job is to fly through the system, measuring the distribution of pressure within it, wind speeds and directions, and locating the eye. Radioed back to Miami, this information allows interior details of the storm to be added to the charts. NOAA aircraft also join the team. Equipped with sophisticated instruments, these WP3-D “flying laboratories” communicate with the NOAA Aircraft Operations Center, in Miami. Back at the Hurricane Center, the information from satellites, ships, and aircraft is fed into computer programs that predict the future behavior of the system. They estimate the likelihood that it will grow into a hurricane, and of what size and strength, and the track it will follow. If it is heading for an inhabited island or the U.S. coast, the relevant authorities are alerted.

Radar Still closer to land, the tropical cyclone comes within range of onshore radar. There is a radar network covering the entire east coast of the United States, from Texas to Maine, and it extends seaward as far as the Lesser Antilles, the most easterly group of Caribbean islands, extending southward in an arc with its northern end to the east of Puerto Rico. Radar is electromagnetic radiation—the same type of radiation as visible light and radio waves—that is emitted from a transmitter and reflected from certain surfaces. The reflected radiation is detected by a receiver and provides two kinds of information. The first is an image, displayed on a screen, of the shape of the object scanned by the radar. The second is the distance to the scanned object. This is calculated by measuring the time that elapses between the emission of the signal and the arrival of its reflection. Like all electromagnetic radiation, radar travels at the speed of light, so the time it takes for the round trip reveals how far it has traveled. Different radar wavelengths are used to scan different objects, and a wavelength of 3.94 inches (10 cm) is strongly reflected by water droplets. Once the storm is within range, radar can reveal its clouds and rain in great detail. Nowadays it can do more, because the shore-based radars are being upgraded to Doppler systems. These measure the frequency of the reflected waves very precisely. Radar waves all travel at the same speed, but in 1842 the Austrian physicist Christian Johann Doppler (1803–53) made an interesting discovery, originally about sound waves but extended later to electromagnetic

How hurricane damage is predicted waves. If an object moving toward or away from an observer emits waves traveling at a constant speed, from the point of view of the observer their frequency will change. This happens because the distance the waves travel is changing. If the source is approaching, the frequency will increase, because each pulse has a shorter distance to travel than its predecessor. If the source is receding, each pulse will have farther to travel and the frequency will decrease. With sound waves, increasing the frequency raises the pitch and decreasing the frequency lowers it. This is why the sound of a speeding train rises in pitch as it approaches, then falls in pitch after it has passed. With light waves, increasing the frequency makes the light more blue, and decreasing it makes the light more red. Astronomers have made use of this discovery for many years to tell how rapidly remote galaxies are receding from us (are red-shifted). Now meteorologists also use it, with the help of systems that display radar signals as patches of color on their computer screens. With Doppler radar they can add details of movement to the radar images they already have of the size of clouds and type and intensity of rain. They can tell how fast the storm is rotating, because one side will be retreating and the other approaching. Color the retreating side red and the approaching side blue, based on the frequencies of the radar reflections from water droplets, and the rotation becomes clearly visible. What is more, the stronger the colors the faster is the movement. Wind speeds inside the storm can be calculated from the rate the storm is rotating. The radar also reveals the direction the storm as a whole is moving and its speed. Monitoring is now very advanced, but not all of the Tropics are covered so well as the seas off the eastern United States. Satellites observe the whole world and ships and aircraft much of it, but planes equipped as meteorological laboratories, powerful computers, and radar networks are expensive. Until these are available in all the countries that lie along tropical cyclone tracks, some communities will be less prepared than others for severe storms.

How hurricane damage is predicted Wind, rain, and the raging sea can wreak havoc, but their effects vary widely. A hurricane with 135-MPH (217-km/h) winds that crossed Texas in 1949, for example, caused two deaths, but it remained in rural areas, destroying crops but not cities. Hurricane Beulah, in 1967, produced winds gusting to more than 100 MPH (160 km/h) in Texas, but of the 15 deaths 10 were due to floods and five to the 155 tornadoes the hurricane triggered. Hugo, on the other hand, in 1989, was the most destructive hurricane in U.S. history. With winds gusting to 220 MPH (354 km/h) over the

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Hurricanes Caribbean islands and more than 80 MPH (129 km/h) over the United States, accompanied by storm surges and tornadoes, Hugo killed a total of 43 people and caused damage costing $10.5 billion in the United States, as well as destroying almost all the homes on several of the islands it crossed. Mitch, in October 1998, generated winds of 180 MPH (290 km/h) that were sustained for 15 hours and in the six to eight days it remained over land it dumped up to 75 inches (1,905 mm) of rain. As it swept across Central America, touching the United States, Mitch killed between 10,000 and 12,000 people, making it the most deadly storm for more than 200 years (see “What happened when Mitch struck?” page 1). It was the storm surge (see “Storm surges” page 122) that made the 1900 Galveston hurricane so lethal (see page 133). The costliest storm in U.S. history was Andrew, 1992. It completely destroyed several South Florida cities and caused damage in the Bahamas, Louisiana, and Florida that cost more than $30 billion.

Advance warning These are among the most devastating hurricanes ever recorded. Others produce far less destruction, despite generating winds, rain, and waves that may be just as fierce. Clearly, when local communities find themselves in the path of an approaching tropical cyclone they need to know the scale of the destruction they should expect. The emergency services, too, can function much more efficiently if they are given advance warning of the type and extent of the incident with which they will have to deal. This need was recognized long ago, and the first U.S. hurricane warning was issued in 1873, when a tropical cyclone was seen approaching the coast between New Jersey and Connecticut. Instruments mounted on orbiting satellites and on aircraft mean that potentially dangerous storms can now be tracked for much longer and in more detail as they form, intensify, and move over the ocean (see the section “How a hurricane begins” page 57). At the same time, their characteristics can be studied and their effects predicted. Much of the necessary information is made available to the public and emergency services, and adequate preparation can be made in time at least to minimize the number of casualties. The results have been dramatic. In 1925, hurricanes caused approximately 16 deaths for every million dollars of property damage. That number has been reduced drastically. For Camille, in 1969, there was one U.S. death for every $284 million of damage, and for both Gilbert in 1988 and Hugo in 1989, one death for every $2 billion of damage. Hurricane Andrew, 1992, caused one death for every $1.3 billion dollars of damage. Although these are hurricanes that crossed the coast of the United States, the same pattern is being repeated throughout the world. A cyclone that struck Bangladesh in 1991 killed more than 130,000 people. A similar cyclone struck in 1994, but that one caused only 200 deaths. According to the Bangladesh government, the reduction was due to improvements in

How hurricane damage is predicted warning systems and the timely evacuation of people in the path of the storm. Prediction allows time for preparation and preparation saves lives. Although the ratio of deaths to property damage has improved and the actual number of deaths has decreased, the cost of hurricane damage to property in the United States increased sharply during the last century. An average of more than 800 people were killed by hurricanes each year between 1900 and 1910. By the 1990s the average was about five. The cost of property damage, however, increased from almost nothing in the first years of the century to around $500 million a year in the 1930s and $2.6 billion in the 1990s. In the world as a whole, damage from tropical cyclones cost around $3–$4 billion a year in the 1960s, but $25–$30 billion a year in the early 1990s. Figures for the cost of damage to property are based on insurance claims, and the size of the claims that followed Andrew alerted insurers to the fact that they had seriously underestimated the amount of damage a single storm could do. Andrew cost $15.5 billion in claims for property damage—and a total of more than $30 billion when damage to uninsured property is included. Between 1986 and 1992, hurricanes and tropical storms were responsible for 53 percent of such claims. They caused more damage than the 35 percent due to tornadoes and windstorms. Fires, explosions, earthquakes, riots, and other disasters accounted for the remaining 12 percent of claims. Improved precautionary measures account for the reduction in the number of deaths. The increasing risk to property is due to the rising popularity of Florida and the Gulf coast as places to live or vacation. The population of Florida increased by 37 percent between 1980 and 1993, that of North Carolina by 25 percent, and that of Texas by 10 percent—although the population of Louisiana fell by about 4 percent during the same period. The National Oceanic and Atmospheric Administration (NOAA) predicts that by 2010 there will be more than 73 million people living in hurricane-prone areas. Most of them will insure their property, and consequently the cost of hurricane damage is almost certain to increase, even though the intensity of hurricanes may decrease.

Measuring the approaching storm Measurements of the essential features of a tropical storm begin from the time it forms. At first these amount to an alert among meteorologists at the National Hurricane Center, in Miami, which is part of NOAA. When it appears that the storm is intensifying and heading for land, closer observation begins and the resources devoted to its study increase greatly as it continues to approach. The atmospheric pressure in the center is monitored, because from this the meteorologists can calculate the wind speed. Cloud formations are observed, because this allows the intensity of the rainfall to be estimated. The temperature is also watched closely, because it is its warm rather than

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Hurricanes cool center that distinguishes a hurricane from a tropical storm. It is also important to measure the size of the entire storm system. This determines the width of the area it will affect as it moves inland and also allows scientists to calculate wind speeds at varying distances from the center. As these measurements and calculations are being made, the track of the storm is being plotted. This plot, together with knowledge of the prevailing winds outside the disturbance and experience of the behavior of past hurricanes, provides a basis for the future track to be predicted. All of the data is fed into computer models held on some of the most powerful supercomputers in the world, located at Princeton, New Jersey, Washington, D.C., and at Bracknell, in Great Britain. Output from the models is fed back to the forecasters at Miami. Hurricanes can change course for no apparent reason, so there is an inevitable element of uncertainty in the predicted track. To allow for this, warnings are issued for a belt that is considerably wider than the diameter of the hurricane itself. At this stage the meteorologists know the size and strength of the hurricane and have predicted where it will cross the coast and, from measurements of its speed, when it will arrive. Now other considerations become important. The pressure in the eye tells them by how much the sea will rise, and its time of arrival must be related to the state of the tide to anticipate the total rise in sea level. The size of sea waves can be calculated from knowledge of the wind speeds and must be related to the shape of the coast and slope of the seabed. Together, the rise in sea level due to pressure, the tide, and the growth in the size of waves as they approach the shore make it possible to predict the size of any storm surges. The elevation of the ground behind the shore determines the distance the storm surges will penetrate inland, and when account is taken of the effect of the rainfall on natural drainage systems it becomes possible to predict the severity of any flooding.

Grading the hurricane Hurricanes are graded according to their eye pressure, wind speeds, and height of their storm surges, usually on the Saffir/Simpson scale of 1 to 5 (see sidebar, page 114). At this point in the investigation it is possible to assign the hurricane to a category, and it is its category that determines the precautionary measures that should be taken before it arrives. Its category on the Saffir/Simpson scale indicates the kind and extent of the damage the storm will cause, but only in general terms. Obviously, a hurricane that moves across a city center will cause far more damage than one passing only across sparsely populated countryside. No monetary values are attached to damage predictions, of course. Those come after the storm has passed and are based on valuations made by insurance companies. Warnings in advance of the storm are of such things as “mobile homes destroyed,” “flooding to six miles inland,” or “extensive

How hurricane damage is predicted damage to roofs.” They are meant to suggest degrees of severity, but without being specific.

Issuing warnings Typically, the first “hurricane watch” warning, issued one or two days before the storm is forecast to arrive, affects a belt of coast and its hinterland that is about six times wider than the diameter of the hurricane. Much of this belt, to either side of the hurricane center, will miss the full force of the storm, although it may still experience strong gales. Because predictions of hurricane behavior are imprecise, it is quite likely that one side or other of the belt will escape entirely. This “overwarning” is not like “crying wolf” when there is no wolf. There is a hurricane all right, even if happily it turns out to be somewhere else. The actual diameter of the tropical cyclone determines the width of the belt that will be most seriously affected. With an allowance for unpredicted deviations to either side of this, a hurricane warning is issued for this belt. As the hurricane nears the coast, this warning is regularly updated, with details of wind, rain, and storm surges as well as the speed and direction of its movement. The warnings must also distinguish the effects to either side of the center. Places near the eye wall and to the right of the track will experience the strongest winds (see “What is a hurricane?” page vii) and the heaviest rain will fall close to the eye wall.

Preparing for the storm If people act on the warnings, preparations for the storm will be completed by the time it arrives. Offshore installations, such as oil rigs, will have been evacuated. Fishing boats will be in harbors, tied up as securely as possible and their decks cleared of anything that is not fastened down. Larger ships will have moved into the most sheltered positions they can find. Factories in the path of the hurricane will have been closed and their furnaces extinguished. Offices will have been closed. All employees will have been advised to remain at home. Windows will have been boarded, on homes as well as on shops and other business premises. People living on islands and along the coast will have been evacuated. Their numbers may have run into tens of thousands. If one or more cities are affected, the cost of these preparations runs into millions of dollars in materials, evacuation transport and accommodation, and lost production, and if uncertainties in the strength or track of the hurricane mean the affected belt has to be widened, the costs increase still more. Preparing for a hurricane costs an average $0.5–$1.0 million per mile of shoreline ($0.3–$0.6 million per kilometer), and warnings usually extend for 300–400 miles (483–644 km), so the total cost averages $150–$400 million. Because of the high cost, forecasters try not to exaggerate the risk when issuing warnings.

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Hurricanes Unfortunately, warnings are not always heeded. People are understandably reluctant to make time-consuming and expensive preparations and prefer to trust that they will be lucky or that the forecasters are mistaken and the hurricane will pass them by. By the time they start to appreciate the danger they are in, they may have left it too late. These are often people who have no previous experience of conditions in the eye wall of a major hurricane (one of category 3 or higher on the Saffir/Simpson scale). It is easy to dismiss the warnings when the sky is blue, the air calm, and the hurricane, if it exists at all, is far away over the horizon and out at sea. Mostly, these folk are recent arrivals, enjoying the apparently reliable sunshine, warmth, and beaches around their new homes. They will have seen TV news stories about hurricanes, but these cannot describe what it is like to be there, what the hurricane really looks, feels, and sounds like, and the ease with which it can pluck homes from the ground and smash them to firewood. Such people are unwise. It is not only their own lives they risk, but also those of the emergency personnel who may have to rescue them. When a hurricane warning is issued, it means a hurricane is approaching. Warnings are not issued lightly and must always be taken seriously and acted on appropriately and, above all, promptly.

Will global climate change bring more hurricanes? Throughout the late summer and fall of 1995 the hurricane trackers were kept very busy. It was the worst hurricane year for several decades. There was speculation that the increase in the number of hurricanes was linked to global warming, a worldwide increase in temperatures caused by increasing atmospheric concentrations of so-called greenhouse gases, primarily carbon dioxide. Such speculation was premature. There have always been more tropical cyclones in some years than in others, although 1995 was certainly a very bad year, with a total of 19 Atlantic hurricanes. That is the greatest total number since 1933, when there were 21. There were 18 in 1969 and 14 in 1990, but the number in most years has been much lower. Hurricanes vary in severity. Five of the 1995 hurricanes were rated “intense”— category 3–5 on the Saffir/Simpson scale. This is the greatest number of intense hurricanes since 1969, the other record year. There were more hurricanes than average in the five-year periods 1891–5, 1931–5, 1946–50, 1951–5, 1961–5, and 1966–70, but since then there have been fewer. The average is now 5.9 hurricanes and 2.3 intense hurricanes each year. There were nine hurricanes in 2001 and four of them were intense, so 2001 was an active year. Scientists predicted there

Will global climate change bring more hurricanes? would be six hurricanes and two intense hurricanes in 2002, making it an average year. Unfortunately, it was during the period of low hurricane frequency in the 1970s and 1980s that so many people moved into coastal areas of the southeastern United States (see “How hurricane damage is predicted” page 147). That period may be coming to an end.

Hurricanes linked to other weather phenomena There have been some powerful hurricanes since 1995, and forecasters think it likely that the period of low frequency has ended, at least for a time, and that we may expect more hurricanes during the first decades of the 21st century. Hurricane frequency seems to follow four weather cycles. The number of storms that cross over land each year depends on the position of each of these cycles. There are more hurricanes, and more that make landfall, when all four cycles are at their peaks simultaneously. This is what happened in 1995. The cycles combine in such a way that the frequency and intensity of hurricanes rise and fall over a cycle of 20 years. What causes the cycles? No one knows, but they are probably related to periodic changes in the circulation of surface water in the North Atlantic and Pacific Oceans. The best known of these changes is called El Niño (the Christ child, so named because it usually arrives around Christmastime). In fact, El Niño is part of a longer cycle that includes its opposite, La Niña. Both are linked to a change in the distribution of pressure over the Tropics called the southern oscillation, so that the complete cycle is called the El Niño–Southern Oscillation, or ENSO (see sidebar, page 154). Scientists have compared the number of Atlantic hurricanes in El Niño and La Niña years over the last 112 years. They found that over this period an average of 3.23 hurricanes struck the United States coast each year. During El Niño years, however, the average fell to 2.47. Another study, of the period from 1925 to 1997, found that the amount of hurricane damage during El Niño years was approximately half that during La Niña years. It also found that the average wind speed in hurricanes is about 13 MPH (22 km/h) lower during El Niño years. The other cycles are the quasi-biennial oscillation (QBO), the North Atlantic oscillation (NAO), and the periodic increase and decrease in the amount of rainfall falling in the Sahel region, along the southern border of the Sahara Desert. The QBO is an alternation in the easterly and westerly winds in the stratosphere over the Tropics. This changes over a period of 26–30 months. The NAO, also called the Arctic Oscillation, takes place over decades. It is a change in the difference in atmospheric pressure between a permanent region of low pressure centered over Iceland and a region of high pressure centered over the Azores. The prediction that 2002 would be an average hurricane year was based on the observation that the sea-surface temperature in the North Atlantic was cooler than average in the first half of the year and on the weak El Niño that was developing in the eastern Pacific.

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El Niño At intervals of between two and seven years, the weather changes across much of the Tropics, and especially in southern Asia and western South America. The weather is drier than usual in Indonesia, Papua New Guinea, eastern Australia, northeastern South America, the Horn of Africa, East Africa and Madagascar, and in the northern part of the Indian subcontinent. It is wetter than usual over the central and eastern tropical Pacific, parts of California and the southeastern United States, eastern Argentina, central Africa, southern India, and Sri Lanka. The phenomenon has been occurring for at least 5,000 years. The change is greatest at around Christmas— midsummer in the Southern Hemisphere, of course. That is how it earned its name of El Niño (the Christ child) in Peru, where its effects are most dramatic. Ordinarily, the western coastal regions of South America have one of the driest climates in the world, but El Niño brings heavy rain. Farm crops flourish, but many communities rely on fishing, and the fish disappear. Most of the time, the prevailing low-level winds on either side of the equator are the trade winds, blowing from the northeast in the Northern Hemisphere and from the southeast in the South-

ern Hemisphere. At high level, the winds flow in the opposite direction, from west to east. This is known as the Walker circulation, in memory of Sir Gilbert Walker (1868–1958), who discovered it in 1923. Walker also discovered that air pressure is usually low over the western side of the Pacific, near Indonesia, and high on the eastern side, near South America. This pressure distribution helps drive the trade winds, and the trade winds drive the Equatorial Current, which flows from east to west, carrying warm surface water toward Indonesia. The warm water accumulates around Indonesia, in a warm pool. In some years, however, the pressure distribution changes. Pressure rises over the western Pacific and weakens in the east. The trade winds then slacken. They may cease to blow altogether or even reverse direction, so they blow from west to east instead of east to west. This causes the Equatorial Current to weaken or reverse direction. Water then begins to flow out of the warm pool, moving eastward, and the depth of warm water increases off the South American coast. This suppresses upwelling cold water in the Peru Current and deprives fish and other marine life of the nutrients in the cold water. Air moving toward South America is warmed and

That was the forecast for the short term. The long-term forecast was based on the observation that rainfall in the Sahel has been increasing in recent years. This suggests that hurricane frequency is likely to increase, returning to levels seen in the 1940s, when there were an average of 8.3 hurricanes a year, and the 1950s, when the average was 10.5. Scientists do not believe this increase is linked to global warming. It is just part of a natural cycle.

Global warming It does not mean global warming will not happen. Most scientists believe that releasing certain gases into the atmosphere may affect the global climate, and some think they have detected the first signs of warming

Will global climate change bring more hurricanes?

carries a great deal of moisture. This brings heavy rain to the coastal region. This is an El Niño. In other years the low pressure deepens in the west and the high pressure in the east rises. This accelerates the trade winds and Equatorial Current, increasing the rainfall over southern Asia and the

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dry conditions along the South American coast. This is called La Niña. The periodical change in pressure distribution is known as the Southern Oscillation and the complete cycle is an El Niño–Southern Oscillation (ENSO) event. The diagram illustrates how this happens.

H high pressure L low pressure normal

H

120° W

180°

120° E

60° E

El Niño

H

120° W

L

180°

120° E

60° E

El Niño. A reversal of pressure distribution allows warm water to flow eastward.

(though others disagree). Over the last 100 to 130 years the average temperature is believed to have increased by 0.54–1.08°F (0.3–0.6°C), and the 1980s and 1990s have been the warmest periods on record. Most of the increase happened in the early years of the 20th century, and there was a slight fall in temperature between 1940 and 1980. The global average temperature rose by 0.355°F (0.197°C) between 1979 and 2002, although much of that increase was due to an exceptionally strong El Niño event in 1998. If the world does become a warmer place, some people suggest that there may be more tropical cyclones and their effects may be more severe. So far, the changes are small. Average temperatures vary naturally from one year to another, and even during the warm years of the 1980s and 1990s they remained within the limits of natural variability. The average

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Hurricanes temperature in 1995 was only 1.44°F (0.8°C) warmer than the average for 1861–90 and 0.7°F (0.4°C) warmer than the 1961–90 average. Such a small change is difficult to detect and even more difficult to interpret. It is the kind of warming scientists expect if the global climate is changing as predicted, but the warming will need to go on for another 10 or 20 years before anyone can be certain it is due to the greenhouse effect. Complicating the picture still further, the warming, small as it is, has not been spread evenly. It has been especially strong in winter in northwestern North America and northeastern Siberia, but the center of Antarctica has been growing colder for several decades while the Antarctic Peninsula has become markedly warmer.

The radiation budget Most of the radiation we receive from the Sun is at short wavelengths, concentrated in the waveband we see as visible light (see sidebar below). The atmosphere is almost completely transparent to this radiation, but some is reflected back into space from the tops of clouds and light-colored ground surfaces, such as snow and sandy desert. Most of the radiation is absorbed, however, and warms the land and sea. When a body—such as the Earth—is at a higher temperature than its surroundings—such as space—the warm body radiates heat at a wavelength that is inversely proportional to its temperature. In other words, the warmer the body, the shorter the wavelength of the radiation it emits. That is why the (hot) Sun radiates most intensely at short wavelengths and the (cool) Earth at long wavelengths. The Earth’s surface

The solar spectrum Light, radiant heat, gamma rays, X rays, and radio waves are all forms of electromagnetic radiation. This radiation travels at the speed of light as waves. The various forms differ in their wavelengths, which is the distance between one wave crest and the next. The shorter the wavelength, the more energy the radiation has. A range of wavelengths is called a spectrum. The Sun emits electromagnetic radiation at all wavelengths, so its spectrum is wide. Gamma rays are the most energetic form of radiation, with wavelengths of 10–10–10–14 µm (a micron, µm, is one-millionth of a meter, or about

0.00004 inch; 10–10 is 0.00000000001). Next come X rays, with wavelengths of 10–5–10–3 µm. The Sun emits gamma and X radiation, but all of it is absorbed high in the Earth’s atmosphere and none reaches the surface. Ultraviolet (UV) radiation is at wavelengths of 0.004–4 µm; the shorter wavelengths, below 0.2 µm, are absorbed in the atmosphere but longer wavelengths reach the surface. Visible light has wavelengths of 0.4–0.7 µm, infrared radiation 0.8 µm–1 mm, and microwaves 1 mm–30 cm. Then come radio waves with wavelengths up to 100 km (62.5 miles).

Will global climate change bring more hurricanes? radiates some of its heat upward and air in contact with the surface is warmed and rises by convection. As it cools, the rising air also radiates heat back into space. Heat radiation, from the Earth and its atmosphere, is at long (infrared) wavelengths. This exchange of radiation occurs during the day, when the Sun warms the surface and the surface radiates its heat away. The surface loses heat a little more slowly than it acquires it, so it warms during the day, reaching a peak in the middle of the afternoon. At night the Sun no longer shines on the surface and so it absorbs no heat. It continues to radiate its own heat, however, so it grows steadily cooler during the course of the night, reaching a minimum shortly before dawn. Then the Sun rises again and the warming starts over.

The greenhouse effect In a greenhouse, the glass allows solar radiation to enter. The radiation heats the air inside the greenhouse and the glass prevents the warm air from escaping. Consequently, the interior of the greenhouse grows steadily warmer. This is how the “greenhouse effect” acquired its nickname. The nickname is not really accurate, because although the result is similar, the reasons for it are quite different. A greenhouse does not trap radiation, but certain atmospheric gases do. The atmosphere consists mainly of nitrogen (about 78 percent) and oxygen (about 21 percent). These gases are transparent to radiation at all wavelengths, but the air also contains very small amounts of other gases that are not. Their molecules are larger than those of nitrogen and oxygen and they absorb radiation at particular infrared wavelengths depending on their size. Water vapor is by far the most important of these gases. Others include carbon dioxide, methane, CFCs (chlorofluorocarbon compounds, the manufacture and use of which is now being phased out because of their effect on the ozone layer), ozone, nitrous oxide, and carbon tetrachloride (a solvent formerly used in dry-cleaning, that is also being phased out). These are the greenhouse gases. The contribution each gas makes to the total absorption of infrared radiation is calculated as its global warming potential (GWP), and carbon dioxide is given a GWP value of 1. On this scale, methane has a GWP of 11, nitrous oxide 270, and CFCs and related compounds from 1,200 to 7,100. Molecules of these gases absorb long-wave radiation, each at certain wavelengths. It warms them and they start radiating their own heat. They radiate in all directions. Some radiation goes upward, into space, but most does not. It goes sideways, eventually to be absorbed by other greenhouse-gas molecules and radiated in all directions again, or downward. The overall effect is to warm the lower part of the atmosphere. The gases are rather like a blanket and, like a real blanket, they allow some heat to escape through windows. These are wavelengths at which no

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Hurricanes molecules absorb infrared radiation. A blanket on your bed keeps you warmer than you would be without it, but it does not keep your temperature rising indefinitely, until your body cooks. In the same way, greenhouse gases keep the air warmer than it would be without them, but they do not make temperatures continue to rise until the oceans boil and the rocks melt.

The enhanced greenhouse effect People think of the greenhouse effect as a threat, but without it life on Earth would be very difficult and probably impossible. If the air contained no naturally occurring greenhouse gases, the average temperature at the surface would be about –4°F (–20°C). Plants would not grow at this temperature, and most of the oceans would be covered by ice. This is the natural greenhouse effect. The threat arises because we release greenhouse gases into the air, so the air contains more of them than it did and the concentration is increasing. If this continues, more long-wave radiation will be trapped in the atmosphere, causing the average temperature to rise. Climate scientists refer to this as the enhanced greenhouse effect, to distinguish it from the natural greenhouse effect. It is this that may lead to global warming, but the situation is far from simple. Carbon dioxide is the most important of the greenhouse gases we emit, not because it is more absorptive than the others, but because we release it in much larger amounts. It is produced whenever we burn anything containing carbon, because combustion involves oxidizing the carbon to carbon dioxide (C + O2 → CO2), a chemical reaction that releases energy in the form of heat. All plant material, as well as peat, coal, natural gas, and oil, contain carbon. Of the carbon dioxide released by burning, however, only about half accumulates in the atmosphere. Scientists are uncertain about what happens to the remainder. Some dissolves in the oceans and some is taken up by plants in the process of photosynthesis, but a large amount—about 2 billion tons a year—cannot be accounted for. Much still remains to be learned about the effect of warming on the oceans. Ocean currents transport heat from low to high latitudes, so they have a very important effect on climates, but there is considerable uncertainty about the detailed consequences of a general rise in sea-surface temperatures. Nor can scientists predict just how and where clouds will form. Some clouds reflect incoming solar radiation, others absorb outgoing infrared radiation, so it is very important to know how warming may affect cloud formation. If temperatures rise, more water will evaporate from the surface, so cloudiness will increase and there will be more rain and snow. This might have several consequences. The polar icecaps might thicken, for example, because more snow would fall on them, so that rather than the icecaps melting and raising sea levels, sea levels might remain much as they are

Will global climate change bring more hurricanes? now, or even fall. More rain and snow falling in high latitudes, leading to an increased flow of freshwater into the sea from rivers and more falling on the sea itself, might reduce the density of the surface layer of the sea, because freshwater is less dense than salt. Were this to happen in the North Atlantic, it might interrupt the Atlantic conveyor (see “Ocean currents and sea-surface temperature” page 24), in which case the warm North Atlantic Drift might cease to break away from the Gulf Stream. That would reduce any warming in northwestern Europe and might even cause temperatures to fall. As it is, the warming observed so far is significantly smaller than was estimated around 1990. This is believed to be due to sulfur dioxide. In the air, sulfur dioxide (SO2) attracts atmospheric water vapor and dissolves to form sulfurous acid (H2SO3) and then tiny droplets of sulfuric acid (H2SO4). These reflect incoming solar radiation, and in moist air more water vapor condenses onto them, so they help form clouds. In both cases, by reflecting incoming radiation and increasing cloud formation, sulfur dioxide has a cooling effect. Volcanoes and several biological processes release sulfur dioxide, but it is also released when fuel-containing sulfur, such as certain qualities of coal and oil, is burned. There is much more industrial activity in the Northern Hemisphere than the Southern. This may be why the Northern Hemisphere has warmed more slowly, but the difference is quite small. It may also explain why nighttime minimum temperatures have risen in the Northern Hemisphere, but not daytime maximum temperatures. During the day, sulfuric acid droplets and cloud cool the surface by reflecting incoming radiation. Cloud also reflects outgoing heat radiated from the surface, and at night, when there is no incoming radiation and the ground cools, this reduces the rate at which heat is lost. The combined effect is to make the days cooler and the nights warmer. Atmospheric sulfur dioxide has also been found to alter the track of the jet stream, leading to colder winds over the North Atlantic and North Pacific. Many climatologists believe that by 2100, with a doubling of the atmospheric concentration of greenhouse gases, the average global temperature will have risen by 2.5–10.4°F (1.4–5.8°C). Sea levels are expected to rise by 4.3–30 inches (0.11–0.77 m) between 1990 and 2100. This is the “official” prediction, but some climate scientists disagree with the higher estimate, believing temperatures are unlikely to rise more than about 2.7°F (1.5°C).

The uncertainties Studying the global climate is very difficult. Measurements of pressure, temperature, humidity, cloudiness, and so forth are now made in many parts of the world, but conditions in some large, sparsely populated areas are not reported so regularly. Reliable records of past weather conditions are even more scattered, and none of them are continuous for very long.

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Hurricanes They are also hard to interpret. During the period covered by a particular record the thermometers may have been changed. Temperatures may not always have been read at the same time of day. The weather station may have been moved, perhaps to a different elevation. A city may have expanded to surround a station that had once been in a rural area—urban areas are warmer and drier than the countryside surrounding them. Given these difficulties, it is hard to tell whether the climate is growing warmer or cooler. Recent rises in temperature have been reported mainly by surface weather stations. Measurements made in the upper atmosphere by instruments carried on balloons show much less warming or none at all, and some satellite observations—said to be accurate to one-hundredth of a degree Celsius—show a slight cooling since 1979 (when the first satellite measurements were made). If both sets of measurements are correct, it suggests that any warming is confined to the lowest part of the atmosphere, below about 5,000 feet (1,500 m), and that it is insufficient to warm the bulk of the atmosphere (about 80 percent of it) above this height. Predicting how warming on a global scale will affect particular areas is even more complicated. Scientists use the most powerful supercomputers in the world to calculate what is likely to happen, but the results are meant to give only a general impression. They are not like the evening weather forecast. It is important to remember the uncertainties, but few scientists doubt that the greenhouse effect is real and that we should try to respond to global warming by reducing the amounts of greenhouse gases we discharge into the air. The Kyoto Protocol, drawn up in 1997 under the auspices of the United Nations, aims to achieve this, but even if it succeeds, its effect will be to reduce the average temperature by only 0.27°F (0.15°C) by 2100. The Kyoto agreement is intended only as a first step, of course, but it has proved so difficult to achieve, and there is so much uncertainty about whether its targets will be met, that there can be no guarantee of any future “Kyoto 2” agreement.

More hurricanes? If global warming does occur, sea-surface temperatures will rise and the area of warm sea will expand into higher latitudes on either side of the equator. If the sea is warmer, more water will evaporate from it. This will produce more storm clouds. This seems to suggest that tropical cyclones may become more frequent and more violent. In fact, this is unlikely. The predicted warming is expected to occur mainly over the continents in high latitudes. This is because these continents are often covered by very dry air. Dry air contains little water vapor and so experiences much less natural greenhouse warming than moister air does. Consequently, any increase in its content of carbon dioxide will exert a strong greenhouse effect. As the air warms, more water will evaporate

Protection and safety into it, increasing the amount of water vapor and adding still more greenhouse warming. The Tropics, where the air is very moist, are expected to warm much less. Even if they do warm, there is probably a limit to the temperature the sea surface can attain. As the water temperature rises, evaporation increases, but evaporation absorbs latent heat (see sidebar, page 50). The latent heat is taken from the surface water. This cools the water, thereby limiting the extent to which its temperature rises. Despite the warming that has been detected since the late 19th century, the frequency of tropical cyclones has not increased. It has varied over the years, with periods of high and low storm activity, but there is no overall trend. Hurricanes are expected to be more frequent in the early decades of the 21st century than they were in the 1970s and 1980s, but this is due to climate cycles that have nothing to do with global warming.

Protection and safety If you live in an area that experiences tropical cyclones, you should prepare for them well in advance. A little trouble taken in the winter and early spring could greatly improve your chances of escaping injury when the storm arrives. How do you know if your area is at risk? Ask people who have lived there for many years or check with the local newspaper or public library. If you live near a low-lying Atlantic or Pacific coast in a low latitude you can be fairly certain that a hurricane or typhoon will visit you sooner or later. The United States coast from Virginia to Florida and the Gulf coast are especially at risk. If you are new to the area, do not make the mistake of underestimating the power of a full-scale tropical cyclone. Hurricanes, typhoons, and cyclones occur throughout the Tropics. The precautions you can take apply anywhere. Wherever you live, the authorities will broadcast warnings and instructions before, during, and after the storm, but the warning system varies in detail from country to country.

Know the local geography Start by studying your local geography. Find out how high your home is above sea level, the height of the ground between you and the coast, and how storm surges have affected the district in the past. This will tell you what to expect from the sea. Torrential rains will probably make rivers overflow. Where is your nearest river and is your home high above it? If you live on low ground near a river, you may have to leave home in a hurry, so work out the best route to high ground inland. Remember that if your only escape route

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Hurricanes crosses low-lying land or bridges, these may be impassable once the storm arrives, so you should plan to leave ahead of the bad weather. Try to arrange in advance to stay during an emergency with friends or relatives who live on high ground inland. The authorities will have allocated places in your area for use as emergency shelters. If you are unable to stay with friends or family, find out where the shelters are.

Being prepared You will need to board up windows and secure all external doors. Lay in adequate stocks of lumber, plywood sheeting, polythene sheeting, nails, and rope. Have good-quality flashlights and a reliable battery-powered radio and make sure these are in working order and that you have spare batteries. Check that you have a Weather Radio receiver and that it is in working order. If you do not have a Weather Radio receiver, you should buy one. There are several models, obtainable from reputable dealers across the country, and they cost from $20 to $200. This is a sound investment. Have a camping stove for cooking and plenty of fuel for it. A camping cooler box, with gel packs, will be useful for keeping fresh foods cool. Prepare a first aid box (any manual on first aid will tell you what it should contain). The box should be clearly marked and easily seen. Ideally, paint it white with a large red cross; if it is made of white plastic, use red adhesive tape to make a cross. You will need to store water. Keep enough clean, airtight containers to hold at least 14 gallons (53 liters) of water for each member of the household. Lay in supplies of dry or canned food. You will need enough to feed all of you for at least two weeks (including household pets). You will also need other household items, such as soap, toilet paper, toothpaste, and kitchen towels. If you are evacuated, you will need blankets or sleeping bags. Keep up with the house maintenance and gardening. Make sure there are no loose or missing roof tiles or slates. Keep gutters and drainpipes clear of obstructions. If there are any old or weak trees or shrubs near the house, remove them. Remove all weak branches from trees, trying to open up the trees so air can move freely through them.

As the storm approaches When you learn that a storm is approaching, listen to broadcasts from your local radio or TV station, or in the United States better still from the Weather Radio, a public service provided by the National Oceanic and Atmospheric Administration (NOAA) of the Department of Commerce. Check the broadcasts frequently, if possible using electrical power to save the batteries.

Protection and safety In the United States, the first alert will be a Tropical Storm Watch or Hurricane Watch announcement. A storm means sustained winds—not occasional gusts—of up to 74 MPH (119 km/h). A hurricane means winds stronger than that. The warning will tell you when the storm or hurricane is expected to arrive. Usually it will give you about 36 hours to complete your preparations. You may also receive a Flash Flood Watch, warning of possible flooding. Check whether your car has a full fuel tank. If not, fill it. If you are taking medication, obtain an extra supply sufficient to last two weeks. Take out the materials you have stored for securing windows and doors and have them ready to hand. If you have a mobile home, tie it down securely. Clear away any loose objects outdoors, such as garden furniture. Check your emergency supplies. Freeze the gel packs for your cooler box. Make sure you have a supply of cash.

When you hear the warning A Tropical Storm Warning or Hurricane Warning will be issued when the storm is expected within 24 hours or less. Board up windows. Leave the radio or TV turned on so you will hear any instructions. Obey these at once. If you receive a Flash Flood Warning, it means rapid flooding has either started or is imminent. If possible, you should move away from low-lying land immediately. If possible, try to travel in daylight, but in any case leave as soon as you can, because the roads may be crowded and some may be closed. If your home is liable to be flooded and you are remaining in it, move upstairs.

Evacuation The warning may advise you to evacuate your home. If it does, do so immediately. You are likely to have to leave if you live within a few hundred yards of the coast, on an island, on the floodplain of a river, or if the land around your home has been flooded in the past by a storm surge. You should also leave if you live in a high rise, because the hurricane may weaken the structure. Do not remain in a mobile home. No matter how securely you tied it down, the hurricane may be able to wreck it. Before you leave, turn off the gas, electricity, and water supply. Unplug all electrical appliances. Take personal identification with you, as well as important private documents and cash. If you are not moving to friends or relatives, try to book accommodations at a motel or hotel. Do not delay, because many people will be seeking rooms in safe buildings. Let a friend or relative who lives well away from the affected area know where you are going. Leave family pets behind. Make sure they are shut indoors and that you have left them ample supplies of food and water.

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Emergency shelter Do not go to an emergency shelter until you hear from the radio or TV that the nearest one to you is open. If you do go to a shelter, take with you blankets or sleeping bags, toilet articles, your first aid box, a flashlight, and a radio. You will also need identification, some cash may be useful, and you should take any important personal documents. You may be in the shelter for some time, so take things to pass the time, such as books, games, or cards. Do not expect the shelter to be comfortable or to find much privacy there. The authorities will do their best, but it is likely to be crowded and there may be no electricity.

Staying at home Stay at home only if you have not been told to leave. If you are staying at home, unplug small electrical appliances and turn off the gas supply. Place fresh food in the refrigerator, turn it to full power, and open the door only when you must. Food in the refrigerator will remain safe to eat for a few days. Fill your containers with drinking water and fill the bathtub with water for washing. Close all doors and brace exterior doors so they cannot blow open. Keep listening to the radio and obey any instructions. You may be asked to turn off the electricity or water supply. Move to the safest part of the building, taking your flashlight and radio with you. Keep as far as you can from windows. If possible, move into a room with no outside wall or, in a multistory building, close to (or beneath) the stairs. When the strong winds arrive, lie on the floor, if possible beneath some protection, such as a strong table.

When the storm has passed After a time, the wind may drop and the sky clear. Do not be tempted to go outdoors. This could be the eye of the storm. If it is, the winds will return (from the opposite direction), possibly within a few minutes. Take no chances with the wind. Even if it seems safe, remember that hurricanes often trigger tornadoes and these could appear anywhere without warning. When the storm has passed, a radio announcement will tell you it is safe to go outdoors. Until then, stay inside. If you are away from home, do not try to return until you are told you may do so. You may have to produce identification before being allowed into your home. If your home has been damaged, do not enter it until someone in authority tells you it is safe. Outdoors, watch out for power lines that have been brought down in the wind. They may be alive. Be especially careful of pools of water with

Protection and safety power cables lying in them. Look out for snakes. Floods may have driven them into the open. Keep clear of loose, overhanging objects and branches of trees. When you are allowed back into your home, do not use naked flames, such as candles. Use the telephone only if it is essential to do so. The emergency services need all the telephone lines. If you use fresh food that you bought before the storm started, be sure it is still edible. Dried or canned food is safer. Do not drink or cook food in tap water until you are told it is safe. When power is restored and the risk of fire has passed, you may be advised to boil water before use. All tropical storms, hurricanes, typhoons, and cyclones are dangerous. Their destructive power is immense and their threat to life considerable, but with adequate preparation, a good warning system, and suitable precautions it is possible to survive them unharmed. Safety depends on knowing what to do, when to do it, and acting promptly.

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Hurricanes in history 1281 Typhoon in the China Sea destroyed most of an invading fleet headed for Japan; became known as the “divine wind” (kamikaze).

1703 November 26–27, hurricane-force winds in the English Channel killed 8,000 people and destroyed 14,000 homes.

1740 November 1, hurricane struck London, England.

1780 October, the “Great Hurricane” swept through the West Indies.

1831 Hurricane killed 1,477 people in Barbados.

1854 November 14, hurricane devastated the British fleet at anchor off Sevastopol, Crimea.

1876 Bakarganj cyclone coincided with high waters in the Ganges, due to the monsoon, drowning 100,000 people in half an hour.

1881 October 8, typhoon in China killed an estimated 300,000 people.

1892 January 6, hurricane struck Georgia.

1900 September 8, hurricane killed 6,000 people and destroyed half of all the buildings at Galveston, Texas.

1919 Hurricane killed 900 people in the Florida Keys.

Appendixes

1922 Hurricane struck Tamanrasset, Algeria.

1928 Hurricane killed 1,836 people in Florida when it caused Lake Okeechobee to overflow.

1935 Labor Day hurricane killed 408 people in the Florida Keys.

1938 Hurricane killed 600 people in New England.

1944 Typhoon Cobra killed 790 sailors, sank three ships, and destroyed 150 aircraft belonging to a U.S. fleet in the Philippine Sea.

1953 Typhoon killed 100 people and left 1 million homeless in Nagoya, Japan.

1954 Typhoon killed 1,600 people in Hokkaido, Japan. October 12, Hurricane Hazel killed an estimated 1,175 people in Haiti, the eastern United States, and Canada, before crossing the Atlantic and bringing heavy rain and strong winds to Scandinavia.

1955 Hurricanes killed 180 people in New England.

1956 June 27, Hurricane Audrey killed nearly 400 people on the Gulf coast.

1957 August, Hurricane Diane followed Hurricane Connie, between them killing more than 190 people in the United States.

1959 September, Typhoon Vera killed nearly 4,500 people in Honshu, ¯ Japan. Hurricane killed 2,000 people in Mexico. Cyclone left 100,000 people homeless on islands in the Ganges Delta.

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1961 September, Typhoon Muroto II killed 32 people in Osaka, Japan. September, severe hurricane killed more than 40 people in Galveston, Texas.

1969 August 17–18, Hurricane Camille killed about 250 people near the Mississippi and Louisiana coast and caused floods in which a further 125 people died.

1970 November, cyclone struck Bangladesh, killing about 500,000 people.

1972 Hurricane Agnes struck Florida and New England.

1973 January 17, hurricane-force winds killed at least 19 people in Spain and Portugal. April 12, typhoon-force winds killed up to 200 people in Bangladesh. November 10–11, typhoons killed at least 60 people in Vietnam. November 18–24, typhoon killed 54 people in the Philippines. December 9, cyclone left 1,000 people missing in Bangladesh.

1974 June 17, Hurricane Dolores killed at least 13 people in Mexico. July 6–7, typhoon killed 33 people in southern Japan. August 15, cyclone killed at least 20 people in West Bengal, India. September 20, Hurricane Fifi killed an estimated 5,000 people in Honduras. December 25, Cyclone Tracy killed more than 50 people and destroyed 90 percent of the city of Darwin, Australia.

1975 August, Typhoon Phyllis killed 68 people in Shikoku, Japan. August, Typhoon Rita killed 26 people in Japan. September 16, Hurricane Eloise killed 71 people in Puerto Rico, Hispaniola, Haiti, the Dominican Republic, and Florida, then moved into the northeastern United States, where a state of emergency was declared. October 24, Hurricane Olivia killed 29 people in Mexico.

Appendixes

1976 January 2–3, hurricane-force winds killed 55 people in northwestern Europe. May, Typhoon Olga caused rains in which 215 people died in Luzon, Philippines. September 8–13, Typhoon Fran killed 104 people in Japan. October 1, Hurricane Liza killed 630 people in Mexico.

1977 February, cyclone killed 31 people and destroyed more than 230 square miles (596 km2) of rice fields in Madagascar. April 24, cyclone killed 13 people in Bangladesh. June, a cyclone killed 2 people and destroyed 98 percent of the buildings on the island of Masirah, Oman. July 25, Typhoon Thelma killed 31 people in Taiwan. July 31, typhoon killed at least 38 people in Taiwan. November 12, cyclone killed more than 400 people in Tamil Nadu, India. November 14, typhoon killed at least 30 people in the Philippines. November 19, cyclone and storm surge killed an estimated 20,000 people in Andhra Pradesh, India.

1978 October 26, Typhoon Rita killed nearly 200 people in the Philippines. November 23, cyclone killed at least 1,500 people in Sri Lanka and southern India.

1979 March 27, Cyclone Meli killed at least 50 people in Fiji. April 16–17, typhoon killed at least 12 people in the Philippines. May 12–13, cyclone killed more than 350 people in India. August 25–26, Typhoon Judy killed nearly 60 people in South Korea. August–September, Hurricane David killed more than 1,000 people in the Caribbean, Florida, Georgia, and New York. September, Hurricane Frederic killed eight people in the southeastern United States. October 19, Typhoon Tip killed at least 36 people in Japan (this typhoon produced the lowest surface air pressure ever recorded).

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1980 January, Cyclone Hyacinthe killed at least 20 people in Réunion. April, Cyclone Wally killed at least 13 people in Fiji. July 23, Typhoon Joe killed more than 130 people in North Vietnam. August, Hurricane Allen killed more than 270 people in the Caribbean. September 11, Typhoon Orchid killed seven people in South Korea. September 15, Typhoon Ruth killed at least 164 people in Vietnam. September, a cyclone killed at least 12 people in India.

1981 July 1, Typhoon Kelly killed about 140 people in the Philippines. July 19, Typhoon Maury killed 26 people in Taiwan. August 23, Typhoon Tad killed 40 people in Japan. September 1, Typhoon Agnes killed 120 people in South Korea. September 21, hurricane-force winds killed at least 12 people in Britain. September 21, Typhoon Clara struck China. November 24, Typhoon Irma killed more than 270 people in the Philippines. December 11, typhoon killed at least 27 people in Bangladesh and India.

1982 January–March, Cyclones Benedict, Frida, Electra, Gabriel, and Justine killed more than 100 people in Madagascar. March, Typhoons Mamie and Nelson killed at least 90 people in the Philippines. May 4, cyclone killed 11 people in Myanmar. June 4, cyclone killed 200 people in Orissa, India. June 26–27, hurricane-force winds killed at least 43 people in Brazil. August 12–13, typhoon killed 38 people in South Korea. August, Typhoon Cecil killed at least 35 people in South Korea. September 11–12, Typhoon Judy killed 26 people in Japan. September 30, Hurricane Paul struck Mexico. October 14–15, typhoon killed 68 people in the Philippines. November 8, cyclone killed at least 275 people in India.

Appendixes

1983 April, cyclone killed 76 people in West Bengal, India. April 12, cyclone killed at least 50 people in India. August 18, Hurricane Alicia killed at least 17 people in southern Texas. September 29, Typhoon Forest killed 16 people in Japan. October 15, cyclone killed at least 25 people in Bangladesh. October 20, Hurricane Tico killed 105 people in Mexico.

1984 January 30–31, hurricane killed 13 people in Swaziland. January 31–February 2, Cyclone Domoina killed at least 124 people in southern Africa. April 12, cyclone killed at least 15 people and destroyed 80 percent of the town of Mahajanga, Madagascar. September, Typhoon Ike killed more than 1,300 people in the Philippines and 13 people in China. November, Typhoon Agnes killed at least 300 people in the Philippines. November 24, hurricane-force winds killed at least 14 people in northwestern Europe.

1985 January 22, Cyclones Eric and Nigel killed 23 people in Fiji. May 25, cyclone killed more than 2,500 people in Bangladesh. July 1, Typhoon Irma killed 19 people in Japan. July 30, typhoon killed 177 people in China. August, typhoons killed more than 500 people in China. August 30, Typhoon Pat killed 15 people in Japan. October, two typhoons caused floods that killed 16 people in Thailand. October 19, Typhoon Dot killed 63 people and destroyed 90 percent of the buildings in Cabanatuan, Philippines. November 19–21, Hurricane Kate killed at least 24 people in Cuba and Florida.

1986 March 17, Cyclone Honorinnia killed 32 people in Madagascar. March 24, hurricane-force winds killed at least 17 people in western Europe. May 16, cyclone killed 11 people in Bangladesh.

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Hurricanes May 19, Typhoon Namu killed more than 100 people in the Solomon Islands. July 9–11, Typhoon Peggy killed more than 70 people in the Philippines and more than 170 in China. August 22, typhoon killed 22 people in Taiwan. August 25, Hurricane Charley killed at least 11 people in Britain. September 4, typhoon killed 400 people in Vietnam. September 19, Typhoon Abby killed 13 people in Taiwan.

1987 February 7, Cyclone Uma killed 45 people in Vanuatu. July 15, Typhoon Thelma killed at least 111 people in South Korea. July 28, Typhoon Alex killed at least 38 people in China. October 15, hurricane-force winds killed 13 people in Britain. October 24, Typhoon Lynn struck Taiwan. November 3–6, cyclone-force winds killed at least 34 people in India. November 26, Typhoon Nina killed 500 people in the Philippines.

1988 September 12–17, Hurricane Gilbert killed about 460 people in Jamaica, Mexico, and Texas. October 22–27, Hurricane Joan killed at least 111 people in Central and South America. October 24–25, Typhoon Ruby killed about 500 people in the Philippines. November 7, Typhoon Skip killed at least 129 people in the Philippines. November 29, cyclone killed up to 3,000 people in Bangladesh and India.

1989 January 28–29, Cyclone Firinga killed at least 10 people in Réunion. February 25–26, hurricane-force winds killed at least 12 people in Spain. May 25–26, Typhoon Cecil killed 140 people in Vietnam. May 27, cyclone killed 200 people in Bangladesh and India. May, Typhoon Brenda killed 26 people in China. July 16, Typhoon Gordon killed 33 people in the Philippines. July 24, Typhoon Irving killed at least 200 people in Vietnam. July, Typhoon Judy killed at least 17 people in South Korea.

Appendixes September 11, Typhoon Sarah killed 13 people in Taiwan. September 16, Typhoon Vera killed 162 people in China. September 17–21, Hurricane Hugo killed 32 people in the Caribbean and southeastern United States and destroyed 99 percent of the homes in Montserrat. October, Typhoon Angela killed at least 50 people in the Philippines. October 2–13, three typhoons killed 63 people in China. October 10, Typhoon Dan killed 43 people in the Philippines. October 19, Typhoon Elsie killed 30 people in the Philippines. November 4–5, Typhoon Gay killed 365 people in Thailand. November 9, cyclone killed 50 people in India.

1990 January, cyclone killed at least 12 people in Madagascar. February 3, hurricane-force winds killed 29 people in France and Germany. February 26, hurricane-force winds killed at least 51 people in Europe. May 9, cyclone killed at least 962 people in India. June 23–24, Typhoon Ofelia killed 57 people in the Philippines, Taiwan, and China. August, typhoon killed 108 people in China. August, hurricane caused floods in Mexico in which 23 people died. August, Typhoon Yancy killed 216 people in China and 12 in the Philippines. August 31, Typhoon Abe killed 48 people in China. September 16–17, Typhoon Flo killed 32 people in Japan. October 23, typhoon killed 15 people in Vietnam. November 14, Typhoon Mike killed 190 people in the Philippines.

1991 April 30, cyclone killed at least 131,000 people in Bangladesh. July 20–21, Typhoon Amy killed at least 35 people in China. August 18–20, Hurricane Bob killed 16 people in the United States. August 23, Typhoon Gladys killed 72 people in South Korea. September 27, Typhoon Mireille killed 45 people in Japan. October 27, Typhoon Ruth killed 43 people in the Philippines. December 6–10, Typhoon Val killed 12 people in Western Samoa.

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1992 August 23–26, Hurricane Andrew killed 38 people in the Bahamas and United States. This was the most costly hurricane in U.S. history.

1993 January 2–3, Cyclone Kina killed 12 people in Fiji. July 6–7, Hurricane Calvin killed 28 people in Mexico. September, Typhoon Yancy killed 41 people in Japan. November 23, Typhoon Kyle killed at least 45 people in Vietnam. December, cyclone killed 47 people in India. December, hurricane-force winds killed 12 people in Britain. December 25–26, Typhoon Nell killed at least 47 people in the Philippines.

1994 February 2–4, Cyclone Geralda killed 70 people in Madagascar and destroyed 95 percent of the buildings in Toamasina. March, cyclone killed 34 people in Mozambique. May 2, cyclone killed 233 people in Bangladesh. August, typhoon killed 10 people in Taiwan. August 20–21, Typhoon Fred killed about 1,000 people in China. October 23, Typhoon Teresa killed 25 people in the Philippines. November, cyclone killed 30 people in Somalia.

1995 July, Typhoon Faye killed at least 16 people in South Korea. September 4–6, Hurricane Luis killed at least 15 people in Puerto Rico and the U.S. Virgin Islands. September 14, Hurricane Ismael killed at least 107 people in Mexico. September 15–16, Hurricane Marilyn killed nine people in Puerto Rico and the U.S. Virgin Islands. September 27, Hurricane Opal killed 63 people in Guatemala, Mexico, and the United States. October, Hurricane Roxanne killed 14 people in Mexico. November 3, Typhoon Angela killed more than 700 people in the Philippines.

Appendixes

1996 June 16, cyclone killed at least 100 people in India. July 8, Hurricane Bertha killed at least seven people in the Caribbean and United States. July 18, Typhoon Eve struck Ky ush ¯ u, ¯ Japan. July 25–26, Typhoon Gloria killed at least 30 people in the Philippines and three in Taiwan and China. July 31, Typhoon Herb killed at least 41 people in Taiwan. August 14–15, Typhoon Kirk struck Honshu, ¯ Japan. September 1, Hurricane Edouard killed 2 people in New Jersey. September 6, Hurricane Fran killed at least 34 people in the southeastern United States. September 10, Typhoon Sally killed more than 130 people in China. September 22, Typhoon Violet killed at least seven people in Japan. September, Typhoon Willie killed at least 38 people on the island of Hainan, China. September 28, Typhoon Zane killed two people in Taiwan.

1997 March, Cyclone Gavin killed at least 26 people in Fiji. May 19, cyclone killed at least 100 people in Bangladesh. August, Typhoon Victor killed 49 people in China. August 18–19, Typhoon Winnie killed at least 37 people in Taiwan, at least 140 in China, and 16 in the Philippines. September 27, cyclone killed at least 60 people in Bangladesh. October 8–10, Hurricane Pauline killed 217 people in Mexico. November, Cyclone Martin killed nine people in the Cook Islands. November, Typhoon Linda killed 484 people in Vietnam, Cambodia, and Thailand.

1998 March, cyclone killed at least 200 people in India. May 22, cyclone killed at least 25 people in Bangladesh. June 9, cyclone killed about 100 people in India. August, floods and landslides caused by Typhoon Rex killed 11 people in Japan.

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Hurricanes September 21–28, Hurricane Georges killed at least 300 people in the Caribbean and U.S. Gulf Coast. October, Typhoon Zeb killed at least 111 people in the Philippines, Taiwan, and Japan. October, Hurricane Mitch killed more than 8,800 people in Central America. October, Typhoon Babs killed at least 132 people in the Philippines. November 19–23, Typhoon Dawn killed more than 100 people in Vietnam.

1999 May–June, cyclone killed 128 people in Pakistan. August, Typhoon Olga killed more than 111 people in the Philippines. September, Hurricane Floyd killed about 50 people in the eastern United States. September 24, Typhoon Bart killed at least 26 people in Honshu, ¯ Japan. October–November, cyclone killed 9,463 people in Orissa, India.

2000 February–March, Cyclone Eline struck southern Africa. August 23, Typhoon Bilis killed at least 11 people in Vietnam. September 1, Typhoon Maria killed at least 47 people in China. November 1–2, Typhoon Xangsane killed at least 58 people in Taiwan. November 3, Typhoon Bebinca caused landslides and floods that killed 40 people in the Philippines.

2001 June 23–24, Typhoon Chebi killed 82 people in Taiwan and China. July, Typhoon Utor killed about 145 people in Taiwan, the Philippines, and China. July 30, Typhoon Toraji killed 77 people in Taiwan. September 16–19, Typhoon Nari killed 94 people in Taiwan. October 8–9, Hurricane Iris killed 22 people in Belize.

Appendixes

Cyclone names (If a tropical cyclone has a major impact, the country or countries most affected may ask the World Meteorological Organization to retire its name from the list. This allows the name to be associated unambiguously with a particular storm in historical references and for the purposes of insurance claims and legal actions. Once retired, a name cannot be used for at least 10 years. An alternative name of the same gender and language—English, French, or Spanish—is then substituted for the retired name.)

Atlantic 2003

2004

2005

2006

2007

2008

Ana Bill Claudette Danny Erika Fabian Grace Henri Isabel Juan Kate Larry Mindy Nicholas Odette Peter Rose Sam Teresa Victor Wanda

Alex Bonnie Charley Danielle Earl Frances Gaston Hermine Ivan Jeanne Karl Lisa Matthew Nicole Otto Paula Richard Shary Tomas Virginie Walter

Arlene Bret Cindy Dennis Emily Franklin Gert Harvey Irene Jose Katrina Lee Maria Nate Ophelia Philippe Rita Stan Tammy Vince Wilma

Alberto Beryl Chris Debby Ernesto Florence Gordon Helene Isaac Joyce Keith Leslie Michael Nadine Oscar Patty Rafael Sandy Tony Valerie William

Allison Barry Chantal Dean Erin Felix Gabrielle Humberto Iris Jerry Karen Lorenzo Michelle Noel Olga Pablo Rebekah Sebastien Tanya Van Wendy

Arthur Bertha Cristobal Danny Edouard Fay Gustav Hanna Isidore Josephine Kyle Lili Marco Nana Omar Paloma Rene Sally Teddy Vicky Wilfred

Adrian Beatriz Calvin Dora Eugene Fernanda Greg Hilary

Aletta Bud Carlotta Daniel Emilia Fabio Gilma Hector

Adolph Barbara Cosme Dalila Erick Flossie Gil Henriette

Alma Boris Cristina Douglas Elida Fausto Genevieve Hernan

Eastern North Pacific Andres Blanca Carlos Dolores Enrique Felicia Guillermo Hilda

Agatha Blas Celia Darby Estelle Frank Georgette Howard

(continues)

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(continued) 2003 2004

2005

2006

2007

2008

Ignacio Jimena Kevin Linda Marty Nora Olaf Patricia Rick Sandra Terry Vivian Waldo Xina York Zelda

Irwin Jova Kenneth Lidia Max Norma Otis Pilar Ramon Selma Todd Veronica Wiley Xina York Zelda

Ileana John Kristy Lane Miriam Norman Olivia Paul Rosa Sergio Tara Vicente Willa Xavier Yolanda Zeke

Israel Juliette Kiko Lorena Manuel Narda Octave Priscilla Raymond Sonia Tico Velma Wallis Xina York Zelda

Iselle Julio Kenna Lowell Marie Norbert Odile Polo Rachel Simon Trudy Vance Winnie Xavier Yolanda Zeke

Isis Javier Kay Lester Madeline Newton Orlene Paine Roslyn Seymour Tina Virgil Winifred Xavier Yolanda Zeke

Central North Pacific List 1 List 2 List 3

List 4

(The names are used in sequence. When the last name in one list is reached, the next cyclone is given the first name on the next list, regardless of the year. The last name used was Paka [1997], so the next will be Upana.) Akoni Aka Alika Ana Ema Ekeka Ele Ela Hana Hali Huko Halola Io Iolana Ioke Iune Keli Keoni Kika Kimo Lala Li Lana Loke Moke Mele Maka Malia Nele Nona Neki Niala Oka Oliwa Oleka Oko Peke Paka Peni Pali Uleki Upana Ulia Ulika Wila Wene Wali Walaka

Western North Pacific (There are five lists. Names are used in sequence regardless of the year. Each row of names is contributed by a nation in the region.) Country I II III IV V Cambodia China DPR Korea

Damrey Longwang Kirogi

Kong-rey Yutu Toraji

Nakri Fengshen Kalmaegi

Krovanh Dujuan Maemi

Sarika Haima Meari

Appendixes

Country Hong Kong Japan Lao PDR Macau Malaysia Micronesia Philippines Rep. of Korea Thailand U.S.A. Vietnam Cambodia China DPR Korea Hong Kong Japan Lao PDR Macau Malaysia Micronesia Philippines Rep. of Korea Thailand U.S.A. Vietnam

I Kai-Tak Tenbin Bolaven Chanchu Jelawat Ewinlar Bilis Gaemi Prapiroon Maria Saomai Bopha Wukong Sonamu Shanshan Yagi Xangsane Bebinca Rumbia Soulik Cimaron Chebi Durian Utor Trami

II Man-yi Usagi Pabuk Wutip Sepat Fitow Danas Nari Vipa Francisco Lekima Krosa Haiyan Podul Lingling Kaziki Faxai Vamei Tapah Mitag Hagibis Noguri Ramasoon Chataan Halong

III Fung-wong Kanmuri Phanfone Vongfong Rusa Sinlaku Hagupit Changmi Megkhla Higos Bavi Maysak Haishen Pongsona Yanyan Kuzira Chan-hom Linfa Nangka Soudelor Imbudo Koni Hanuman Etau Vamco

IV Choi-wan Koppu Ketsana Parma Melor Nepartak Lupit Sudal Nida Omais Conson Chanthu Dianmu Mindule Tingting Kompasu Namtheun Malou Meranti Rananin Malakas Megi Chaba Kodo Songda

V Ma-on Tokage Nock-ten Muifa Merbok Nanmadol Talas Noru Kularb Roke Sonca Nesat Haitang Nalgae Banyan Washi Matsa Sanvu Mawar Guchol Talim Nabi Khanun Vicete Saola

Western Australia (The names are used in sequence for all Australian storms. When all the listed names have been used for any part of the country, the same list is used again, starting at the beginning.) Adeline Alison Alex Bertie Billy Bessie Clare Cathy Chris Daryl Damien Dianne Emma Elaine Errol Floyd Frederic Fiona Glenda Gwenda Graham Hubert Hamish Harriet Isobel Ilsa Inigo Jacob John Jana Kirsty Kirrily Ken Lee Leon Linda (continues)

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(continued) Melanie Nicholas Ophelia Pancho Rhonda Selwyn Tiffany Victor Zelia

Marcia Norman Olga Paul Rosita Sam Taryn Vincent Walter

Eastern Australia Alfred Ann Blanch Bruce Charles Cecily Denise Dennis Ernie Edna Frances Fergus Greg Gillian Hilda Harold Ivan Ita Joyce Justin Kelvin Katrina Lisa Les Marcus May Nora Nathan Owen Olinda Polly Pete Richard Rona Sadie Sandy Theodore Tessi Verity Vaughan Wallace Wylva Northern Australia Amelia Alistair Bruno Bonnie Coral Craig Dominic Debbie Esther Evan Ferdinand Fay Gretel George Hector Helen

Monty Nicky Oscar Phoebe Raymond Sally Tim Vivienne Willy

Abigail Bernie Claudia Des Erica Fritz Grace Harvey Ingrid Jim Kate Larry Monica Nelson Odette Pierre Rebecca Steve Tania Vernon Wendy

Appendixes

Jason Irma Kay Laurence Marian Neville Olwyn Phil Rachel Sid Thelma Vance

Jasmine Ira Kim Laura Matt Narelle Oswald Penny Russel Sandra Trevor Valerie

Fiji (Lists A–D are used in sequence, regardless of year. List E is a standby list of replacement names should these be needed.) A B C D E Ami Beni Cilla Dovi Eseta Fili Gina Heta Ivy Judy Kerry Lola Meena Nancy Olaf Percy Rae Sheila Tam Urmil Vaianu Wati Yani Zita

Arthur Becky Cliff Daman Elisa Funa Gene Hettie Innis Joni Ken Lin Mick Nisha Oli Pat Rene Sarah Tomas Usha Vania Wilma Yasi Zaka

Atu Bobby Cyril Drena Evan Freda Gavin Helene Ian June Keli Lusi Martin Nute Osea Pam Ron Susan Tui Ursula Veli Wes Yali Zuman

Alan Bart Cora Dani Ella Frank Gita Hali Iris Jo Kim Leo Mona Neil Oma Paula Rita Sam Trina Uka Vicky Walter Yolande Zoe

Amos Bune Chris Daphne Eva Fanny Garry Hagar Irene Julie Koko Louise Mike Nat Odile Pami Reuben Solo Tuni Ula Victor Winston Yalo Zena (continues)

181

182

Hurricanes

(continued)

Papua New Guinea (The lists are used sequentially.) A B Epi Guba Ila Kama Matere Rowe Tako Upia

Abdul Emau Gule Igo Kamit Tiogo Ume

Appendixes

SI units and conversions Unit

Quantity

Symbol

Conversion

Base units meter kilogram second ampere kelvin candela mole

Supplementary units radian steradian Derived units coulomb cubic meter farad henry hertz joule kilogram per cubic meter lumen lux meter per second meter per second squared mole per cubic meter newton ohm

length mass time electric current thermodynamic temperature luminous intensity amount of substance

m kg s

1 m = 3.2808 inches 1 kg = 2.205 pounds

A K

1 K = 1°C = 1.8°F

cd mol

plane angle solid angle

rad sr

quantity of electricity volume capacitance inductance frequency energy density

C m3 F H Hz J kg m–3

luminous flux illuminance speed

lm lx m s–1

1 m s–1 = 3.281 ft. s–1

m s–2 mol m–3 N

1 N = 7.218 lb. force

acceleration concentration force electric

p/2 rad = 90°

1 m3 = 1.308 yards3

1 J = 0.2389 calories 1 kg m–3 = 0.0624 lb. ft.–3

183

184

Hurricanes

Unit

Quantity

Symbol

Conversion

Derived units resistance pressure angular velocity angular acceleration area magnetic flux density electromotive force power magnetic flux

pascal radian per second radian per second squared square meter tesla volt watt weber

Ω Pa

1 Pa = 0.145 lb. in.–2

rad s–1 rad s–2 m2

1 m2 = 1.196 yards2

T V W

1 W = 3.412 Btu h–1

Wb

PREFIXES USED WITH SI UNITS Prefixes attached to SI units alter their value.

Prefix

Symbol

Value

atto femto pico nano micro milli centi deci deca hecto kilo mega giga tera

a f p n µ m c d da h k M G T

× 10–18 × 10–15 × 10–12 × 10–9 × 10–6 × 10–3 × 10–2 × 10–1 × 10 × 102 × 103 × 106 × 109 × 1012

Bibliography and further reading

Bibliography and further reading “Airship.” Available on-line. URL: http://spot.colorado.edu/~dziadeck/ airship/html. Updated February 4, 2002. Alex, Christine. “NWR Receiver Consumer Information.” National Weather Service. Available on-line. URL: http://205.156.54.206/nwr/nwrrcvr.htm. Modified October 7, 2002. Allaby, Michael. A Chronology of Weather. New York: Facts On File, 1998. ———. Elements: Air. New York: Facts On File, 1992. ———. Elements: Water. New York: Facts On File, 1992. ———. Encyclopedia of Weather and Climate. 2 vols. New York: Facts On File, 2001. ———. The Facts On File Weather and Climate Handbook. New York: Facts On File, 2002. Ayscue, Jon K. “Hurricane Damage to Residential Structures: Risk and Mitigation, Natural Hazards Research Working Paper #4.” University of the Colorado. Available on-line. URL: http://www.colorado.edu/hazards/wp/wp94/wp94.html. Accessed November 2002. Barry, Roger G., and Richard J. Chorley. Atmosphere, Weather and Climate. 7th ed. New York: Routledge, 1998. “The Beaufort Scale.” Available on-line. URL: http://www.met-office.gov.uk/ education/historic/beaufort.html. Accessed November 2, 2002. “Beaufort Wind Scale.” Available on-line. URL: http://www.psych.usyd.edu.au/ vbb/woronora/maritime/beaufort/html. Accessed November 2, 2002. “Bernoulli’s Principle.” Avialable on-line. URL: http://www.mste.uiuc.edu/davea/ aviation/bernoulliPrinciple.html. Accessed November 2, 2002. Cane, H. “Hurricane Alley.” Available on-line. URL: http://www.hurricanealley.net/. Accessed November 2, 2002. Capella, Chris. “Dance of the Storms: The Fujiwhara Effect.” Available on-line. URL: http://www.usatoday.com/weather/wfujiwha/htm. January 6, 1999. Danish Wind Industry Association. “Aerodynamics of Wind Turbines: Stall and Drag.” Available on-line. URL: http://www.windpower.dk/tour/wtrb/stall.htm. Updated August 6, 2002. “Early Warning Saves Grief and Money.” World Meteorological Organization. Available on-line. URL: http://www.wmo.ch/web/Press/warning.html. Accessed November 2, 2002. “European Storms Kill 136 people.” Available on-line. URL: http://europe.cnn.com/ 1999/WORLD/europe/12/30/europe.storms.01/. Posted December 30, 1999. Fink, Micah. “Extratropical Storms.” Available on-line. URL: http://www.pbs.org/ wnet/savageplanet/02storms/01extratropical/indexmid.html. Accessed November 2002. “Food Safety in Hurricanes and Floods.” Clemson University Cooperative Extension Service. Available on-line. URL: http://hgic.clemson.edu/factsheets/ HGIC3800.htm. Revised December 1999. Gjevik, Bjørn, Halvard Moe, and Atle Ommundsen. “The Lofoten Maelstrom.” Available on-line. URL: http://www.math.uio.no/maelstrom/. Accessed November 2, 2002.

185

186

Hurricanes Grossman, Daniel J. “Airship: DJ’s Zeppelin Page.” Available on-line. URL: http://www.airships.net/index.shtml. Accessed November 2, 2002. Guiney, John L., and Miles B. Lawrence. “Preliminary Report: Hurricane Mitch 22 October–05 November 1998.” National Hurricane Center. Available online. URL: http://www.nhc.noaa.gov/1998mitch.html. Posted January 28, 1999. “Gulf of Corryvreckan—Legends and Facts.” Available on-line. URL: http://www.gemini-crinan.co.uk/corryvreckan.html. Accessed November 2, 2002. Haby, Jeff. “Calculation of Earth, Shear and Curvature Vorticity.” Available online. URL: http://www.theweatherprediction.com/habyhints/194/. Accessed November 2, 2002. Hamblyn, Richard. The Invention of Clouds. New York: Farrar, Straus, and Giroux, 2001. Heidorn, Keith C. “Luke Howard.” Available on-line. URL: http://www.islandnet. com/~see/weather/history/howard.htm. Posted May 1, 1999. Helfferich, Carla. “Beaufort’s Scale.” University of Alaska, Fairbanks. Available online. URL: http://www.gi.alaska.edu/ScienceForum/ASF9/911.html. February 2, 1989. Henderson-Sellers, Ann, and Peter J. Robinson. Contemporary Climatology. Harlow, U.K.: Longman, 1986. Herring, David, and Robert Kannenberg. “The Mystery of the Missing Carbon.” NASA Earth Observatory. Available on-line. URL: http://earthobservatory. nasa.gov/Study/BOREASCarbon/. Accessed November 2, 2002. Houghton, J.T., et al. Climate Change 2001: The Scientific Basis. Cambridge, U.K.: Cambridge University Press, 2001. “How Hurricanes Do Their Damage.” Available on-line. URL: http://hpccsun.unl. edu/nebraska/damage.html. Accessed November 2, 2002. “Hurricane.” American Red Cross. Available on-line. URL: http://www.redcross.org/ services/disaster/keepsafe/readyhurricane.html. Accessed November 2, 2002. “Hurricane Gilbert.” Available on-line. URL: http://www.csc.noaa.gov/crs/cohab/ hurricane/gilbert/gilbert.htm. Accessed November 2, 2002. “Hurricane Mitch Reports from the Disaster Center.” Available on-line. URL: http://www.disastercenter.com/hurricmr.htm. Accessed November 1, 2002. “Hurricane Mitch Special Coverage.” Available on-line. URL: http://www.osei. noaa.gov/mitch.html. Accessed November 1, 2002. Hurricanes 2001. Available on-line. URL: http://www.hurricanes2000.com/ Summary01.html. Accessed November 1, 2002. “International Cloud Atlas.” Available on-line. URL: http://www.wmo.ch/web/ catalogue/New%20HTML/frame/engfil/407.html. Accessed November 2, 2002. “JOIDES Resolution: Ocean Drilling Program Drill Ship.” Available on-line. URL: http://www-odp.tamu.edu/resolutn.html. Modified July 22, 2002. “Jonathan Dickinson Shipwreck.” USGenWeb Project and FLGen Web Project. Available on-line. URL: http://www.rootsweb.com/~findian/jondick.htm. Updated November 28, 1999. “Jonathan Dickinson State Park.” Available on-line. URL: http://www.ona1a.com/ Parks/State/dickenson.html. Accessed November 2, 2002. Kermann, Jochen. “Tropical Cyclone Ando.” Available on-line. URL: http://www.eumetsat.de/en/area2/image/may2001/page007.html. Posted May 14, 2001.

Bibliography and further reading Knauss, John A. Introduction to Physical Oceanography. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 1997. Landsea, Christopher W. “FAQ: Hurricanes, Typhoons, and Tropical Cyclones; Part B: Tropical Cyclone Names.” Atlantic Oceanic and Meteorological Laboratory. Available on-line. URL: http://www.aoml.noaa.gov/hrd/tcfaq/ tcfaqB.html. Posted October 16, 2002. Lawrence, Miles B, and Michelle M. Mainelli. “Tropical Cyclone Report: Hurricane Juliette, 21 September–3 October, 2001.” National Hurricane Center. Available on-line. URL: http://www.nhc.noaa.gov/2001juliette.html. Posted November 30, 2001. Lutgens, Frederick K., and Edward J. Tarbuck. The Atmosphere 7th ed. Upper Saddle River, N.J.: Prentice Hall, 1998. McIlveen, Robin. Fundamentals of Weather and Climate. London: Chapman & Hall, 1992. Maher, Brian, and Jack Beven. “Hurricane Gilbert.” Available on-line. URL: http://www.nhc.noaa.gov/1988gilbert.html. Accessed November 2002. Michaels, Patrick J. “Carbon Dioxide: A Satanic Gas?” Testimony to the Subcommittee on National Economic Growth, Natural Resources and Regulatory Affairs, U.S. House of Representatives. Available on-line. URL: http://www.cato.org/testimony/ct-pm100699.html. Accessed November 2, 2002. Michaels, Patrick J., and Robert C. Balling, Jr. The Satanic Gases: Clearing the Air About Global Warming. Washington, D.C.: Cato Institute, 2000. “Names of Notable Hurricanes Are Retired.” USA Today. Available on-line. URL: http://www.usatoday.com/weather/whretire.htm. October 17, 2001. National Oceanic and Atmospheric Administration. “Hurricane Mitch Special Coverage.” Available on-line. URL: http://www.osei.noaa.gov/mitch.html. Accessed November 2002. “Natural Disaster Survey Report: Hurricane Marilyn September 15–16, 1995.” National Weather Service, National Oceanic and Atmospheric Administration. Available on-line. URL: http://www.nws.noaa.gov/om/service_assessments/ marilyn.pdf. Posted January 1996. “1935 Labor Day Hurricane.” Storms of the Century. Available on-line. URL: http://www.weather.com/newscenter/specialreports/sotc/storm 1/page2.html. Accessed November 2, 2002. “NOAA Weather Radio.” National Weather Service. Available on-line. URL: http://205.156.54.206/nwr/. Modified October 24, 2002. Oliver, John E., and John J. Hidore Climatology: An Atmospheric Science. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2002. Padgett, Gary. “A Review of the 2000 Tropical Cyclone Season.” Available on-line. URL: http://australiasevereweather.com/cyclones/2001/summ2000.txt, and http://australiasevereweather.com/cyclones/2001/summ2000-2001.txt. Accessed November 1, 2002. Rekenthaler, Doug. “The Storm That Changed America: The Galveston Hurricane of 1900.” DisasterRelief.org. Available on-line. URL: http://www.disasterrelief. org/Disasters/980813Galveston/. Posted August 15, 1998. Rockett, Paul, and Mark Saunders. “June Forecast Update for Northwest Pacific Typhoon Activity in 2002.” Available on-line. URL: http://forecast.mssl. ucl.ac.uk/docs/TSRNWPFForecastJune2002.pdf, and http://forecast.mssl. ucl.ac.uk/for_typh.html. Posted June 7, 2002.

187

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Hurricanes Rockett, Paul, Mark Saunders, and Frank Roberts. “Summary of 2001 NW Pacific Typhoon Season and Verification of Authors’ Seasonal Forecasts.” Available online. URL: http://forecast.mssl.ucl.ac.uk/docs/TSRNWP2001Verification.pdf. Posted January 25, 2002. Schlatter, Thomas. “Vexing Vorticity.” Available on-line. URL: http://www. weatherwise.org/qr/qry.vort.html. Accessed November 2, 2002. “Scientists: Future Atlantic Hurricane Picture Is Highly Complex.” NASA Earth Observatory. Available on-line. URL: http://earthobservatory.nasa.gov/ Newsroom/MediaAlerts/2001/200109205219.html. September 20, 2001. Spindler, Todd. “The 2001 Atlantic Hurricane Season.” National Hurricane Center. Available on-line. URL: http://www.nhc.noaa.gov/2001.html. Updated January 23, 2002. “Storm Prediction Based on Human Experience, Sophisticated Machines.” USA Today. Available on-line. URL: http://www.usatoday.com/weather/hurricane/ 1999/atlantic/wfnhc.htm. Posted September 15, 1999. “Tropical Cyclone ‘Ando’ in the Indian Ocean.” Available on-line. URL: http://www.eumetsat.de/en/area5/special/cyclone_06012001.html. Accessed November 2, 2002. “Up Close and Personal with Hurricanes.” USA Today. Available on-line. URL: http://www.usatoday.com/weather/wac1.htm. Posted June 16, 1999. U.S Geological Survey. “USGS Hurricane Mitch Program Hurricane Overview.” Available on-line. URL: http://mitchnts1.cr.usgs.gov/overview.html. Updated November 1, 2002. “What Is ODP?” Available on-line. URL: http://www.oceandrilling.org/ODP/ ODP.html. Revised June 14, 2000. Wheeler, Dave. “The Beaufort Wind Scale.” Available on-line. URL: http://www.zetnet.co.uk/sigs/weather/Met_Codes/beaufort.htm. Updated January 9, 1999. World Meteorological Organization. Available on-line. URL: http://www.wmo.ch/ homeframe.html. Accessed November 2, 2002.

Index

189

Index Page numbers in italic refer to illustrations.

A Abe (typhoon) 103 absolute humidity 49 absolute instability 6 absolute stability 6 absolute vorticity 67 abyssal plain 128, 128 adiabatic cooling and warming 40–42 adiabatic lapse rates 6, 40 Agnes (typhoon) 138 agriculture 109–110 air, rising 53, 56, 58, 61, 63 airborne expendable bathythermograph system (AXBT) buoys 143 air masses 91, 91 airplanes hurricane damage to 117 hurricane monitoring by 143, 146 principles of flight 119–121 air pressure 42–44, 59–60, 110–112, 124–125. See also highs (high pressure); lows (low pressure) airships 37–38 air temperature 6, 36 Alabama 83–84, 123 Alberto (tropical storm) 123 Allison (tropical storm) 80, 109 altocumulus (Ac) clouds 47, 48 altostratus (As) clouds 47, 48 Amy (typhoon) 103 ana-fronts 20

Ando (cyclone) 100 Andrew (hurricane) 109, 148 anemometers 77, 78 angular momentum 13, 68, 71–72 angular velocity 71 Antarctic and Arctic extratropical hurricanes 104–108 anticyclones 10, 17 anticyclonic flow 59 Archimedes principle 39, 39 Arctic and Antarctic extratropical hurricanes 104–108 Asia. See typhoons; specific countries Atlantic Ocean currents 27–28, 29, 30–32 hurricanes 16–17, 23, 35, 89 atmospheric circulation 26–27, 35, 44–45 atmospheric gases 157–159 atmospheric pressure. See air pressure Audrey (hurricane) 135 Australia 9, 97 Bathurst Bay Hurricane 124 supercyclones 98 Tracy (cyclone) 102, 109, 138 AXBT buoys 143

B baguios 9, 97 Bahamas 109, 136–137, 148

Bangladesh 100, 101, 115, 139, 148–149 barometers 42, 110 Bathurst Bay Hurricane 124 Beaufort, Francis 78 Beaufort scale 77, 78 Belgium 91–92 Belize 5, 8, 57, 82 Bengal, Bay 9, 17, 139 Benguela Current 31 Bergeron, Tor 105 Bernoulli, Daniel 118 Bernoulli principle 118–120 Bess (typhoon) 137 Beulah (hurricane) 147 Brahmaputra River 101 buoyancy 38, 38–42 Burns Night storm 91–92 Buys Ballot, Christoph 59–60, 61 Buys Ballot’s law 59–60, 61

C Camille (hurricane) 88, 135–136, 148 Canada 91, 91, 127, 136–137, 141 carbon dioxide 158 Caribbean Sea 15, 16 Carol (hurricane) 136 categories of hurricanes 114–115, 150–151 CCN (cloud condensation nuclei) 50, 52 Cecil (typhoon) 109–110, 137 centripetal force 125 charge separation 55 Charles, Jacques A. C. 38

190

Hurricanes

Charles law 39 Charley (hurricane) 90–91 Charybdis whirlpool 65 China 96, 103, 109, 123, 137 China Sea (South China Sea) 102, 103, 137. See also East China Sea Christmas Day storm (Cyclone Tracy) 102, 109, 138 circulation. See general circulation of the atmosphere; three-cell model of atmospheric circulation cirrocumulus (Cc) clouds 47, 48 cirrostratus (Cs) clouds 47, 48 cirrus (Ci) clouds 47, 48 cloud condensation nuclei (CCN) 50, 52 clouds classification 46–47, 48 droplets 52, 54 formation of 49–53 in hurricanes 76–77 thunder and lightning 54–56, 55, 56 upcurrents and downcurrents 54 coastal shelving 127–129, 128 Cobra (typhoon) 116, 139 condensation 6, 50–54 conditional instability 6 Connecticut 136 conservation of angular momentum 71 conservation of mass 67, 111 continental shelf 127–129, 128 continental slope 128, 128 convection 4 Coriolis, Gaspard Gustave de 68, 69–70 Coriolis effect 13, 59–60, 68, 69–70

Corryvrechan whirlpool 65–66 Costa Rica 8 Crimean hurricane 132 crop destruction 109–110 Cuba 82 cumulonimbus (Cb) clouds 47, 48 cumulus (Cu) clouds 47, 48, 49 currents. See ocean currents curvature vorticity 67 cyclones (Bay of Bengal, southern Pacific Ocean) 9, 10, 10, 17, 97, 100–102, 138 cyclones (low pressure) 58, 92–95 cyclonic flow 58, 59

D DALR (dry adiabatic lapse) 6 damage from hurricanes. See hurricane damage Danas (typhoon) 96 dangerous semicircle 79 Darling, Grace 139 dart leaders 55–56 Defoe, Daniel 131–132 Denmark 89–90, 91–92 depressions vii–viii, 9, 10–12, 14, 58, 61, 61, 94–95, 95 dewpoint temperature 50–51 Dickinson, Jonathan 131 direction of vorticity 67 divergence 58 doldrums 33, 34 Dominica 84, 84–85 Dominican Republic 57, 82 Doppler, Christian Johann 146–147 Doppler radar 146–147 droplets 52, 54 dropsondes 137, 143

dry adiabatic lapse rate (DALR) 6 Durian (typhoon) 96

E Earth vorticity 67 East China Sea 102, 103 easterly waves 12, 57–58, 61 eddies 112, 113 Edna (hurricane) 136 Eline (cyclone) 109 El Niño–Southern Oscillation (ENSO) events 153, 154–155 ELR (environmental lapse rate) 6 El Salvador 5, 8 emergency shelters 164 energy of hurricanes 72 England 91, 110, 131–132, 139 enhanced greenhouse effect 158–159 environmental lapse rate (ELR) 6 Equatorial Current 154 equatorial trough 11–12, 14, 57, 99 Europe 23, 89–92. See also specific countries evacuation 151–152, 162 evaporation 50–54 extratropical hurricanes (Arctic and Antarctic) 104–108 extratropical hurricanes (previously tropical cyclones) 106 eye and eyewall (hurricane) 63–64, 75–76

F fatalities, reduction of 148–149 Ferrel, William 35, 59

Index fetch 115 flight, principles of 119–121 flooding 122–123 Florida 1696 hurricane 131 Alberto (tropical storm) 123 Andrew (hurricane) 109, 148 Gordon (tropical storm) 123 Labor Day storm 135 Mitch (hurricane) 5 Opal (hurricane) 83–84, 122 Flo (tropical storm) 123 Floyd (hurricane) 91, 109 Force 1 through 12 (Beaufort scale) 78 forests and trees 110 Forfarshire 139 France 89–90, 91–92 Fran (typhoon) 137 Frederic (hurricane) 123 frontal storms 92–95 fronts. See weather fronts Fujiwara, Sakuhei 103 Fujiwara effect 103 Fundy, Bay of 127

G Galveston (Texas) 124, 133–134 Ganges River 101 Gay (typhoon) 115 Gay-Lussac, Joseph L. 38 Gay-Lussac’s law 39 general circulation of the atmosphere 26–27 Georgia 123, 135–136 geostationary orbits 144, 145 geostrophic winds 43, 60, 60 Germany 89–90, 91–92 Gilbert (hurricane) 86–87, 148

global warming 154–161 greenhouse effect 157–159 increasing temperatures 154–156, 159, 160 Kyoto Protocol 160 solar radiation and 156–157 uncertainties about 159–160 global warming potential (GWP) 157 global wind systems 22–23 Gloria (tropical storm) 109 GOES (Geostationary Operational Environmental Satellite) 144 Gordon (tropical storm) 123 gradient winds 43 Grand Cayman Islands 87 gravity 110–111 Great Conveyor 28–32, 32 greenhouse effect 157–159 greenhouse gases 157–158 Guadeloupe 85, 86 Guam 137 Guatemala 5, 8, 83–84 Gulf Stream 27, 32, 34 gusts 112–113 GWP (global warming potential) 157 gyres 27–28, 29

H Hadley, George 35, 44–45 Hadley cells 33, 34, 35, 44–45, 45 hailstones 54 Hazel (hurricane) 136–137 heat transport. See atmospheric circulation; ocean currents heroism 138–139 highs (high pressure) 10, 26–27, 42–45. See also anticyclones

191

Himawari 144 Hindenburg 37 historic hurricanes 130–139, 166–176 homes, destruction of 109 Honduras 2, 5, 7 horse latitudes 33, 34 hot-air balloons 38–40 Howard, Luke 46 Hugo (hurricane) 84, 84–85, 110, 147–148 humidity 49–50 hurricane areas 9–16, 10 hurricane categories 114–115, 150–151 hurricane damage fatalities, reduction of 148–149 flooding 122–123 predicting 147–152 storm surges 123–130, 135–136 wind. See wind damage hurricane identification and monitoring 143–152 airplane surveillance 143, 146 measurements 149–150 predicting damage 147–152 radar 146–147 warnings, decision to issue 151–152 weather satellites 144–146 hurricane preparations 161–164 hurricanes areas of occurance 9–16, 10 cloud bands in 76–77 death of 79 description of typical vii–ix energy of 72

192

Hurricanes

eye and eyewall 63–64, 75–76 formation vii–viii, 10–16, 57–64 frequency 36, 88, 89, 152–154, 160–161 naming 9–10, 140–142, 177–182 seasons 16–17, 23, 34–37 structure of 72, 74–77, 76 waves 77–79 wind speed (force) and direction ix, 43, 60–63, 62, 66–72, 77, 113–115 hurricane safety 161–164 Hurricane Trackers 146 hurricane tracks 16, 22–23 hurricane warnings 151–152, 162–164 hygrometers 50

I Iceland Scotland Overflow Water (ISOW) 31 Ike (typhoon) 137 India 100, 138, 148 Indian Ocean 17, 22, 35, 100, 138 Indonesia 102, 154 inertia 125 Insat 144 instability and stability of air 6, 45, 50, 52–53 International Cloud Atlas 46 intertropical convergence zone (ITCZ) 11–12 inverse square law 125 inversions. See temperature inversions ionosphere 54 ions 54 Ireland 92 Iris (hurricane) 57, 82, 103 isobars 43

ISOW (Iceland Scotland Overflow Water) 31 Italy 92 ITCZ (intertropical convergence zone) 11–12

J Jamaica 57, 82, 86–87 Japan Danas (typhoon) 96 Fran (typhoon) 137 kamikaze 132–133, 133 Muroto II (typhoon) 123 Tip (typhoon) 137 Vera (typhoon) 103, 137 jet stream (subtropical jet stream) 14–16, 15, 61, 94–95, 99. See also polar front jet stream Joe (typhoon) 137 JOIDES Resolution 104–105, 116 Juliette (hurricane) 81, 81

K kamikaze 132–133, 133 Karen (tropical storm) 103 kata-fronts 20 kinetic energy 40, 111–112 Kuroshio Current 32, 34 Kyoto Protocol 160

L Labor Day storm 135 Lamarck, Jean 46 Langley, Samuel Pierpoint 37 La Niña 153, 156 lapse rates 6, 40 latent heat 50–51 Lekima (typhoon) 97 leverage 112–113 lift 120, 120–121 lifting condensation level 6

lightning and thunder 54–56, 55, 56 Lingling (tropical storm) 97 Lofoten Maelstrom whirlpool 65 Louisiana 80, 88, 109, 135, 148 lows (low pressure) 26–27, 42–45, 57–63, 66–68, 106–107. See also depressions; equatorial trough Luis (hurricane) 85–86

M Madagascar 100, 109 Maelstrom whirlpool 65 magnitude of vorticity 67 Maine 136 Maria (typhoon) 109 Marilyn (hurricane) 116, 117 Mars, gravity on 111 mass 67, 110–111 Massachusetts 136–137 mass mixing ratio 49 Meteosat 144 Mexico air masses 91, 91 Gilbert (hurricane) 86–87 Juliette (hurricane) 81, 81 Mitch (hurricane) 8 Opal (hurricane) 83–84, 122 Tico (hurricane) 139 Michelle (hurricane) 82 millibars 43 Mississippi 83–84, 88 Mitch (hurricane) 1–8 damage 5, 7–8, 148 formation of 1–2 recovery from 9 track of 2–3, 3 mixing ratio 49

Index monitoring hurricanes. See hurricane identification and monitoring monsoons 99 Moon and tides 125–126 Mozambique 101 Muroto II (typhoon) 123

N NADW (North Atlantic Deep Water) 31–32 names of tropical cyclones 9–10, 140–142, 177–182 NAO (North Atlantic Oscillation) 153 Nari (typhoon) 96–97 National Hurricane Center 144, 149 National Oceanic and Atmospheric Administration (NOAA) 143, 144 navigable semicircle 79 negative vorticity 66, 67 Netherlands 91–92, 131 New England 136 New Jersey 136 Newton, Isaac 125 New York 135, 136, 137 Nicaragua 7, 82 nimbostratus (Ns) clouds 47, 48 NOAA (National Oceanic and Atmospheric Administration) 143, 144 NOAA-class satellites 144 North America, air masses 91, 91 North Atlantic Deep Water (NADW) 31–32 North Atlantic Drift 27, 31 North Atlantic Oscillation (NAO) 153 North Carolina Allison (tropical storm) 80

Camille (hurricane) 135–136 Floyd (hurricane) 109 Hugo (hurricane) 84, 84–85, 110

O occluded fronts 21, 107 occlusions 21 ocean currents 26–37 Great Conveyor 28–32, 32 gyres 27–28, 29 winds and 32–34 Ocean Drilling Program (ODP) 104 oceans. See also ocean currents; specific oceans salinity 28, 30–31 storm surges 122–130, 135–136 thermal capacity of 25–26 tides 125–127, 126 October Storm 91 Oklahoma 86 Olga (typhoon) 109 Oman 138 Oonk, Edwin G. 104 Opal (hurricane) 83–84, 122, 123 Orchid (typhoon) 139 oscillations, weather 153–155

193

Lingling (tropical storm) 97 Olga (typhoon) 109 rainfall record 98 Utor (typhoon) 96 plane of the ecliptic 24 Poland 89–90 polar front jet stream 16, 94 polar fronts 35, 94, 106–107 polar lows 106–107 polar orbits 144 Polly (tropical storm) 109, 123 positive vorticity 66, 67 potential temperature 36 preparations and safety 151–152, 161–164 pressure gradient 43, 67 pressure-gradient force (PGF) 59–60, 73–74 Puerto Rico Hazel (hurricane) 136–137 Hugo (hurricane) 84, 84–85, 110 Luis (hurricane) 85–86 Marilyn (hurricane) 116 Santa Ana (hurricane) 141

Q quasi-biennial oscillation (QBO) 153

P

R

Pacific Ocean 16, 35, 97–98, 100, 102, 137 PGF (pressure-gradient force) 59–60, 73–74 Philippines 102 Agnes 138 Durian (typhoon) 96 Flo (tropical storm) 123 Ike (typhoon) 137

radar 146–147 radiation budget 156–157 rain 98, 122–123 relative humidity (RH) 50, 51 relative vorticity 67 return strokes 55 RH (relative humidity) 50, 51 Rhode Island 136 ridges 94

194

Hurricanes

Romania 89–90 Roxanne (hurricane) 86

S safety and preparations 151–152, 161–164 Saffir/Simpson Hurricane Scale 114–115, 150–151 SALR (saturated adiabatic lapse rate) 6 salt 28, 30, 30–31 Santa Ana (hurricane) 141 satellites, weather 144–146 saturated adiabatic lapse rate (SALR) 6 Saxby’s Gale 141 Scotland 131 sea cliffs 127, 128 sea-surface temperatures 12–13, 34–37 shearing forces 15, 64, 65, 66, 66 shear vorticity 67 sheet lightning 55 shelters, emergency 164 ships and hurricanes 77, 104–105, 115–116, 138–139 sodium chloride 28, 30, 30–31 solar radiation 24, 40, 156–157 solar spectrum 156 Solomon Browne 139 South Carolina 84–85, 110, 136 South China Sea. See China Sea Southern Oscillation 153, 155 South Korea 137, 139 Spain 90 specific humidity 49 spindrift 77, 115 spray gun 119 Sri Lanka 138 stability and instability of air 6, 45, 50, 52–53

stepped leaders 55 storm surges 122–130, 135–136 coastal shelving and 127–130, 128 damage from 123–124, 135–136 falling air pressure and 124–125 tides and 125–127 waves and 127, 129 storm tracks 16–22, 18 stratocumulus (Sc) clouds 47, 48 stratosphere 42 stratus (St) clouds 47, 48, 49 sublimation 50, 51 subtropical jet stream. See jet stream sulfur dioxide 159 Sun and tides 125–126 Sun-synchronous orbits 144 supercyclones 98 supertyphoons 98 surface runoff 122–123 Sweden 90 Switzerland 90, 92

T ta feng (tai fung) 97 Taiwan 96–97, 103 temperature inversions 58 Teresa (typhoon) 115 Texas Allison (tropical storm) 80, 109 Beulah (hurricane) 147 Galveston 124, 133–134 Gilbert (hurricane) 86–87 Theophrastus 46 thermal capacity 25–26 thermal equator 12 thermal winds 16, 94 three-cell model of atmospheric circulation 35, 44–45

thunder and lightning 54–56, 55, 56 Tico (hurricane) 139 tidal waves 126–127 tides 125–127, 126 Tip (typhoon) 97–98, 137 TIROS (Television and Infrared Observation Satellite) 144 Toraji (typhoon) 96 tornadoes 5, 18, 86, 113 Torricelli, Evangelista 42, 110 tracking hurricanes. See hurricane identification and monitoring Tracy (cyclone) 102, 109, 138 trade wind inversion 58 trade winds 11–12, 32–34, 35 trees and forests 110 tropical cyclones. See cyclones (Bay of Bengal, southern Pacific Ocean); hurricanes; typhoons tropical depressions, hurricane development and vi–vii, 9–16, 58, 61, 61 tropical disturbances 58 tropical storms 58, 60–61, 61 Tropics 9–13, 24, 26, 34–37 tropopause 42 troughs 11–12, 58, 94 typhoons areas of 9, 10, 10, 16–17, 22–23, 96–98 historic 137–138 monsoons and 99 names 141–142

U Union Star 139 United Kingdom 90–92, 110, 131–132, 139 United States 87–88, 91, 91, 136–137. See also specific states

Index U.S. Air Force Reserve 146 U.S. Navy Task Force 38 77–79, 116, 139 U.S. Virgin Islands 85, 86, 116, 117 Utor (typhoon) 96

V Vera (typhoon) 103, 137 Vietnam 96, 97, 109–110, 137 Viper (typhoon) 139 Virginia 85, 88, 135–136 Virgin Islands 85, 86 vorticity 13, 64–72 and bad weather 67–68 and bathtub water 64 components of 66 negative and positive 66, 67 types of 67 whirlpools 64–65

W Wales 91 Walker, Gilbert 154 Walker circulation 154 warm pool 154 warm sectors 95 warnings 151–152, 162–164 water, properties of 28, 30, 30–31, 50–51

water vapor 49–50, 51, 51 waves characteristics of 129–130 storm surges 123–130, 135–136 tidal 126–127 wind and 77–79, 115–116 weather cycles 153–15ñ weather fronts 19, 20–21, 92–95, 106–107 weather satellites 144–146 weight, definition of 110–111 West Wind Drift 31 whirlpools 65–66 willy-willys (willy-nillys) 9–10, 10 wind damage 102–122 air pressure and 110–112 crop destruction 109–110 gusts 112–113 to homes 109 kinetic energy and 111–112 to roofs 116–122 Saffir/Simpson Hurricane Scale 114–115, 150–151 tornadoes 5, 18, 86, 113 trees and forests 110 winds. See also wind damage air pressure and 42–44, 73–74

195

Beaufort wind scale 113–114 Buys Ballot’s law 59–60 easterlies and westerlies 17–22 geostrophic 43, 60, 60 global wind belts 22–23 gradient 43 hurricane, direction and speed of ix, 42–44, 60–63, 62, 66–72, 77, 78, 113–115 Saffir/Simpson Hurricane Scale 114–115 speed (force), hurricane 62, 78, 113–115 thermal 16, 94 trade 11–12, 32–34, 35 and waves 77–79, 115–116 wind shear 15 World Meteorological Organization (WMO) 46

Y Yancy (typhoon) 103 Yutu (typhoon) 96

Z zero vorticity 65