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Tropical Cyclone

Tropical cyclone

Tropical cyclones are large circulating storm systems consisting of multiple bands of intense showers and thunderstorms and extremely high winds. These storm systems develop over warm ocean waters in the tropical regions that lie within about 25° latitude of the equator. Tropical cyclones may begin as isolated thunderstorms. If conditions are favorable, they grow and intensify to form the storm systems known as hurricanes in the Americas, typhoons in East Asia , willy-willy in Australia , cyclones in Australia and India, and baguios in the Philippines. A fully developed tropical cyclone is a circular complex of thunderstorms about 403 mi (650 km) in diameter and over 7.5 mi (12 km) high. Winds near the core of the cyclone can exceed 110 mph (50 meters/second). At the center of the storm is a region about 912.5 mi (1520 km) across called the eye, where the winds are light and skies are often clear. After forming and reaching peak strength over tropical seas , tropical cyclones may blow inland, causing significant damage and loss of life. The storm destruction occurs by high winds and forcing rapid rises in sea level that flood low lying coastal areas. Better forecasting and emergency planning has lowered the death tolls in recent years from these powerful storms.

Several ocean areas adjacent to the equator possess all the necessary conditions for forming tropical cyclones. These spots are: the West Indies/Caribbean Sea, where most hurricanes develop between August and November; the Pacific Ocean off the west coast of Mexico, with a peak hurricane season of June through October; the western Pacific/South China Sea, where most typhoons, baguios, and cyclones form between June and December; and south of the equator in the southern Indian Ocean and the south Pacific near Australia, where the peak cyclone months are January to March. Note that in each area the peak season is during late summer (in the Southern Hemisphere, summer runs from December to March). Tropical cyclones require warm surface waters at least 80°F (27°C). During the late summer months, sea surface temperatures reach their highest levels and provide tropical cyclones with the energy they need to develop into major storms.

The annual number of tropical cyclones reported varies widely between regions and from year to year. The West Indies recorded 658 tropical cyclones between 18861966, an average of about eight per year. Of these, 389, or about five per year, grew to be of hurricane strength. The Atlantic hurricane basin has a 50-year average of ten tropical storms and six hurricanes annually.

In the United States, the National Weather Service names hurricanes from an alphabetic list of alternating male and female first names. New lists are drawn up each year to name the hurricanes of western Pacific and the West Indies. Other naming systems are used for the typhoons and cyclones of the eastern Pacific and Indian oceans .

In some ways, tropical cyclones are similar to the low pressure systems that cause weather changes at higher latitudes in places like the United States and Europe . These systems are called extratropical cyclones and are marked with an "L" on weather maps. These weather systems are large masses of air circulating cyclonically (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere). Cyclonic circulation is caused by two forces acting on the air: the pressure gradient and the Coriolis force.

In both cyclone types air rises at the center, creating a region of lower air (barometric) pressure. Because air is a fluid, it will rush in from elsewhere to fill the void left by air that is rising off the surface. The effect is the same as when a plug is pulled out of a full bathtub: water going down the drain is replaced by water rushing in from other parts of the tub. This is called the pressure gradient force because air moves from regions of high pressure to lower pressure. Pressure gradient forces are responsible for most day-to-day winds. As the air moves toward low pressure, the Coriolis force turns the air to the right of its straight-line motion (when viewed from above). In the Southern Hemisphere, the reverse is true: the Coriolis force pushes the moving air to the left. The air, formerly going straight toward a low-pressure region, is forced to turn away from it. The two forces are in balance when the air circles around the low pressure zone with a constant radius creating a stable cyclone rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

All large-scale air movements such as hurricanes, typhoons, extratropical cyclones, and large thunderstorms set up a cyclonic circulation in this manner. (Smaller scale circulations such as the vortex that forms in a bathtub drain are not cyclonic because the Coriolis force is overwhelmed by other forces. The larger a system, the more likely that the Coriolis force will prevail and the rotation will be cyclonic.) The Coriolis force is a consequence of the rotation of Earth. Moving air masses, like any other physical body, tend to move in a straight line. However, we observe them moving over Earth's surface, which is rotating underneath the moving air. From our perspective, the air appears to be turning even though it is actually going in a straight line, and it is we who are moving.

In both tropical and extratropical cyclones, the rising air at the cyclone center causes clouds and precipitation to form. A fully developed hurricane consists of bands of thunderstorms that grow larger and more intense as they move closer to the cyclone center. The area of strongest updrafts can be found along the inner wall of the hurricane. Inside this inner wall lies the eye, a region where air is descending. Descending air is associated with clearing skies; therefore, in the eye the torrential rain of the hurricane ends, the skies clear, and winds drop to nearly calm. In the eye of a hurricane, the eye wall clouds appear as towering vertical walls of thunderstorm clouds, stretching up to 7.5 mi (12 km) in height, and usually completely surrounding the eye. Hurricanes and other tropical cyclones move at the speed of the prevailing winds, typically 1020 mph (1632 kph) in the tropics. A hurricane eye passes over an observer in less than an hour, replaced by the high winds and heavy rain of the intense inner thunderstorms.

Several conditions are necessary to create a tropical cyclone. Warm sea surface temperatures, which reach a peak in late summer, are required to create and maintain the warm, humid air mass in which tropical cyclones grow. This provides energy for storm development through the heat stored in humid air, called latent heat. It takes energy to change water into vapor; that is why one must add heat to boil a kettle of water. The reverse is also true: when vapor condenses back to form liquid water, heat is released that may heat up the surrounding air. In a storm such as a hurricane, many hundreds of tons of humid air are forced to rise and cool, condensing out tons of water droplets and liberating a vast quantity of heat. This warms the surrounding air, causing it to expand and become even more buoyant, that is, more like a hot air balloon. More air begins rising, causing even more humid air to be drawn into the cyclone. This process feeds on itself until it forms a cyclonic storm of huge proportions. The more humid air available to a tropical cyclone, the greater its upward growth and the more intense it will become.

For storm growth to begin, air needs to rise. Because tropical air masses are uniformly warm and humid, the atmosphere over much of the tropics is stable; that is, it does not support rising air and the development of storms. Thunderstorms occasionally develop but tend to be short-lived and small in scale, unlike the severe thunderstorms in the middle latitudes. During the late summer, this peaceful picture changes. Tropical disturbances begin to appear. These can take the form of a cluster of particularly strong thunderstorms or perhaps a storm system moving westward off of the African continent and out to sea. Tropical disturbances are regions of lower pressure at the surface. As we have seen, this can lead to air rushing into the low pressure zone and setting up a vortex, or rotating air column, with rising air at its core.

An additional element is needed for tropical cyclone development: a constant wind direction with height throughout the lower atmosphere. This allows the growing vortex to stretch upward throughout the atmosphere without being sheared apart. Even with all these elements present, only a few of the many tropical disturbances observed each year become hurricanes or typhoons. When a tropical disturbance near the surface encounters a similar disturbance in the air flow at higher levels such as a region of low pressure at about the 3 mi (5 km) level (called an upper low), conditions are favorable for hurricane formation. These upper lows sometimes wander toward the equator from higher latitudes where they were part of a decaying weather system.

Once a tropical disturbance has begun to intensify, a chain reaction occurs. The disturbance draws in humid air and begins rising. Eventually it condenses to form water droplets. This releases latent heat, which warms the air, making it less dense and more buoyant. The air rises more quickly from the surface. As a result, the pressure in the disturbance drops and more humid air moves toward the storm. Meanwhile, the disturbance starts its cyclonic rotation and surface winds begin to increase. Soon, the tropical disturbance forms a circular ring of low air pressure and becomes known as a tropical depression. As more heat energy is liberated and updrafts increase inside the vortex, the internal barometric pressure continues to drop and the incoming winds increase. When wind speeds increase beyond 37 mph (60 kph), the depression is upgraded to a tropical storm. If the winds reach 75 mph (120 kph), the tropical storm is officially classified as a hurricane (or typhoon, cyclone, etc., depending on location). The chain reaction driving this storm growth is efficient. About 5070% of tropical storms intensify to hurricanes.

A mature tropical cyclone is a giant low-pressure system pulling in humid air, releasing its heat, and transforming it into powerful winds. The storm can range in diameter from 60600 mi (1001000 km) with wind speeds greater than 200 mph (320 kph). The central barometric pressure of the hurricane drops 60 millibars (mb) below the normal sea level pressure of 1013 mb. By comparison, the passage of a strong storm front in the middle latitudes may cause a drop of about 2030 mb. The size and strength of the storm is limited only by the air's humidity , which is determined by ocean temperature . It is estimated that for every 1.8°F (1°C) increase in sea surface temperature, the central pressure of a tropical cyclone can drop 12 mb. With such low central pressure, winds are directed inward, but near the center of the storm the winds are rotating so rapidly the Coriolis force prevents any further inward movement. This inner boundary creates the eye of the tropical cyclone. Unable to go in, the air is forced to move upward and then

spread out at an altitude of about 7.5 mi (12 km). Viewed from above by a satellite , the tropical cyclone appears as a mass of clouds diverging away from the central eye.

All of the cyclone development described thus far takes place at sea, but the entire cyclone also is blown along with the prevailing winds. Often this movement brings the storm toward land. As tropical cyclones approach land, they begin affecting the coastal areas with sea swells, large waves caused by the storm's high winds. Swells often reach 33 ft (10 m) in height and can travel thousands of kilometers from the storm. Coastal areas are at risk of severe damage from these swells that destroy piers, beach houses, and harbor structures every hurricane season. Particularly high swells may cause flooding farther inland.

More dangerous than the gradually rising swells are the sudden rises in sea level known as storm surges. Storm surges occur when the low barometric pressure near the center of a cyclone causes the water surface below to rise. Then strong winds blowing toward the coast push this "bulge" of water out ahead of the storm. The water piles up against the coast, quickly raising sea level as much as 16 ft (5 m) or more. The highest storm surge (for Northern Hemisphere storms, hurricanes) generally occurs east of the storm's path. When storm-tossed waves of 2333 ft (710 m) are added to this wall of water, land areas may be inundated. In 1900, the city of Galveston, Texas, was hit with a destructive storm surge during a hurricane. One eyewitness reported that the sea rose 4 ft (1.3 m) in a matter of seconds. Over 5,000 people lost their lives in the Galveston hurricane and resulting flooding, making it the deadliest storm ever recorded in the United States.

Tropical cyclones that travel onto the land immediately begin to weaken as humid air, their source of energy, is cut off. The winds at the base of the cyclone encounter greater friction as they drag across uneven terrain that slows them. Nevertheless, tropical cyclones at this stage are still capable of producing heavy rains, thunderstorms, and even tornadoes. Occasionally, the remnants of a tropical cyclone that has begun to weaken over land will unite with an extratropical low pressure system, forming a potent rain-making storm front that may bring flooding to areas far from the coast.

Until relatively recently, people in the path of a tropical cyclone had little warning of approaching storms. Usually their only warning signs were the appearance of high clouds and a gradual increase in winds. Hurricane watch services were established beginning in the early years of the twentieth century. By the 1930s, hurricanes were detected with weather balloons and ship reports while the 1940s saw the introduction of airplanes as hurricane spotters. Radar became available after World War II and has remained a powerful tool for storm detection. Today, a global network of weather satellites allows meteorologists to identify and track tropical cyclones from their earliest appearance as disturbances over the remote ocean. This improved ability to watch storms develop anywhere in the world has meant that warnings and evacuation orders can be issued well in advance of a tropical cyclone reaching land. Even though coastal areas have more people living near them today than ever before and tropical cyclones remain as powerful as ever, fewer storm-related deaths are now reported due to advances in storm detection and forecasting.

See also Air masses and fronts; Atmospheric pressure; Beach and shoreline dynamics; Beaufort wind scale; Convection (updrafts and downdrafts); El Nino and La Nina phenomena; Meteorology; Ocean circulation and currents; Wave motions; Weather forecasting methods; Weather forecasting; Weather radar; Weather satellite

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tropical cyclone

tropical cyclone (revolving storm) A generally fairly small but intense, closed low-pressure system which develops over tropical oceans. Wind speeds of at least 33 m/s (force 12 on the Beaufort scale, 64 knots or more) define such storms and distinguish them from less intense systems, e.g. tropical depressions (of twice or more than twice the diameter) or tropical storms. The atmospheric pressure gradient in such cyclones commonly ranges from about 950 mb at the centre to about 1000 mb at the margins.

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tropical cyclone

tropical cyclone(revolving storm) A generally fairly small but intense, closed, low-pressure system that develops over tropical oceans. Wind speeds of at least 33 m/s (force 12 on the Beaufort scale, 64 knots or more) define such storms and distinguish them from less intense systems (e.g. tropical depressions (of twice or more than twice the diameter) or tropical storms). The atmospheric pressure gradient in such cyclones commonly ranges from about 950 mb at the centre to about 1000 mb at the margins.

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Tropical Cyclone Programme

Tropical Cyclone Programme (TCP) A project to improve forecasting and warning systems for tropical cyclones. It forms part of World Weather Watch.

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Tropical Cyclone

Tropical Cyclone

Tropical cyclone geography and season

Structure and behavior

Life history of a tropical cyclone

The tropical cyclone on land

The 2004 and 2005 hurricane seasons

Resources

Tropical cyclones are large circulating storm systems consisting of multiple bands of intense showers and thunderstorms and high winds. These storm systems develop over warm ocean waters in the tropical regions that lie within about 25° latitude of the equator. Tropical cyclones may begin as isolated thunderstorms. If conditions are right, they grow and intensify to form the storm systems known as hurricanes in the Americas, typhoons in East Asia, willy-willy in Australia, cyclones in Australia and India, and baguios in the Philippines. A fully developed tropical cyclone is a circular complex of thunderstorms about 400 mi (645 km) in diameter and more than 7.5 mi (12 km) high. Winds near the core of the cyclone can exceed 110 mph (177 km/h). At the center of the storm is a region about 9-12.5 mi (15-20 km) across called the eye, where the winds are light and skies are often clear. After forming and reaching peak strength over tropical seas, tropical cyclones may blow inshore causing significant damage and loss of life. The storm destruction occurs by very high winds and forcing rapid rises in sea level that flood low-lying coastal areas. Better forecasting and emergency planning has

lowered the death tolls in recent years from these extremely powerful storms.

Tropical cyclone geography and season

Several ocean areas adjacent to the equator possess all the necessary conditions for forming tropical cyclones. These spots are: the West Indies/Caribbean Sea where most hurricanes develop between August and November; the Pacific Ocean off the west coast of Mexico with a peak hurricane season of June through October; the western Pacific/South China Sea where most typhoons, baguios, and cyclones form between June and December; and south of the equator in the southern Indian Ocean and the south Pacific near Australia where the peak cyclone months are January to March. Note that in each area the peak season is during late summer (in the southern hemisphere summer runs from December to March). Tropical cyclones require warm surface waters at least 80° F (27° C). During the late summer months, the sea surface temperatures reach their highest levels and provide tropical cyclones with the energy they need to develop into major storms.

The annual number of tropical cyclones reported varies widely between regions and from year to year. The West Indies recorded 658 tropical cyclones between 1886 and 1966, an average of about eight per year. Of these, 389, or about five per year, grew to be of hurricane strength. The Atlantic hurricane basin has a 50-year average of ten tropical storms and six hurricanes annually.

In the United States, the National Weather Service names hurricanes from an alphabetic list of alternating male and female first names. New lists are drawn up each year to name the hurricanes of western Pacific and the West Indies. Other naming systems are used for the typhoons and cyclones of the eastern Pacific and Indian oceans.

Structure and behavior

In some ways tropical cyclones are similar to the low-pressure systems that cause weather changes at higher latitudes in places like the United States and Europe. These systems are called extratropical cyclones and are marked with an L on weather maps. These weather systems are large masses of air

circulating cyclonically (counterclockwise in the northern hemisphere and clockwise in the southern hemisphere). Cyclonic circulation is caused by two forces acting on the air: the pressure gradient and the Coriolis force.

In both cyclone types, air rises at the center, creating a region of lower air (barometric) pressure. Because air is a fluid, it will flow in from elsewhere to fill the void left by air that is rising off the surface. The effect is the same as when a plug is pulled out of a full bathtub: water going down the drain is replaced by water rushing in from other parts of the tub. This is called the pressure gradient force because air moves from regions of high pressure to lower pressure. Pressure gradient forces are responsible for most of the day-to-day winds. As the air moves toward low pressure, the Coriolis force turns the air to the right of its straight line motion (when viewed from above). In the southern hemisphere the reverse is true: the Coriolis force pushes the moving air to the left. The air, formerly going straight toward a low-pressure region, is forced to turn away from it. The two forces are in balance when the air circles around the low pressure zone with a constant radius creating a stable cyclone rotating counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

All large scale air movements such as hurricanes, typhoons, extratropical cyclones, and large thunderstorms tend to set up a cyclonic circulation in this manner. (Smaller scale circulations such as the vortex that forms in a bathtub drain are not cyclonic because the Coriolis force is overwhelmed by other forces. One can make a bathtub drain vortex rotate clockwise or counterclockwise simply by stirring the water the right way. The larger a system is, the more likely that the Coriolis force will prevail and the rotation will be cyclonic.) The Coriolis force is a consequence of the rotation of Earth. Moving air masses, like any other physical body, tend to move in a straight line. However, they are observed moving over Earths surface, which is rotating underneath the moving air. From the perspective of an observer on Earths surface, the air appears to be turning even though it is actually going in a straight line, and it is the observer that is moving.

The Coriolis effect can be demonstrated by two people riding across from each other on a merry-go-round. If one person throws a ball straight at his friend, she will rotate out of position while the ball is moving and will be unable to catch it. To the two observers, the ball seemed to curve away from the catcher as if some force pushed it. Of course the ball actually went perfectly straight but the observers rotating frame of reference made it appear that a force was at work. On the surface of the rotating Earth this apparent forcethe Coriolis force makes moving air masses curve with respect to the surface and sets up cyclonic circulation.

In both tropical and extratropical cyclones, the rising air at the cyclone center causes clouds and precipitation to form. A fully developed hurricane consists of bands of thunderstorms that grow larger and more intense as they move closer to the cyclone center. The area of strongest updrafts can be found along the inner wall of the hurricane. Inside this inner wall lies the eye, a region where air is descending. Descending air is associated with clearing skies, therefore, in the eye the torrential rain of the hurricane ends, the skies clear, and winds drop to nearly calm. To someone in the eye of a hurricane, the eye wall clouds appear as just that: towering vertical walls of thunderstorm clouds, stretching up to 7.5 mi (12 km) in height, and usually completely surrounding the eye. Hurricanes and other tropical cyclones move at the speed of the prevailing winds, typically 10-20 mph (16-32 km/h) in the tropics. A hurricane eye passes over an observer in less than an hour, replaced by the high winds and heavy rain of the intense inner thunderstorms.

Life history of a tropical cyclone

Several conditions are necessary to create a tropical cyclone. Warm sea surface temperatures, which reach a peak in late summer, are required to create and maintain the warm, humid air mass in which tropical cyclones grow. This provides energy for storm development through the heat stored in humid air called latent heat. It takes energy to change water into vapor; that is why one must add heat to boil a kettle of water. The reverse is also true: when vapor condenses back to form liquid water, heat is released that may heat up the surrounding air. In a storm such as a hurricane, many hundreds of tons of humid air are forced to rise and cool, condensing out tons of water droplets and liberating a vast quantity of heat. This warms the surrounding air causing it to expand and become even more buoyant, that is, more like a hot air balloon. More air begins rising, causing even more humid air to be drawn into the cyclone. This process feeds on itself until it forms a cyclonic storm of huge proportions. The more humid air available to a tropical cyclone, the greater its upward growth will be and the more intense it will become.

For storm growth to begin, air needs to begin rising. Because tropical air masses are so uniformly warm and humid, the atmosphere over much of the tropics is fairly stable; that is, it does not support rising air and the development of storms. Thunderstorms occasionally develop but tend to be short-lived and small in scale, unlike the severe thunderstorms in the middle latitudes. During the late summer this peaceful picture changes. Tropical disturbances begin to appear. These can take the form of a cluster of particularly strong thunderstorms or perhaps a storm system moving westward off of the African continent and out to sea. Tropical disturbances are regions of lower pressure at the surface. This can lead to air rushing into the low-pressure zone and setting up a vortex, or rotating air column, with rising air at its core.

An additional element is needed for tropical cyclone development: a constant wind direction with height throughout the lower atmosphere. This allows the growing vortex to stretch upward throughout the atmosphere without being sheared apart. Even with all these elements present, only a few of the many tropical disturbances observed each year become hurricanes or typhoons. Some sort of extra kick is necessary to start the growth of a hurricane. This often comes when a tropical disturbance near the surface encounters a similar disturbance in the airflow at higher levels such as a region of low pressure at about the 3 mi (5 km) level (called an upper low). These upper lows sometimes wander toward the equator from higher latitudes where they were part of a decaying weather system.

Once a tropical disturbance has begun to intensify, a chain reaction occurs. The disturbance draws in humid air and begins rising. Eventually it condenses to form water droplets. This releases latent heat, which warms the air, making it less dense and more buoyant. The air rises more quickly off of the surface. As a result, the pressure in the disturbance drops and more humid air moves toward the storm. Meanwhile, the disturbance starts its cyclonic rotation and surface winds begin to increase. Soon the tropical disturbance forms a circular ring of low air pressure and becomes known as a tropical depression. As more heat energy is liberated and updrafts increase inside the vortex, the internal barometric pressure continues to drop and the incoming winds increase. When wind speeds increase beyond 37 mph (60 km/h), the depression is upgraded to a tropical storm. If the winds reach 75 mph (120 km/h), the tropical storm is officially classified as a hurricane (or typhoon, cyclone, etc., depending on location). The chain reaction driving this storm growth is very efficient. About 50-70% of tropical storms intensify to hurricanes.

A mature tropical cyclone is a large low pressure system pulling in humid air, releasing its heat, and transforming it into powerful winds. The storm can range in diameter from 60-600 mi (100-1000 km) with wind speeds greater than 200 mph (320 km/h). The central barometric pressure of the hurricane drops 60 millibars (mb) below the normal sea level pressure of 1,013 mb. By comparison, the passage of a strong storm front in the middle latitudes may cause a drop of about 20-30 mb. The size and strength of the storm is limited only by the airs humidity, which is determined by ocean temperature. It is estimated that for every 1.8° F (1° C) increase in sea surface temperature, the central pressure of a tropical cyclone can drop 12 mb. With such low central pressure, winds are directed inward, but near the center of the storm, the winds are rotating so rapidly the Coriolis force prevents any further inward movement. This inner boundary creates the eye of the tropical cyclone. Unable to go in, the air is forced to move upward then spread out at an altitude of about 7.5 mi (12 km). Viewed from above by a satellite, the tropical cyclone appears as a mass of clouds diverging away from the central eye.

The tropical cyclone on land

All of the cyclone development described thus far takes place at sea, but the entire cyclone also is blown along with the prevailing winds. Often this movement brings the storm toward land. As tropical cyclones approach land, they begin affecting the coastal areas with sea swells, large waves caused by the storms high winds. Swells often reach 33 ft (10 m) in height and can travel thousands of kilometers from the storm. Coastal areas are at risk of severe damage from these swells that destroy piers, beach houses, and harbor structures every hurricane season. Particularly high swells may cause flooding farther inland.

Perhaps more dangerous than the gradually rising swells are the sudden rises in sea level known as storm surges. Storm surges occur when the low barometric pressure near the center of a cyclone causes the water surface below to rise. Then strong winds blowing toward the coast push this bulge of water out ahead of the storm. The water piles up against the coast, quickly raising sea level as much as 16 ft (5 m) or more. The highest storm surge generally occurs to the right of the storms path. When storm-tossed waves 23-33 ft (7-10m) high are added to this wall of water, land areas may be inundated. In 1900 the city of Galveston, Texas, was struck by a storm surge during a hurricane. One eyewitness reported that the sea rose 4 ft (1.2 m) in a matter of seconds. Over 5,000 people lost their lives in the Galveston hurricane and resulting flooding, making it the deadliest storm ever recorded in the United States.

Tropical cyclones that travel onto the land immediately begin to weaken since humid air, their source of energy, is cut off. The winds at the base of the cyclone encounter greater friction as they drag across uneven terrain that slows them. Nevertheless tropical cyclones at this stage are still capable of producing heavy rains, thunderstorms, and even tornadoes. Occasionally, the remnants of a tropical cyclone that has begun to weaken over land will unite with an extratropical low-pressure system, forming a very potent rain-making storm front that may bring flooding to areas far from the coast.

Until relatively recently, people in the path of a tropical cyclone had little warning of approaching storms. Usually their only warning signs were the appearance of high clouds and a gradual increase in winds. Hurricane watch services were established beginning in the early years of the twentieth century. By the 1930s, hurricanes were detected with weather balloons and ship reports while the 1940s saw the introduction of airplanes as hurricane spotters. Radar became available after World War II and has remained a powerful tool for storm detection in the years since. In the early twenty-first century, a global network of weather satellites allows meteorologists to identify and track tropical cyclones from their earliest appearance as disturbances over the remote ocean. This improved ability to watch storms develop anywhere in the world has meant that warnings and evacuation orders can be issued well in advance of a tropical cyclone reaching land. Even though coastal areas have more people living near them today than ever before and tropical cyclones remain just as powerful as they have always been, far fewer storm-related deaths are reported each year than were in the mid-twentieth century thanks to advances in storm detection and forecasting.

KEY TERMS

Coriolis force An apparent force that seems to push moving air masses into curving paths. The Coriolis effect is not a true force but is due to observing air motion on the surface of the rotating Earth.

Extratropical cyclone Circulating columns of air that may bring storms to areas in the middle latitudes. Often called low-pressure systems.

Eye A calm, rain-free region at the very center of a tropical cyclone.

Hurricane (typhoon, cyclone, etc.) A tropical cyclone with winds that have reached the speed of 75 mph (120 km/h).

Latent heat The heat given off when water vapor condenses to form liquid water.

Midlatitudes The portion of Earths surface midway between the tropics and the polar regions lying about 35-65° north or south of the equator.

Pressure gradient force The force that pushes air from regions of higher pressure to regions of lower pressure.

Swell The rise of sea level near coastal areas due to the low barometric pressure; winds and wave activity of a tropical cyclone. Also called surge.

Tropical depression An early stage in the development of a hurricane, typhoon, or cyclone.

Tropical storm A tropical cyclone with wind speeds 3775 mph (60120 km/h).

Tropics The region around Earths equator spanning 23.5° north latitude to 23.5° south latitude.

The 2004 and 2005 hurricane seasons

The 2004 and 2005 hurricane seasons produced storms of notable intensity and unprecedented damage to coastal areas along the Gulf of Mexico, with four of the ten most intense Atlantic hurricanes on record occurring during those years. Hurricane Ivan (2004) was the ninth most intense hurricane on record, with a minimum atmospheric pressure of 910 mbar. Hurricanes Katrina (902 mbar), Rita (895 mbar), and Wilma (882 mbar), which all occurred during 2005, were the sixth, fourth, and first most intense hurricanes recorded. By way of comparison, the atmospheric pressure used in many scientific and engineering calculations for typical atmospheric conditions is 1,014 mbar. As they crossed the warm waters of the Gulf of Mexico, all four of these storms reached Category 5 on the Saffir-Simpson hurricane scale, which is characterized by winds of at least 156 mph (250 km/hr) and storm surges greater than 18 ft (5.5 m). Hurricane Wilma was also notable for the speed with which it developed, increasing from a Category 1 hurricane (winds 74-95 mph or 119-153 km/hr) to a Category 5 hurricane over the course of 24 hours. Over about the same period of time, atmospheric pressure at the center of the storm decreased 97 mbar. The most rapid pressure decrease ever recorded on Earth was 100 mb in 24 hours for Super Typhoon Forrest, which crossed the western Pacific Ocean in 1983.

Hurricane Katrina, which made landfall three times during August 2005, was by far the most damaging and deadly of the 2004 and 2005 storms. It caused more that $60 billion in damage to insured properties and killed more than 1,800 people, making it the most expensive (but not the most deadly) natural disaster in U.S. history. Estimates of the total economic damage, which include uninsured property, range as high as $125 billion. The most deadly natural disaster in U.S. history was the Galveston hurricane of 1900, which, according to the National Oceanic and Atmospheric Administration (NOAA), killed 6,000 to 12,000 people. Katrina first crossed Florida as a Category 1 hurricane on August 23 and weakened to a tropical storm over land. It regained strength as it continued into the Gulf of Mexico and increased to Category 5 by the morning of August 28. It decreased to a Category 3 hurricane as it made landfall again east of New Orleans in Plaquemines Parish, Louisiana, on the morning of August 29, crossed open water along the coast, and made landfall for the third time as a Category 3 hurricane near the Louisiana-Mississippi border. Little more than two weeks later, Hurricane Ivan made landfall along the Alabama coast on September 16, at which point it had decreased from a Catetory 5 to a Category 3 storm.

The approach of Katrina as a category 5 hurricane prompted a mandatory evacuation of New Orleans, a city in which many areas are below sea level. Despite warnings, many residents remained in New Orleans either by choice or necessity (for example, some residents did not have cars or were hospitalized). Most of the city was flooded when heavy rains and the storm surge, which was measured at 12 ft (3.6 m) to 14 ft (4.3 m) in different locations, caused levees between New Orleans and Lake Ponchartrain to fail after the storm had passed. Damage from rain and wind were minor compared to that caused by flooding after the storm. An independent review panel from the University of California inferred that two storm surges were likely to have occurred (one along the Mississippi River and the other from Lake Pontchartrain). The panel concluded that some flooding occurred when levees were overtopped and supporting soil was eroded by fast moving water, which had not been anticipated by the engineers who designed the levees. In other cases, the panel concluded that levees failed before they were overtopped because of weak soils that had not been recognized by levee designers. Some levees, moreover, were not as high as had been intended, a situation that may have been exacerbated by regional land subsidence (a gradual lowering of Earths surface).

See also Atmosphere observation; Atmospheric circulation; Atmospheric pressure; Cyclone and anticyclone; Weather forecasting.

Resources

BOOKS

Ahrens, Donald C. Meteorology Today. Pacific Grove, CA:
Brooks Cole, 2006.

Daniels, R. J.; D. F. Kettl; and H. Kunreuther, eds. On Risk
and Disaster: Lessons from Hurricane Katrina.

Philadelphia: University of Pennsylvania Press, 2006.

Palmer, Tim, and Renate Hagedorn, eds. Predictability of Weather and Climate. New York: Cambridge University Press, 2006.

van Heerden, I., and M. Bryan. The Storm: What Went
Wrong and Why During Hurricane Katrina.
New York: Viking, 2006

OTHER

Independent Levee Investigation Team Final Report.
Department of Civil Engineering, University of California, Berkeley. July 31, 2006. <http://www.ce.berkeley.edu/new_orleans/> (accessed November 14, 2006).

New Orleans Hurricane Protection Projects Data. U.S.
Army Corps of Engineers. June 2006. <https://ipet.wes.army.mil/> (accessed November 14, 2006).

James Marti

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Tropical Cyclone

Tropical Cyclone

Introduction

A tropical cyclone can be fearsome. The high winds, violent thunderstorms, pounding rains, and wind-blown surges of water onto coastal areas that are associated with such a storm system can be destructive and deadly.

These punches are packed by a storm system that is visually similar to a hurricane (which is one type of tropical cyclone) in its spiral shape and, in some cases, by the presence of a zone of clear weather at the center called the eye. The system harbors thunderstorms, strong winds that blow from regions of higher air pressure farther out from the eye to the area of low pressure within the eye, and intense rainfall that can cause flooding when the storm moves ashore.

The term “tropical” refers to the typical location of the storm; they almost always develop in tropical equatorial regions, where rising warm air feeds the storm. The term “cyclone” describes the slowly revolving nature of the storm, which gives a storm the typical pinwheel pattern when viewed from high in the atmosphere.

Although some studies have proposed that climate change observed since the 1950s, specifically the increased warming of the atmosphere, is influencing the frequency

and severity of tropical cyclones, the scientific community remains divided over this suggestion.

Historical Background and Scientific Foundations

Tropical cyclones have likely been a fact of life ever since the modern atmosphere formed approximately 500 million years ago. Recorded descriptions of cyclones in the equatorial region of the Atlantic Ocean and Caribbean by European explorers date back to at least the seventeenth century.

Even then, the devastating power of tropical cyclones was recognized. Coastal areas bear the brunt of the cyclone's punch. Because a cyclone forms over the ocean, it still is fully potent when it makes landfall. Inland regions can be spared a storm's full fury, since the energy of the storm drops as it moves inland and away from the ocean. However, torrential rainfall can still occur inland, which can result in flooding.

At the coast, another danger of tropical cyclones is the wind-driven surge of water that can batter the land. A recent example is the devastating storm surge caused by Hurricane Katrina, which struck the U.S. Gulf Coast in August of 2005. This storm surge was huge; in Gulf-port, Mississippi, calculations revealed that at the shoreline the waves were 28 ft (8.5 m) high.

Similar to the classification system to rate the severity of hurricanes, a tropical cyclone is considered to be the most severe of three classes of tropical storms. According to the classification, a cyclone has sustained winds of at least 74 mph (119 km/h) per hour. Less severe is a tropical depression, with sustained winds of 38 mph (61 km/h), and a tropical storm, with sustained winds ranging from 39 to 73 mph (63 to 117 km/h).

A typical feature of a tropical cyclone is a central, funnel-shaped region. At the base of the funnel is an area of intense low pressure. Indeed, some of the lowest air pressures ever recorded have been in this part of a cyclone. The remainder of the funnel that tightly spirals upward results from the upward movement of warm moist air. At higher altitudes, the water vapor in the air condenses to form clouds that fan outward from the funnel. The interior of a cyclone is warmer than the surrounding air; when a cyclone passes over a region, temperatures climb.

Even though the airflow in the wall of the funnel is upward, the flow of the air in the center of the funnel can be downward. A strong downward flow can cause the base of the funnel to clear, producing a hurricane-like eye. Depending on the size of the cyclone, an eye can vary from several miles to over 200 mi (322 km) in diameter. As well, airflow moves downward in the areas farther away from the central funnel. Over the ocean, this cyclical pattern of the upward movement of warm air and downward flow of cooler air feeds the storm, and it can cause the storm to grow in size and intensity. Once over land, the water-derived heat energy that drives the storm is no longer there, and the cyclone will rapidly become less intense and fade within several days.

Another characteristic feature of tropical cyclones is banding, or the occurrence of periodic regions of rain and thunderstorms that are arranged in a spiral outward from the eye. Heavy rains and high winds, which are most intense nearer to the storm center, are separated by periods of calm weather that occur as the cyclone passes over a given area of land. Tornados can even form in the zones of rainfall.

The rotation of air within a cyclone differs with height. At the surface, the horizontal airflow is determined by the Coriolis effect (which results from the rotation of Earth). If the storm is in the Northern Hemisphere, the airflow will be counterclockwise, and is clockwise if the storm is in the Southern Hemisphere. At higher altitudes, however, the cyclonic flow weakens and can even reverse, so that the airflow at the top of a cyclone is opposite the surface pattern.

A cyclone can release a tremendous amount of energy. Scientists have calculated that the energy produced by a fully formed cyclone is equivalent to the detonation of a 10-megaton bomb every 20 minutes. To put this into perspective, the atomic bomb that obliterated the Japanese city of Hiroshima during World War II was approximately 0.01 megatons. So, a cyclone's energy can be equivalent to the explosion of more than 100 Hiroshima-type atomic bombs every 30 minutes.

Tropical cyclone formation tends to be most active in late summer, when water temperatures are highest. In the Atlantic Ocean north of the equator, tropical cyclones form most frequently from the beginning of June through the end of November; this period is known as the hurricane season. In contrast, in the Southern Hemisphere, the cyclone season in the Indian Ocean runs from April through December.

WORDS TO KNOW

CORIOLIS EFFECT: A pseudoforce describing the deflection of winds due to the rotation of Earth, which produce a clockwise or counterclockwise rotation of storm systems in the Southern and Northern Hemispheres, respectively.

INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE (IPCC): Panel of scientists established by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in 1988 to assess the science, technology, and socioeconomic information needed to understand the risk of human-induced climate change.

LANDFALL: When the center of a storm system formed over the ocean (e.g., hurricane) reaches land, the storm is said to have made landfall. Some hurricanes never make landfall but exhaust themselves over the ocean. Those that do make landfall quickly lose power and dissipate, since they draw their energy from warm ocean surface waters.

STORM SURGE: Local, temporary rise in sea level (above what would be expected due to tidal variation alone) as the result of winds and low pressures associated with a large storm system. Storm surges can cause coastal flooding, if severe.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

IN CONTEXT: FUTURE OF TROPICAL CYCLONES

“It is very likely that hot extremes, heat waves, and heavy precipitation events will continue to become more frequent.”

“Based on a range of models, it is likely that future tropical cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and more heavy precipitation associated with ongoing increases of tropical SSTs. There is less confidence in projections of a global decrease in numbers of tropical cyclones. The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period.”

“Extra-tropical storm tracks are projected to move pole-ward, with consequent changes in wind, precipitation, and temperature patterns, continuing the broad pattern of observed trends over the last half-century.”

Statement of the Intergovernmental Panel on Climate Change (IPCC) as formally approved at the 10th Session of Working Group I of the IPCC in Paris, France, during February 2007.

SOURCE: Solomon, S., et al, eds. Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Despite the devastation of a tropical cyclone, the tremendous rains associated with a storm can be beneficial to a region that is suffering through a drought. Additionally, a cyclone contributes to atmospheric circulation by moving warm tropical air to cooler latitudes north and south of the equator. This mass movement of air assists in maintaining a global atmospheric temperature that is fairly stable.

Impacts and Issues

During the past two centuries, tropical cyclones have directly killed at least two million people. Those who survive a cyclone are not necessarily free of danger. A cyclone can destroy infrastructure such as electricity, transportation, and running water, increasing the chances of the development of infectious diseases such as malaria, which can claim many more lives.

This devastation can occur even in developed countries. Some relief workers who aided victims of Hurricane Katrina in 2005 compared the conditions they encountered in New Orleans, Louisiana, as rivaling the most deplorable conditions they had encountered in the developing world.

Since the formation of cyclones depends on the ocean temperature, the documented warming of the oceans has been a suggested contributor to the increasing frequency and severity of tropical cyclones. Some scientists have gone even further, suggesting that global warming influences cyclone occurrence. In this scenario, the warming ocean provides more heat energy to fuel cyclone activity.

But, this view is not shared by the majority of scientists. In 2006, for example, the World Meteorological Organization (WMO) issued a statement that climate change could not be linked to any one cyclone event in the prior two years, during which cyclones were more severe and did more damage than had ever been recorded before. According to the WMO, data linking climate change and cyclones is still inconclusive.

However, two studies published in 2005 in the journals Nature and Science, which analyzed data on cyclones over the past half century, documented the increasing frequency, severity, and duration of cyclones in response to increasing ocean temperatures. Further-more, a 2007 report by the United Nations' Intergovernmental Panel on Climate Change (IPCC) described how this increased temperature has been greater than that predicted from a normal increase in global temperature (which has happened several times during Earth's existence). These studies imply that human activities do play a role in cyclone formation and ferocity; they highlight that more research is necessary before a firm conclusion can be made.

See Also El Ninño and La Ninña; Extreme Weather; Global Warming; Hurricanes; Sea Temperatures and Storm Intensity.

BIBLIOGRAPHY

Books

Bigg, Grant R. The Oceans and Climate. Cambridge, U.K.: Cambridge University Press, 2004.

Emanuel, Kerry. What We Know About Climate Change. Boston: MIT Press, 2007.

Periodicals

Emanuel, K. “Increasing Destructiveness of Tropical Cyclones over the Past 30 Years.” Nature 436 (2005): 686–688.

Shultz, J. M., J. Russell, and Z. Espinel. “Epidemiology of Tropical Cyclones: The Dynamics of Disaster, Disease, and Development.” Epidemiologic Reviews 27 (2005): 12–14.

Webster, P. J., G. J. Holland, J. A. Curry, and H.-R. Chang. “Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment.” Science 309 (2005): 1844–1846.

Web Sites

“Tropical Cyclone Structure.” National Weather Service, August 29, 2007. <http://www.srh.noaa.gov/jetstream/tropics/tc_structure.htm> (accessed November 10, 2007).

Brian D. Hoyle

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Tropical Cyclone

Tropical cyclone

Tropical cyclones are large circulating storm systems consisting of multiple bands of intense showers and thunderstorms and extremely high winds. These storm systems develop over warm ocean waters in the tropical regions that lie within about 25° latitude of the equator. Tropical cyclones may begin as isolated thunderstorms. If conditions are right they grow and intensify to form the storm systems known as hurricanes in the Americas, typhoons in East Asia , willy-willy in Australia , cyclones in Australia and India, and baguios in the Philippines. A fully developed tropical cyclone is a circular complex of thunderstorms about 403 mi (650 km) in diameter and over 7.5 mi (12 km) high. Winds near the core of the cyclone can exceed 110 MPH (50 km/h). At the center of the storm is a region about 9-12.5 mi (15-20 km) across called the eye , where the winds are light and skies are often clear. After forming and reaching peak strength over tropical seas, tropical cyclones may blow inshore causing significant damage and loss of life. The storm destruction occurs by very high winds and forcing rapid rises in sea level that flood low lying coastal areas. Better forecasting and emergency planning has lowered the death tolls in recent years from these extremely powerful storms.


Tropical cyclone geography and season

Several ocean areas adjacent to the equator possess all the necessary conditions for forming tropical cyclones. These spots are: the West Indies/Caribbean Sea where most hurricanes develop between August and November; the Pacific Ocean off the west coast of Mexico with a peak hurricane season of June through October; the western Pacific/South China Sea where most typhoons, baguios, and cyclones form between June and December; and south of the equator in the southern Indian Ocean and the south Pacific near Australia where the peak cyclone months are January to March. Note that in each area the peak season is during late summer (in the southern hemisphere summer runs from December to March). Tropical cyclones require warm surface waters at least 80°F (27°C). During the late summer months the sea surface temperatures reach their highest levels and provide tropical cyclones with the energy they need to develop into major storms.

The annual number of tropical cyclones reported varies widely between regions and from year to year. The West Indies recorded 658 tropical cyclones between 1886-1966, an average of about eight per year. Of these, 389, or about five per year, grew to be of hurricane strength. The Atlantic hurricane basin has a 50-year average of 10 tropical storms and six hurricanes annually.

In the United States, the National Weather Service names hurricanes from an alphabetic list of alternating male and female first names. New lists are drawn up each year to name the hurricanes of western Pacific and the West Indies. Other naming systems are used for the typhoons and cyclones of the eastern Pacific and Indian oceans.


Structure and behavior

In some ways tropical cyclones are similar to the low pressure systems that cause weather changes at higher latitudes in places like the United States and Europe . These systems are called extratropical cyclones and are marked with an "L" on weather maps. These weather systems are large masses of air circulating cyclonically (counterclockwise in the northern hemisphere and clockwise in the southern hemisphere). Cyclonic circulation is caused by two forces acting on the air: the pressure gradient and the Coriolis force.

In both cyclone types air rises at the center, creating a region of lower air (barometric) pressure. Since air is a fluid, it will rush in from elsewhere to fill the void left by air that is rising off the surface. The effect is the same as when a plug is pulled out of a full bathtub: water going down the drain is replaced by water rushing in from other parts of the tub. This is called the pressure gradient force because air moves from regions of high pressure to lower pressure. Pressure gradient forces are responsible for most of our day-to-day winds. As the air moves toward low pressure, the Coriolis force turns the air to the right of its straight line motion (when viewed from above). In the Southern Hemisphere the reverse is true: the Coriolis force pushes the moving air to the left. The air, formerly going straight toward a low pressure region, is forced to turn away from it. The two forces are in balance when the air circles around the low pressure zone with a constant radius creating a stable cyclone rotating counterclockwise in the northern hemisphere and clockwise in the southern hemisphere.

All large scale air movements such as hurricanes, typhoons, extratropical cyclones, and large thunderstorms tend to set up a cyclonic circulation in this manner. (Smaller scale circulations such as the vortex that forms in a bathtub drain are not cyclonic because the Coriolis force is overwhelmed by other forces. You can make a bathtub drain vortex rotate clockwise or counterclockwise simply by stirring the water the right way. The larger a system is the more likely that the Coriolis force will prevail and the rotation will be cyclonic.) The Coriolis force is a consequence of the rotation of Earth . Moving air masses, like any other physical body, tend to move in a straight line. However, we observe them moving over Earth's surface, which is rotating underneath the moving air. From our perspective the air appears to be turning even though it is actually going in a straight line, and it is we who are moving.

The Coriolis effect can be demonstrated by two people riding across from each other on a merry-go-round. If one person throws a ball straight at his friend she will rotate out of position while the ball is moving and will be unable to catch it. To the two observers the ball seemed to curve away from the catcher as if some force pushed it. Of course the ball actually went perfectly straight but the observer's rotating frame of reference made it appear that a force was at work. On the surface of the rotating Earth this apparent force—the Coriolis force—makes moving air masses curve with respect to the surface and sets up cyclonic circulation.

In both tropical and extratropical cyclones, the rising air at the cyclone center causes clouds and precipitation to form. A fully developed hurricane consists of bands of thunderstorms that grow larger and more intense as they move closer to the cyclone center. The area of strongest updrafts can be found along the inner wall of the hurricane. Inside this inner wall lies the eye, a region where air is descending. Descending air is associated with clearing skies, therefore, in the eye the torrential rain of the hurricane ends, the skies clear, and winds drop to nearly calm. If you are in the eye of a hurricane the eye wall clouds appear as just that: towering vertical walls of thunderstorm clouds, stretching up to 7.5 mi (12 km) in height, and usually completely surrounding the eye. Hurricanes and other tropical cyclones move at the speed of the prevailing winds, typically 10-20 MPH (16-32 km/h) in the tropics. A hurricane eye passes over an observer in less than an hour, replaced by the high winds and heavy rain of the intense inner thunderstorms.


Life history of a tropical cyclone

Several conditions are necessary to create a tropical cyclone. Warm sea surface temperatures, which reach a peak in late summer, are required to create and maintain the very warm, humid air mass in which tropical cyclones grow. This provides energy for storm
development through the heat stored in humid air called latent heat. It takes energy to change water into vapor; that is why one must add heat to boil a kettle of water. The reverse is also true: when vapor condenses back to form liquid water, heat is released that may heat up the surrounding air. In a storm such as a hurricane, many hundreds of tons of humid air are forced to rise and cool, condensing out tons of water droplets and liberating a vast quantity of heat. This warms the surrounding air causing it to expand and become even more buoyant, that is, more like a hot air balloon . More air begins rising, causing even more humid air to be drawn into the cyclone. This process feeds on itself until it forms a cyclonic storm of huge proportions. The more humid air available to a tropical cyclone the greater its upward growth will be and the more intense it will become.

For storm growth to get started some air needs to begin rising. Because tropical air masses are so uniformly warm and humid, the atmosphere over much of the tropics is fairly stable; that is, it does not support rising air and the development of storms. Thunderstorms occasionally develop but tend to be short-lived and small in scale, unlike the severe thunderstorms in the middle latitudes. During the late summer this peaceful picture changes. Tropical disturbances begin to appear. These can take the form of a cluster of particularly strong thunderstorms or perhaps a storm system moving westward off of the African continent and out to sea. Tropical disturbances are regions of lower pressure at the surface. As we have seen, this can lead to air rushing into the low pressure zone and setting up a vortex, or rotating air column, with rising air at its core.

An additional element is needed for tropical cyclone development: a constant wind direction with height throughout the lower atmosphere. This allows the growing vortex to stretch upward throughout the atmosphere without being sheared apart. Even with all these elements present only a few of the many tropical disturbances observed each year become hurricanes or typhoons. Some sort of extra kick is necessary to start the growth of a hurricane. This often comes when tropical disturbance near the surface encounters a similar disturbance in the air flow at higher levels such as a region of low pressure at about the 3 mi (5 km) level (called an upper low). These upper lows sometimes wander toward the equator from higher latitudes where they were part of a decaying weather system.


Once a tropical disturbance has begun to intensify a chain reaction occurs. The disturbance draws in humid air and begins rising. Eventually it condenses to form water droplets. This releases latent heat, which warms the air, making it less dense and more buoyant. The air rises more quickly off of the surface. As a result, the pressure in the disturbance drops and more humid air moves toward the storm. Meanwhile, the disturbance starts its cyclonic rotation and surface winds begin to increase. Soon the tropical disturbance forms a circular ring of low air pressure and becomes known as a tropical depression. As more heat energy is liberated and updrafts increase inside the vortex, the internal barometric pressure continues to drop and the incoming winds increase. When wind speeds increase beyond 37 MPH (60 km/h) the depression is upgraded to a tropical storm. If the winds reach 75 MPH (120 km/h) the tropical storm is officially classified as a hurricane (or typhoon, cyclone, etc., depending on location). The chain reaction driving this storm growth is very efficient. About 50-70% of tropical storms intensify to hurricanes.

A mature tropical cyclone is a giant low pressure system pulling in humid air, releasing its heat, and transforming it into powerful winds. The storm can range in diameter from 60-600 mi (100-1000 km) with wind speeds greater than 200 MPH (320 km/h). The central barometric pressure of the hurricane drops 60 millibars (mb) below the normal sea level pressure of 1013 mb. By comparison, the passage of a strong storm front in the middle latitudes may cause a drop of about 20-30 mb. The size and strength of the storm is limited only by the air's humidity , which is determined by ocean temperature . It is estimated that for every 1.8°F (1°C) increase in sea surface temperature the central pressure of a tropical cyclone can drop 12 mb. With such low central pressure, winds are directed inward, but near the center of the storm the winds are rotating so rapidly the Coriolis force prevents any further inward movement. This inner boundary creates the eye of the tropical cyclone. Unable to go in, the air is forced to move upward then spread out at an altitude of about 7.5 mi (12 km). Viewed from above by a satellite , the tropical cyclone appears as a mass of clouds diverging away from the central eye.


The tropical cyclone on land

All of the cyclone development described thus far takes place at sea, but the entire cyclone also is blown along with the prevailing winds. Often this movement brings the storm toward land. As tropical cyclones approach land they begin affecting the coastal areas with sea swells, large waves caused by the storm's high winds. Swells often reach 33 ft (10 m) in height and can travel thousands of kilometers from the storm. Coastal areas are at risk of severe damage from these swells that destroy piers, beach houses, and harbor structures every hurricane season. Particularly high swells may cause flooding farther inland.

Perhaps more dangerous than the gradually rising swells are the sudden rises in sea level known as storm surges. Storm surges occur when the low barometric pressure near the center of a cyclone causes the water surface below to rise. Then strong winds blowing toward the coast push this "bulge" of water out ahead of the storm. The water piles up against the coast, quickly raising sea level as much as 16 ft (5 m) or more. The highest storm surge generally occurs to the right of the storm's path. When storm-tossed waves 23-33 ft (7-10 m) high are added to this wall of water land areas may be inundated. In 1900 the city of Galveston, Texas, was hit with a storm surge during a hurricane. One eyewitness reported that the sea rose 4 ft (1.3m) in a matter of seconds. Over 5,000 people lost their lives in the Galveston Hurricane and resulting flooding, making it the deadliest storm ever recorded in the United States.

Tropical cyclones that travel onto the land immediately begin to weaken since humid air, their source of energy, is cut off. The winds at the base of the cyclone encounter greater friction as they drag across uneven terrain that slows them. Nevertheless tropical cyclones at this stage are still capable of producing heavy rains, thunderstorms, and even tornadoes. Occasionally, the remnants of a tropical cyclone that has begun to weaken over land will unite with an extratropical low pressure system, forming a very potent rain-making storm front that may bring flooding to areas far from the coast.

Until relatively recently, people in the path of a tropical cyclone had little warning of approaching storms. Usually their only warning signs were the appearance of high clouds and a gradual increase in winds. Hurricane watch services were established beginning in the early years of the twentieth century. By the 1930s hurricanes were detected with weather balloons and ship reports while the 1940s saw the introduction of airplanes as hurricane spotters. Radar became available after World War II and has remained a powerful tool for storm detection in the years since. Today a global network of weather satellites allows meteorologists to identify and track tropical cyclones from their earliest appearance as disturbances over the remote ocean. This improved ability to watch storms develop anywhere in the world has meant that warnings and evacuation orders can be issued well in advance of a tropical cyclone reaching land. Even though coastal areas have more people living near them today than ever before and tropical cyclones remain just as powerful as they have always been far fewer storm related deaths are reported each year than 60 years ago thanks to advances in storm detection and forecasting.

See also Atmosphere observation; Atmospheric circulation; Atmospheric pressure; Cyclone and anticyclone; Weather forecasting.


Resources

books

Battan, Louis J. Weather. Engelwood Cliffs: Prentice-Hall Inc., 1985.

Battan, Louis J. Weather in Your Life. New York: W.H. Freeman & Co., 1983.

Fisher, David E. The Scariest Place on Earth: Eye to Eye with Hurricanes. New York: Random House, 1994.

Gedzelman, Stanley D. The Science and Wonders of the Atmosphere. New York: John Wiley & Sons, 1980.

Hardy, Ralph, Peter Wright, John Kington, and John Gribben. The Weather Book. Boston: Little, Brown and Co., 1982.

McNeill, Robert. Understanding the Weather. Las Vegas: Arbor Publishers, 1991.

Wallace, John M., and Hobbs, Peter V. Atmospheric Science: An Introductory Survey. New York: Academic Press, 1977.


periodicals

"Cyclolysis: A Diagnosis Of Two Extratropical Cyclones." Monthly Weather Review 129, no. 11 (2001): 2714-2729.

Leroux, M. "The Meteorology And Climate Of Tropical Africa." Journal of Meteorology 27, no. 271 (2002): 274.

Rodgers, Edward B. "Contribution of Tropical Cyclones to the North Atlantic Climatology." Journal of Applied Meteorology 40, no. 11 (2001): 1785-1800.


James Marti

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Coriolis force

—An apparent force that seems to push moving air masses into curving paths. The Coriolis effect is not a true force but is due to our observing air motion on the surface of the rotating Earth.

Extratropical cyclone

—Circulating columns of air which may bring storms to areas in the middle latitudes. Often called low pressure systems.

Eye

—A calm, rainfree region at the very center of a tropical cyclone.

Hurricane (typhoon, cyclone, etc.)

—A tropical cyclone with winds that have reached the speed of 75 MPH (119 km/h).

Latent heat

—The heat given off when water vapor condenses to form liquid water.

Midlatitudes

—The portion of the earth's surface midway between the tropics and the polar regions lying about 3565° north or south of the equator.

Pressure gradient force

—The force that pushes air from regions of higher pressure to regions of lower pressure.

Swell

—The rise of sea level near coastal areas due to the low barometric pressure; winds and wave activity of a tropical cyclone. Also called surge.

Tropical depression

—An early stage in the development of a hurricane, typhoon, or cyclone.

Tropical storm

—A tropical cyclone with wind speeds 37–75 MPH (60–120 km/h).

Tropics

—The region around Earth's equator spanning 23.5° north latitude to 23.5° south latitude.

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