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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." A Dictionary of Earth Sciences. . Retrieved December 13, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/tropical-cyclone

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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