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CYCLONES ARE METEOROLOGICAL systems consisting of areas of low pressure where the winds in the Northern Hemisphere and Southern Hemisphere rotate counter-clockwise and clockwise, respectively. The biggest storms around the world, such as cyclones, are intensifying. It is reported that the number of category four and five tropical cyclones, which are very intense, have increased over time. One reason for this is the impact of global warming, in particular, the effect of increased sea-surface temperature. Cyclones are often subdivided into other types, which include tropical cyclones and extratropical cyclones. Other types of cyclones include polar, polar low, subtropical, and mesoscale cyclones.

Tropical cyclones are low-pressure systems consisting of large rotating systems of clouds and winds, which occur over tropical and subtropical waters. They also have thunderstorm activity and cyclonic surface wind circulation. Depending on their location and strength, there are various terms by which tropi

cal cyclones are known, such as hurricane, typhoon, tropical storm, cyclonic storm. and tropical depression. These cyclones have no fronts and the center of the storm is warmer than the surrounding air. They are known as tropical storms if the wind reaches 56 ft. (17 m.) per second and hurricanes if the winds reach 108 ft. (33 m.) per second. Hurricanes occur in the North Atlantic Ocean and the northeast Pacific Ocean, east of the dateline. There are several environmental conditions favoring tropical cyclones, such as warm ocean waters, winds that do not vary greatly with height, and a minimum distance of 311 mi. (500 km.) from the equator. There are seven tropical cyclone basins where storms occur on a regular basis. These are the Atlantic basin, Northeast Pacific and Northwest Pacific basins, North Indian basin, South west Indian basin, Southeast Indian/Australian basin, and Australian/Southwest Pacific basin.

Extratropical cyclones, sometimes called mid-latitude cyclones, are storms that form outside the tropics, sometimes as a tropical storm or hurricane changes, such that they have neither tropical nor polar characteristics. They form where cold and warm air masses come into contact with each other, and are formed in middle or high latitudes, in frontal zones. The center of the storm is generally colder than the surrounding air.

A subtropical cyclone is a non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Compared with tropical systems, they have a broad zone of maximum winds located further from the center. They also have a less symmetric wind field and distribution of convection.

They can be formed in a wide band of latitude, from the equator to 50 degrees both North and South. A subtropical cyclone is known as a depression if the winds reach wind speed of 38 mi. (62 km.) per hour or less and as a storm if the speed reaches 39 mi. (63 km.) per hour or more. Many times, these subtropical storms transform into true tropical cyclones.

There is another type of subtropical cyclone known as a mesoscale. This is a cyclonic vortex of air with an average diameter of 1-6 mi. (2-10 km.) within a con-vective storm. They are often associated with super-cells, which are rare thunderstorms with a rotating updraft (current of rising air).

Some supercell thunderstorms can maintain themselves for several hours and are responsible for many extreme weather events, such as tornadoes. There are many other characteristics that are associated with supercell thunderstorms, such as a flanking line (a line of cumulus clouds) and a thunderstorm with back-shaped anvil (the flat top of a cumulonimbus cloud; back-shaped anvils have high severe weather potential).

A mesocyclone is an area of vertical atmospheric rotation, typically 2-6 mi. (3-8 km.) across. Meso-cyclones are formed when change in wind speed or direction occur dramatically, with changes in height; this sets the lower part of the atmosphere spinning horizontally. Polar cyclones are low-pressure systems in which the air circulates counterclockwise in the Northern Hemisphere.

Usually, polar cyclones range from 60-600 mi. (96965 km.) across, with surface winds of30-100 mi. (48160 km.) per hour. Polar cyclones may occur through any time of the year, however, as they usually occur in areas with little or no population, they are viewed as less destructive than other types of cyclones.

SEE ALSO: Atmospheric General Circulation Models; Hurricanes and Typhoons; Thunderstorms; Weather; Wind Driven Circulation.

BIBLIOGRApHY. F.H. Ludlam, The Cyclone Problem: A History of Models of the Cyclonic Storm: Inaugural Lecture 8 November 1966: London University (Imperial College of Science and Technology, 1967).

Farhana Akter King's College, London

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