The term cyclone, in common use, is sometimes applied to a tornado. In the science of meteorology, however, the term has a different meaning. For meteorologists, a cyclone — and its counterpart, an anticyclone — is a large-scale system of air circulation in the atmosphere in the zones between the equator and either of the poles. It can be considered as either producing or resulting from differences in air pressure in those zones. In a cyclone the central air pressure is lower than that of the surrounding environment, and the flow of circulation is clockwise in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. Cyclones are also characterized by low-level convergence and ascending air within the system.

An anticyclone system has characteristics opposite to that of a cyclone. That is, an anticyclone's central air pressure is higher than that of its surroundings, and the airflow is counterclockwise in the Southern Hemisphere and clockwise in the Northern Hemisphere. Anticyclones are usually characterized by low-level divergence and subsiding air.

Semipermanent Systems. Semipermanent cyclone systems rarely vary during a season. One example is the Bermuda High in the northern subtropical region. Others include the Siberian High and the Aleutian Low, which dominate winter in the middle and high latitudes of Asia and North America.

The subtropical high-pressure belts in the atmosphere coincide with the descending legs of the air-circulation mechanisms known as Hadley cells. Subsiding air heats the atmosphere by adiabatic compression, producing an intense subsidence inversion within the first 2 km (1.2 mi) of the atmosphere. The inversion, characterized by an extremely warm layer in the atmosphere, forms a stable lid that creates air-pollution problems in many cities. These semi-permanent subtropical centers of high pressure develop as direct responses to surface-heating anomalies, such as those produced by the differential heating of continents and oceans or by variations in the sea's surface temperature. Due to the effect of the Hadley cell, the subtropics remain at a fairly high pressure throughout the year. The centers change intensity and adjust their longitudinal position, however, to compensate for changing temperature and pressure gradients between land and ocean.

Surface-pressure anomalies develop at higher latitudes by similar processes. During summer, land areas are considerably warmer than adjacent oceans, producing rising air over the land and subsidence over the oceans. The resulting pressure gradient causes cool ocean air to flow toward the warm land surface. The Coriolis effect deviates this flow, producing cyclonic flow over the land and anticyclonic flow over the sea. During winter the situation is reversed. The land cools quickly, having little stored heat. Consequently high-pressure regions form over the land, while low-pressure regions dominate the ocean. With the clear atmosphere of the subsident region, the land surface can continue cooling. The loss of heat is compensated for by an increase of energy that flows into the system, as a warm airflow, from the oceanic low-pressure region. When the amount of energy radiated to space matches the inflow, an equilibrium is reached, but by that time a very deep high-pressure region has developed.

Transient Systems. The second cyclonic group consists of transient cyclones and anticyclones associated with weather systems. Located in the equatorial and middle latitudes, they may grow, mature, and decay within a few days.

Depressions in middle latitudes are cyclonic systems that develop rapidly and move eastward against the basic westerly flow, over distances from 500 to 2,000 km (30 to 1,200 mi). Central pressures often fall below 990 millibars (mb). Inclement weather, strong winds (connected to the high-pressure gradient), and squalls are associated with such mid-latitude systems, which result from basic instabilities of a heated and rotating atmosphere. Because of the Coriolis effect, the upper tropospheric flow toward the pole in the Hadley cell is forced eastward, developing strong westerlies. The air accelerates as it moves progressively poleward. Because the winds are produced by pressure gradients, which in turn are functions of the temperature distributions, zones of strong winds ought to be associated with strong temperature gradients. Were this situation to continue, the wind and temperature gradients would build up an infinite potential-energy reservoir. If such a system is perturbed, however, so that cold air moves equatorward across the gradient and warm air moves poleward, rapid changes will ensue.

As the light warm air overrides dense cold air and the latter undercuts warm air, a thermal circulation develops that taps the potential-energy store. The perturbation continues to grow, effectively relaxing the north-south temperature gradient and reducing the speed of the intense westerlies. This process, called a baroclinic instability, is the cause of most middle-latitude depressions. Subsequent development continues to move warm air poleward and cold air equatorward, producing adjacent pools of warm and cold air. The resultant large east-west temperature gradient produces a pressure distribution that causes a cyclonic circulation around the low-pressure center and an anticyclonic flow around the high.

In the tropics, cyclonic systems known as tropical depressions may develop with central pressures less than 2 mb lower than the environment. Associated with periods of intense rain, these systems usually move westward. Those which intensify significantly (pressures falling below 950 mb) are called tropical cyclones or hurricanes. Because their horizontal scale is far less than that of their middle-latitude counterparts, the pressure gradient is tighter, resulting in more intense winds.

by P. J. Webster

Bibliography: Anthes, R., Tropical Cyclones (1982); Holton, J., Introduction to Dynamic Meteorology, 3d ed. (1992); Lutgens, F., and Tarbuck, L., The Atmosphere, 5th ed. (1991); Newton, C., and Holopainen, E., eds., Extratropical Cyclones (1990).