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Cyclone and Anticyclone
A cyclone is a storm or system of winds that rotates around a center of low atmospheric pressure. An anticyclone is a system of winds that rotates around a center of high atmospheric pressure. Distinctive weather patterns tend to be associated with both cyclones and anticyclones. Cyclones (commonly known as lows) generally are indicators of rain, clouds, and other forms of bad weather. Anticyclones (commonly known as highs) are predictors of fair weather.
Winds in a cyclone blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Winds in an anticyclone blow just the opposite. Vertical air movements are associated with both cyclones and anticyclones. In cyclones, air close to the ground is forced inward toward the center of the cyclone, where pressure is lowest. It then begins to rise upward, expanding and cooling in the process. This cooling increases the humidity of the rising air, which results in cloudiness and high humidity in the cyclone.
In anticyclones, the situation is reversed. Air at the center of an anticyclone is forced away from the high pressure that occurs there. That air is replaced in the center by a downward draft of air from higher altitudes. As this air moves downward, it is compressed and warmed.
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 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.
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.
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).
How Cyclones Work
Cyclones are areas of low pressure. Since air moves from areas of high pressure to low pressure, cyclones produce a convergence at the surface. This converging air is forced upwards into the atmosphere, creating a divergence aloft. As warm, moist air is sucked into the low and forced aloft, it produces an unstable atmosphere. This warm, moist air cools, condenses and forms storm clouds. Cyclones can be tropical in nature, such as a hurricane, or a low-pressure system over a land mass, such as the United States. Cyclones spin in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Effects of Cyclones
In general, cyclones are associated with clouds, rain and thunderstorms. They produce steep pressure gradients, creating strong surface winds. Over the United States, cyclones will draw in warm, moist air from the Gulf of Mexico, creating a warm front. This generally produces light, steady rain to the northeast of a low, ahead of the warm front. Cyclones also draw in cold air from the north. This colder air forms a cold front, which collides with the warm, moist air to produce showers and thunderstorms to the southeast of a low, ahead of the cold front.
How Anticyclones Work
Anticyclones are areas of high pressure. The sinking air spreads out when it reaches the ground, producing a divergence at the surface. Aloft, air rushes in to fill the void, creating a convergence aloft. Anticyclones produce a stable atmosphere. Anticyclones, or highs, are also referred to as blocking highs because they tend to force areas of low pressure to travel around them. For example, a hurricane (tropical cyclone) that encounters an area of high pressure will be deflected around the cyclone. Blocking highs have spared the East Coast of the United States from many hurricane strikes, pushing them out over the Atlantic Ocean. Anticyclones spin in a clockwise direction in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
Effects of Anticyclones
In general, anticyclones are associated with fair weather. As the air sinks, it warms and dries. This produces clear skies and increases the air's ability to transmit radiant energy. In the summer, this means high temperatures due to solar heating of the surface. During the winter, this means low temperatures due to the radiation of heat from the surface into space. Cyclones typically have low-pressure gradients, producing light, variable winds at the surface. Cyclones tend to be slow movers, providing extended periods of fair weather. During the summer and fall, a Bermuda High can establish itself off the eastern coast of the U.S. for long periods of time, producing high temperatures in the Southeast and blocking hurricanes.
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