Anticyclones

ANTICYCLONE IS A term for a region of closed circulation of air in the troposphere (the lowest 6 mi. [10 km.] or so of Earths atmosphere) with descending air from aloft and high atmospheric pressure at the surface. Anti-cyclonic circulation is usually accompanied by relatively high atmospheric pressure at the surface, and so the term high is often used interchangeably with anticyclone.

Anticyclones, as the name suggests, are the opposite of cyclones, which are regions of low pressure and typically associated with clouds, precipitation, and storms. On a surface weather map, meteorologists often denote anticyclones with a capital letter “H,” and cyclones by a capital “L.” Unlike cyclones, anticyclones are not usually associated with weather fronts. Anticyclones are sometimes referred to as ridges of high pressure. This is not quite accurate, as an anticyclone is a closed circulation and a ridge is not, but both are associated with regions of high surface pressure.

Anticyclones are typically characterized by descending air, high surface pressure, clear skies, and calm winds. In the mid-latitudes, anticyclones and [Page 66]cyclones are transient eddies steered by the westerly wind belt at the boundary between warm tropical and cold polar air masses. In subtropical and polar regions, anticyclones are semi-permanent features of the general circulation of the atmosphere, associated with some of the worlds major arid regions.

The descending air associated with an anticyclone suppresses convection, and causes the air to become warmer and drier through the process of adiabatic heating. As a result, the weather associated with an anticyclone is generally fair skies and clear weather, with relatively high pressure at the surface. The circulation of surface winds around an anticyclone rotation is outward from the center of high pressure, and is deflected by the Earths rotation. The result is a clockwise spiral outward from the center of high pressure in the Northern Hemisphere, and counterclockwise in the Southern Hemisphere. This difference is the result of the difference in the direction of motion deflection by the Coriolis effect in each hemisphere—to the left of motion in the Southern Hemisphere, and to the right in the Northern Hemisphere.

In the mid-latitudes of both hemispheres, cyclones and anticyclones are transient features that develop and dissipate as embedded eddies in the Rossby (or planetary) waves of the westerly wind belt, steered generally from west to east by the polar jet stream. Regions in which anticyclonic circulation is a primary, semi-permanent feature of climate (persistent and dominant), such as the subtropical highs, are associated with some of the world s great deserts such as the Sahara in Africa, the Atacama in Peru, and central Australia.

Historically, ocean regions at subtropical highs were described as the doldrums or horse latitudes, becalmed areas in which mariners might find themselves stranded at sea for extended periods. In extreme cases, mariners would be forced by food and water shortages to either eat or jettison the horses aboard, thus the horse latitudes. The polar regions are also areas in which persistent anticyclones, or highs, dominate general circulation. The Siberian High is an example of a semi-permanent, high-pressure region that significantly affects climate in the high northern latitudes of Europe.

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