anticyclone(redirected from anticyclones)
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anticyclone,region of high atmospheric pressure; anticyclones are commonly referred to as "highs." The pressure gradient, or change between the core of the anticyclone and its surroundings, combined with the Coriolis effectCoriolis effect
[for G.-G. de Coriolis, a French mathematician], tendency for any moving body on or above the earth's surface, e.g., an ocean current or an artillery round, to drift sideways from its course because of the earth's rotation.
..... Click the link for more information. , causes air to circulate about the core in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. Near the surface of the earth the frictional drag of the surface on the moving air causes it to spiral outward gradually toward lower pressures while still maintaining the rotational direction. This outward movement of air is fed by descending currents near the center of the anticyclone that are warmed by compression as they encounter higher pressures at lower altitudes. The warming, in turn, greatly reduces the relative humidity, so that anticyclones, or "highs," are generally characterized by few clouds and low humidity. Such weather characteristics may extend over an area from a few hundred to a few thousand miles wide. Many low-level anticyclones are swept generally eastward by the prevailing west-to-east flow of the upper atmosphere, usually traversing some 500 to 1,000 mi (800–1,600 km) per day. Other anticyclones are permanent or seasonal features of particular geographic regions. The term anticyclone is derived from the fact that the associated rotational direction and general weather characteristics of an anticylone are opposite to those of a cyclonecyclone,
atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effect, causes air to circulate about the core of lowest pressure in a counterclockwise direction
..... Click the link for more information. .
a region in the atmosphere characterized by increased atmospheric pressure. The pressure distribution in anticyclones is represented on charts by concentric closed isobars (lines of equal pressure) of an irregular, approximately oval shape. The highest pressure is at the center of the anticyclone; it decreases toward the periphery. The pressure at the center of an anticyclone at sea level reaches 1,025–1,040 millibars (mbar), and sometimes (for example, in Asia during the winter) as high as 1,070 mbars. (Average pressure at sea level is 1,010–1,015 mbar; 1,000 mbar ≈ 750 mmHg ≈ 1.02 kg force per sq cm.)
Along with cyclones, anticyclones are formed continuously in the troposphere (the lower part of the atmosphere). Both are part of the general atmospheric circulation, creating inter-latitudinal exchange of air masses. During the year many hundreds of anticyclones develop in each hemisphere. The duration of an anticyclone is several days; in some cases, even weeks. Both cyclones and anticyclones move in the direction of the general exchange of air in the troposphere—that is, from west to east—and in the process are deflected toward the lower latitudes. The average speed of movement of anticyclones is approximately 30 km/hr in the northern hemisphere and approximately 40 km/hr in the southern; however, anticyclones frequently remain almost stationary. In the northern hemisphere, the winds in an anticyclone move in a clockwise direction around the center, and in the southern hemisphere, in a counterclockwise direction, forming a gigantic whirlwind. The diameter of anticyclones is on the order of thousands of km.
Above the so-called friction layer—that is, above 1,000 m on the average—the wind in the anticyclone blows almost entirely along the lines of the isobars; however, in the friction layer the wind is deflected out from the isobars—at the earth’s surface, at an angle of approximately 30°. This flow of air from the anticyclone in the lower layer is accompanied by an influx of air into the anticyclone in the upper layers of the atmosphere and by its gradual settling and subsiding of the anticyclone. In subsiding, the air is heated adiabatically and becomes more unsaturated. Therefore the temperature of the troposphere in an anticyclone increases (only during the winter immediately above the land surface can the temperature be very low) and there is little overcast, and as a rule, no precipitation. The winds in the interior of the anticyclone are weak; however, they increase at the periphery.
As the degree of development and the temperature of the anticyclone increase, so does its height: the closed isobars appear in increasingly higher layers of the troposphere, and even in the lower stratosphere. The stratosphere in anticyclones begins at a higher altitude than in cyclones, and its temperature is lower.
S. P. KHROMOV