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The Highs and Lows of Weather: Part 1 - The High - Page 2

© Keith C. Heidorn

In the Northern Hemisphere, the Coriolis effect causes the wind to turn to the right (to the left in the Southern Hemisphere) as it blows over the earth. Thus, in the ideal situation, winds would circle the high pressure cell in a clockwise manner (known as anticyclonic because it is opposite in direction to flow around a cyclone/low pressure cell) as the Coriolis force balances the pressure gradient force (known as geostrophic flow).

Over the oceans and flat land areas (mountains are great complicators of weather patterns), this balance holds quite well above the surface layer. Within the surface layer, however, friction also plays a role by slightly altering the balance of pressure gradient and Coriolis forces to establish a new balance known as gradient flow. Gradient flow winds cross the pressure gradient (seen as isobars on a weather map) such that the air flows out from the high pressure cell.

Following formation, most Highs are generally elliptical in shape, and often large and sprawling. But as they interact with other air masses and topography, and are distorted by forces of the upper atmosphere, high pressure cells often become long and narrow in shape. When plotted on a surface weather map, these elongated pressure patterns resemble mountain ridges on terrain maps. Meteorologists therefore refer to them as high pressure ridges or simply ridges.


Because the air in a High sinks through a deep layer of atmosphere, the formation of clouds and precipitation are greatly inhibited. Generally if clouds do form, they are spawned by strong solar heating of the surface and are small and of limited vertical extent - usually fair weather cumulus (cumulus humilis). If the air of the High is humid and very warm, such as found in tropical air masses, and solar heating is strong, isolated, weak, isolated showers may form within the High, usually along the cell's outer edges. Otherwise, unless topography throws a "wild card" into play, high pressure regions are generally precipitation free.

Next time, we will look into Low Pressure systems, a quite opposite element.

Copyright 2002, Keith C. Heidorn, All Rights Reserved. (Illustrations courtesy of Spectrum Educational Enteprises, ©2002.)

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