Chapter 13 — CIRCULATION OF THE ATMOSPHERE 



tops of mountains to elevations of about 10,000 

 feet. However, in the trade wind zone, such a3 

 at the Hawaiian Islands, precipitation amounts 

 increase only to about 3,000 feet and then 

 decrease gradually. Even with this decrease 

 in amounts, more rain is received at 6,000 

 feet than at sea level. 



OCEAN CURRENTS. — The ocean currents, 

 which are produced mainly by the prevailing 

 winds of the earth, exert a considerable effect 

 upon the climates of many regions of the earth. 

 Ocean currents will be discussed in detail along 

 with their climatic effects in chapter 16. 



SECONDARY CIRCULATIONS 



The general circulation is modified by the 

 distribution of land and water over the surface 

 of the earth, causing uneven heating and cooling 

 of the earth's surface, and also the changes 

 in heating which result from the change in 

 seasons. 



CENTERS OF ACTION 



When diagrams showing the distribution of 

 mean surface temperatures are compared with 

 those showing mean sea level pressures, (figs. 

 13-1 and 13-2) it can readily be seen that 

 the pressure belts of the primary circulation 

 are rarely continuous, but are broken up into 

 detached areas of high and low pressure cells 

 by the secondary circulation. The break corre- 

 sponds with regions showing differences in 

 temperature from land to water surfaces. These 

 cells which tend to persist in a particular area 

 are called centers of action; that is, they are 

 found at nearly the same location with somewhat 

 similar intensity during the same month each 

 year. 



There is a permanent belt of relatively low 

 pressure along the Equator and another deeper 

 belt of low pressure paralleling the coast of 

 the Antarctic Continent. Permanent belts of 

 high pressure largely encircle the earth, 

 particularly over the oceans, in both the 

 Northern Hemisphere and Southern Hemisphere, 

 with a number of centers of maximums about 

 30 to 35 degrees from the Equator. 



Seasonal Variations 



There are regions where the pressure is 

 predominantly low or high with the changing 



seasons. The pressure over land during the 

 winter is decidedly above the annual average 

 and decidedly below it during the summer. 



In the vicinity of Iceland, pressure is low 

 most of the time. The water surface is warmer 

 than the icecaps of Greenland and Iceland. The 

 Icelandic low is most intense in winter, when 

 the greatest temperature differences occur, but 

 it persists with less intensity through the 

 summer. The Aleutian low is most pronounced 

 when the neighboring areas of Alaska and 

 Siberia are snow-covered and colder than the 

 adjacent ocean. 



These lows are not a continuation of one 

 and the same cyclone. They are regions of 

 low pressure, where lows frequently form or 

 arrive from other regions to remain stationary 

 or move sluggishly for a time, after which they 

 pans on or die out and are replaced by others. 

 Occasionally these regions of low pressure are 

 invaded by traveling high-pressure systems. 



There are semipermanent high-pressure 

 centers over the Pacific to the west of 

 California and in the Atlantic between the African 

 coast and Azores. Pressure is also high, but 

 less persistently so, west of the Azores to 

 the vicinity of Bermuda. 



These subtropical highs reach their greatest 

 intensity during the summer, and the area over 

 which they extend is much greater and the 

 pressure higher in summer. In winter these 

 subtropical highs are found at lower latitudes 

 and migrate poleward with the onset of the 

 summer season and equatorward with the winter 

 season. 



The largest individual circulation cells in 

 the Northern Hemisphere are the Asiatic high 

 in winter and the Asiatic low in summer. In 

 winter, the Asiatic continent is a region of 

 strong cooling and therefore is dominated by 

 a large high-pressure cell. In summer, strong 

 heating is present, and the high-pressure cell 

 becomes a large low-pressure cell. This sea- 

 sonal change in pressure cells gives rise to the 

 monsoonal flow of the India- Burma area. 



Another cell which some consider to be a 

 center of action is the polar high. The polar 

 high in winter is not a cell centered directly 

 over the North Pole, but it appears to be an 

 extension of the Asiatic high and often appears 

 as a wedge extending from the Asiatic Continent. 

 The cell is displaced toward the area of coldest 



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