CHAPTER 13 



CIRCULATION OF THE ATMOSPHERE 



For an understanding of large-scale motions 

 of the atmosphere, it is essential that the 

 Aerographer's Mate study the primary or 

 general circulation of the atmosphere as a 

 whole. 



The sun's radiation is the energy that sets 

 the atmosphere in motion, both horizontally and 

 vertically. The vertical motion is caused by the 

 rising and expanding of the air when it is warmed, 

 or the descending and contracting of the air 

 when it is cooled. The horizontal motion is 

 caused by differences of atmospheric pressure; 

 air moves from areas of high pressure toward 

 areas of low pressure. Differences of tempera- 

 ture, the cause of the pressure differences, are 

 due to the unequal absorption of the sun's radia- 

 tion by the earth's surface. Due to the relative 

 position of the earth with respect to the sun, 

 much more radiation is absorbed near the 

 Equator than at other areas, with the least 

 radiation being absorbed at or near the poles. 

 Consequently, the principal factor affecting the 

 atmosphere is incoming solar radiation, and its 

 distribution depends on the latitude and the 

 season. 



The differences in the type of surface; the 

 differential heating; the unequal distribution of 

 land and water; the relative position of oceans 

 to land, forests to mountains, lakes to sur- 

 rounding land, and the like, cause different 

 types of circulations of the air. 



First, there is the general or primary 

 circulation of the atmosphere. This explains 

 the circulation of the atmosphere as a whole 

 with little attention to details and minor 

 differences from time to time or place to place. 



Then, there are the secondary circulations. 

 These explain the various adjustments the 

 primary circulation makes to major differences 

 in heating which result from the distribution of 

 land and sea on a large, but not a global, scale. 



Finally, there are the various tertiary (third 

 order) circulations, which explain the adjustments 

 of the primary and secondary circulations to 

 strictly local differences in heating. 



GENERAL CIRCULATION 



The general circulation theory attempts to 

 explain the global circulation of the atmosphere 

 with some minor exceptions. Since the earth 

 heats unequally, the heat is carried away from 

 the hot area to a cooler one as a result of 

 the operation of physical laws. This global 

 movement of air which restores a balance of 

 heat on the earth is the general circulation. 

 Since heat is the first cause, the world tem- 

 perature distribution is discussed first. 



WORLD TEMPERATURE GRADIENT 



Temperature gradient is the rate of change 

 of temperature with distance in any given 

 direction at any point. World temperature 

 gradient refers to the change in temperature 

 that exists in the atmosphere from the Equator 

 to the poles. 



The change in temperature, or temperature 

 differential, which causes atmospheric circu- 

 lation can be compared to the temperature 

 differences produced in a pan of water placed 

 over a gas burner. As the water is heated, 

 it expands and its density is lowered. This 

 reduction in density causes the water to rise to 

 the top of the pan. As it rises, it cools and pro- 

 ceeds to the edges of the pan. It cools further and 

 sinks to the bottom, eventually working its way 

 back to the center of the pan where it started. 

 This process sets up a simple circulation 

 pattern due to successive heating and cooling. 



Ideally, the air within the troposphere may 

 be compared to the water in the pan. The most 



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