CHAPTER XVI 

 GENERAL CIRCULATION OF THE ATMOSPHERE 



Inequalities of atmospheric pressure produce 

 unequal heating, which produces unequal pressure, is going on all 

 the time, inequalities of pressure are being renewed constantly. 

 It follows that winds are always blowing. This results in a gen- 

 eral circulation of the atmosphere, the movement being always from 

 a region of greater pressure to one of less pressure, or in other words, 

 down an isobaric slope. 



Causes of Winds 



The general effect of unequal insolation. If the air over the 

 whole earth were quiet at a uniform, low temperature, and if it 

 could then be heated by the sun for a time without any horizontal 

 movement, the effect would be to raise its surface everywhere, 

 and to raise it most where it was heated most, that is, in the low 

 latitudes (Fig. 229). Under these conditions there would be a 

 barometric slope from low latitudes toward high latitudes. Be- 

 fore horizontal movement began, there would be no change of pressure 

 at the bottom of the air, for the same amount (mass) of air would 

 lie over each place, as before the heating. But if the surface of 

 the air had the form shown by the dotted line in Fig. 229, the upper 

 air would move as shown by the arrows. Since the air in low lati- 



90 90 



Fig. 229. The lower line may be taken to represent the surface of the litho- 

 sphere, the upper full line, the upper surface of the atmosphere as it would 

 be if the temperature were low, and everywhere equal. The dotted line 

 at the top shows the effect of heating on the surface of the air. The 

 heating raises the surfaces everywhere, but most in low latitudes. Move- 

 ment would result as indicated by the arrows. 



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