GENERAL CIRCULATION OF THE ATMOSPHERE 273 



IX. THE GENERAL CIRCULATION OF THE ATMOSPHERE 



298. The General Circulation of the Atmosphere. The gen- 

 eral circulation is caused by the unequal heating of the earth 

 by the sun. The tropics receive most heat in a year; this 

 makes the lower air warmest, and therefore lightest, near the 



FIG. 201. 



equator. Because of this, it is crowded upward by the cooler, 

 heavier air on both sides, and spreads out above. Currents are 

 thus set up in the atmosphere somewhat like those in the heated 

 water pan in Fig. 201. If the earth did not rotate, the lower 

 winds from each side would blow straight toward the equator 

 and the upper winds would blow directly away from the equa- 



Ir EARTH DID nor ROTATE On ROTATING 



--- CUPPER. WiriDS -GROIJIiD 



FIG. 202. Principal features of the general circulation of the atmosphere. 

 A. If the earth did not rotate. B. As it is on the rotating earth. 



tor. But the rotation of earth on its axis turns all winds to- 

 ward the right in the northern hemisphere and toward the left 

 in the southern. That causes the permanent wind directions 

 shown on the world map, Fig. 202 B. It also causes the out- 

 flowing winds of a high pressure area to curve spirally outward 

 toward the right (in northern hemisphere), instead of blowing 



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