232 



PHYSIOGRAPHY 



further inflow below. If the heating continues a permanent con- 

 vection current will be established in the heated area (Fig. 212). 



The atmosphere is heated (1) by the absorption ' of the sun's 

 rays as they come through it, and (2) by the absorption of heat 

 radiated from the land and water after they have been warmed by 



Fig. 211. The first rise of air, as a result of heating, is due to the expansion 



of the part heated. 



the absorption of heat radiated from the sun. The amount of heat 

 absorbed by the air from the direct rays of the sun depends on the 

 distance the rays travel in the atmosphere, that is, on the obliquity 

 of the sun's rays (Fig. 210). It is therefore different in different 

 latitudes. When the sun is vertical at the equator, the sun's rays 

 pass through about twice as much atmosphere in latitude 60, 



Fig. 212. The permanent heating of the air over a given region gives rise 

 to permanent convection currents. The numbers on the arrows indi- 

 cate the order in which the several movements start. 



nearly three times as much in latitude 70, and about ten times as 

 much in latitude 85, as they do in latitude 0. In latitude 70, 

 about half as much heat reaches the surface of the land from the 

 sun as in latitude 0, when the sun is vertical there. 



The heat radiated into the air from below is absorbed more 

 readily by the air than that coming from the sun. The atmos- 

 phere is therefore heated by radiation from below more than by 

 direct insolation, and the lower air is heated more than the upper 



