WEATHER 



157 



convection currents are produced which distribute the 

 heated portions throughout the whole atmosphere. 



The air is also heated by compression and by precipi- 

 tation. We have learned that air becomes heated when 

 compressed, as in the chamber of a bicycle pump. As air 

 descends from the higher altitudes, as from the mountain 

 sides to the valleys below, it is compressed and warmed. 

 As it ascends it expands and becomes colder, about 1 C. 

 for every 550 feet of ascent. This heating of the air by 



~12 2 4 6 8 10 IZ Z 4 6 8 1O 12 



O'CLOCK NOON MIDNIGHT 



MIDNIGHT 



FIG. 133. Temperature Curve Showing Variation in Temperature. 



compression and cooling by expansion is known as adia- 

 batic heating and cooling. 



Air is being warmed in areas of high pressure and cooled 

 in areas of low pressure, and it may seem at first as though 

 the air should be warmer in areas of high pressure than in 

 areas of low pressure. This is not the case. The reason 

 for the low pressure is that the area is warmer ; hence the 

 air is lighter. Although the expansion of air is a cooling 

 process, the effect is more than counteracted by the heat 



