THE GLACIERS OF MOUNTAIN AND CONTINENT 269 



To a relatively small degree the atmosphere is heated by the 

 direct absorption of the sun's rays which pass through it. Since 

 air has weight, it compresses the lower layers near the earth, and 

 hence as we ascend from the earth's surface the air becomes con- 

 tinually lighter. Convection currents must, therefore, adjust 

 themselves by the air expanding as it rises. But expansion of a 

 gas always results in its cooling, as every one must have observed 



KILOMETERS 

 18 



17 

 '*- 

 15 



CENTIGRADE 

 -5 



-55 



-55 

 -55 



-55 



-55 

 -53 



.ISOTHERMAL 

 o R 



\ ADVECTIVE 

 ZONE 



_ Ce/7/ng o 



zone 



P N V E C T I V E 



D R Y - 15 XM I D D L E L AYE R 



-/o 



^a^%*y A P OR : ^f' 



:: 



sea 



FIG. 298. The zones of the lower atmosphere as revealed by recent kite and 

 balloon explorations. 



who has placed his finger in the air current which escapes from 

 the open valve of a warm rubber tire. Dry air is cooled a degree 

 Fahrenheit for every six hundred feet of ascent in the atmos- 

 phere. At a height of about seven miles above the earth's sur- 

 face all rising air currents have cooled to about 68 below the 

 zero of the Fahrenheit scale, and exploration with balloons has 

 shown that the currents rise no farther. At this level they 



