TEMPERATURE OF THE AIR 239 



of the sun, and it holds less of the heat radiated from the earth 

 below. 



The temperature over land 10,000 feet high is warmer than 

 the temperature of the air 10,000 feet above low land. A land 

 surface at a high altitude may be heated quite as much by the sun 

 as one at a low altitude, and it then heats the air above it. But 

 since the air over the high land is thin, it does not hold the heat 

 radiated from the land so well as denser air would. The result is 

 that both land and air cool faster in high altitudes than in low alti- 

 tudes, and this makes their average temperature less. 



Isolated elevations like mountain-tops are colder than plateaus 

 of the same elevation, because they are so well exposed to cooling. 



Fig. 214. Diagram to show that the sun's rays may fall less obliquely on a 

 mountain slope than on the plain adjacent. Under these circumstances 

 they have greater heating power, so far as the surface of the land is con- 

 cerned, on the mountain than on the plain. 



In sunny days in summer, the sunny side of a mountain free from 

 snow gets very warm (Fig. 214). If the rock is bare it may become 

 so hot that the hand cannot be held on it. If the air remained 

 long in contact with such a rock surface it would be warmed to the 

 same temperature; but it is commonly moved on quickly by winds 

 before it gets very warm, and its place is taken by colder air. 

 Again, an isolated mountain-peak radiates heat in all directions 

 except downward, while a flat surface radiates it upward only. 

 In mountains, too, there are likely to be many cloudy days, and 

 the clouds shield the rocks from the sun. This tends to lower the 

 average temperature of the mountain, as compared with that of 

 low land. 



Where mountains are covered by snow throughout the year, 

 their surfaces are never warmed above a temperature of 33 F., the 

 melting temperature of snow. All the heat received beyond that 

 necessary to raise them to this temperature is spent in melting and 



