StfOW DEW. 



33 



of which it receives less and less the further it is from the 

 earth, so that at a distance (depending upon the situation 

 and climate) ranging from 3,500 to 17,000 feet from the 

 surface, the air is so cold that it is called " the region of 

 perpetual snow," and all mountains whose tops reach above 

 this altitude are covered with snow (formed by the freezing 

 of the rain and vapour) from this point to their summits 

 (fig. 4), and the sun (although it nearly always shines there, 



FIG. 4. 



these regions being above most of the clouds) is not able to 

 melt it, for the radiation of heat from its surface is greater 

 than its absorption from the sun's rays, white substances 

 absorbing but very slowly, and rough surfaces (as the snow) 

 radiating very rapidly. 



Dew is caused by condensation of the watery vapour held 

 in the air ; when the sun goes down the radiation continues 

 from the surface of the earth, and those surfaces which 

 radiate most rapidly, such as fields of grass, get cooled 

 down below the temperature of the surrounding air, and 



