The Weather 53 



during the day, to a point at which the water vapor in the air 

 immediately surrounding them is condensed and deposited as 

 dew. Dew may be deposited in daytime or at night ; but it 

 is formed more frequently at night when the sky is clear and 

 the air quiet, or only slightly moving. In experiments to deter- 

 mine the dew point, it is found that the temperature at which 

 condensation occurs depends upon the relative humidity of 

 the air. The more moisture the air contains, the higher rela- 

 tively is the temperature at which dew is formed. The less 

 the amount of moisture, the lower is the dew point. This 

 accounts for the very heavy dews in regions near bodies of 

 water, as along the Pacific coast, where in summer the moist 

 sea breezes blow. It also explains the very limited dew de- 

 posits inland. 



Land and vegetation lose heat much more rapidly than air 

 and other gases generally. Thus vegetation, such as grass, 

 which holds but little heat, loses it by radiation soon after 

 sunset. When grass is thus cooled, it chills the air in imme- 

 diate contact with it, and causes its moisture to condense and 

 be deposited as dew. Stones and soil may have deposits of 

 dew, but less frequently, because they receive heat from below 

 as they radiate it from their surfaces, and therefore cool more 

 slowly to the dew point. Air cools still more slowly, which 

 explains why mists are formed later at night. Land radiates 

 as well as absorbs heat more rapidly than water. Make use 

 of this fact in explaining land and sea breezes ; also in ex- 

 plaining why air over large bodies of water may not be cooled 

 as frequently as that over land to the temperature at which 

 dew is deposited. 



Frost. If conditions favor the formation of dew and the 

 temperature is likely to fall to freezing or below during the 

 night, frost is to be expected. When the temperature of the 

 land and of the air near the land surface is at or below freezing, 

 the condensed moisture particles are frozen to form small ice 

 crystals or frost. If the temperature is at 28 F. o.r lower when 



