RAIN, SNOW, FROST, DW 37 



hovering near the earth's surface, or rising far above the 

 level of the mountains, it does not remain there permanently. 

 When this vapor meets a cold wind or is chilled in any way, 

 condensation takes place, and a mass of tiny drops of water 

 or of small particles of snow is formed. When these drops 

 or particles become large enough, they fall to the earth as 

 rain or snow, and in this way the earth is compensated for 

 the great loss of moisture due to evaporation. Fog is formed 

 when vapor condenses near the surface of the earth, and when 

 the drops are so small that they do not fall but hover in the 

 air, the fog is said ''' not to lift " or " not to clear." 



If ice water is poured into a glass, a mist will form on the 

 outside of the glass. This is because the water vapor in the 

 air becomes chilled by contact with the glass and condenses. 

 Often leaves and grass and sidewalks are so cold that the 

 water vapor in the atmosphere condenses on them, and we 

 say a heavy dew has formed. If the temperature of the air 

 falls to the freezing point while the dew is forming, the vapor 

 is frozen and frost is seen instead of dew. 



The daily evaporation of moisture into the atmosphere 

 keeps the atmosphere more or less full of water vapor; but 

 the atmosphere can hold only a definite amount of vapor at 

 a given temperature, and as soon as it contains the maximum 

 amount for that temperature, further evaporation ceases. If 

 clothes are hung out on a damp, murky day they do not 

 dry, because the air contains all the moisture it can hold, 

 and the moisture in the clothes has no chance to evaporate. 

 When the air contains all the moisture it can hold, it is said 

 to be saturated, and if a slight fall in temperature occurs 

 when the air is saturated, condensation immediately begins 

 in the form of rain, snow, or fog. If, however, the air is 

 not saturated, a fall in temperature may occur without 

 producing precipitation. The temperature at which air is 



