THE MOISTURE OF THE AIR 257 



average relative humidity for five months when the record was 

 kept was 23. The average relative humidity of air over the land 

 is probably about 60; that over the ocean about 85. The part of 

 our country which is productive, agriculturally, without irrigation, 

 is chiefly where the relative humidity is more than 65. 



Dew-point. If saturated air is cooled, some of its moisture is 

 condensed. The temperature at which the water vapor of the air 

 begins to condense is the dew-point. Saturated air is therefore 

 at the dew-point. The temperature of the dew-point is not fixed, 

 but is influenced by the amount of water vapor in the air. When 

 this amount is large, the temperature of the dew-point is relatively 

 high; when the amount is small, the temperature of the dew-point 

 is relatively low. 



Air may be brought to the dew-point in various ways: (1) It 

 may be blown where the temperature is lower, as up to a higher 

 latitude or altitude; (2) it may be cooled by having cooler air 

 brought to it/as by a cold wind; (3) it may be cooled by radiation, 

 or (4) by expansion, as when it rises. 



Condensation. When the temperature of saturated air is 

 reduced, some of the water vapor is condensed. If the tempera- 

 ture of condensation is above 32, the vapor condenses into liquid 

 water, which at first takes the form of little droplets, such as those 

 of which fog is made. If the temperature of condensation is less 

 than 32, the water becomes solid (crystallizes) as it condenses, and 

 takes the form of ice particles. These ice particles may be the 

 beginnings of snowflakes, or they may be particles of frost. 



The condensation of water vapor sets free an amount of heat 

 equal to that absorbed in its evaporation. 



Fog, frost, and cloud. The water droplets made by the conden- 

 sation of water vapor make fog (Fig. 1, PL XLIV, p. 256) if the 

 condensation takes place in the lower part of the atmosphere at a 

 temperature of more than 32 F., and the form of frost if the tem- 

 perature is less than 32 F. The water droplets and ice particles 

 take the form of clouds if the condensation takes place above the 

 bottom of the atmosphere (Fig. 2, PL XLIV, p. 256). Fog and 

 frost in the air are the same as clouds, except that the clouds are 

 higher. Fog may be said to be cloud resting on the surface of the 



