CHAPTER XIV 

 THE MOISTURE OF THE AIR 



The atmosphere always contains water in the form of vapor. 

 This is true even in the desert, where the air seems driest. We 

 cannot see or smell or feel water vapor, though air with much 

 water vapor has a different feeling from air with little. 



The presence of moisture in the air may be proved in various 

 ways. If a pitcher of ice-water stands in a warm room, drops of 

 water often appear on the outside of it. This water came from the 

 air. Water vapor sometimes condenses into water in the air, and 

 the water then becomes visible as clouds from which rain may fall. 



Water vapor is lighter than dry air. That is, a cubic foot of 

 it weighs much less than a cubic foot of dry air at the same temper- 

 ature and under the same pressure. The water vapor of the air 

 displaces some of the oxygen and nitrogen, and its presence makes 

 the air lighter. 



Function of atmospheric moisture. The moisture of the at- 

 mosphere is of great importance to all animals and plants, for 

 without it no life could exist. It furnishes the rain and the snow 

 which supply all springs and rivers, and it serves a most important 

 function in connection with temperature, as already indicated, 

 for it absorbs heat radiated from the sun and from the earth. It 

 increases the average temperature at the bottom of the atmosphere, 

 and reduces the extremes of heat and cold which would exist if the 

 air were altogether dry. 



Sources of water vapor : evaporation. Water left standing in 

 an open dish disappears presently, and muddy roads and wet 

 streets soon become dry when the rain ceases. We conclude, 

 therefore, that the water vapor is passing constantly from all moist 

 surfaces into the air. The change of liquid water into water vapor 

 is evaporation. Evaporation also takes place from snow and ice, 



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