MOISTURE IN AIR 125 



and the one in which the dew point was determined showed 

 that when heated air was cooled it deposited moisture. 



This property that air has of taking up a large amount 

 of water when heated and giving it out when cooled is the 

 cause of our clouds and rain. If it were not for this there 

 would be no circulation of moisture over the land, no rain, 

 and without rain there could be no vegetation and no 

 animal life. Thus this simple property of the air furnishes 

 the means for the support of practically all the animate life 

 on the earth. 



61. Moisture in the Atmosphere Experiment 74. Carefully 



weigh a dish of water and place it in a convenient place where there 

 is a free access of air. After some hours weigh it again. What causes 

 the change of weight? Try this experiment with a test tube, watch- 

 glass and a wide-mouthed beaker under various conditions and in 

 various places. 



The 'atmosphere at all tim.es and under all conditions 

 contains some moisture. When its temperature has been 

 raised, its capacity to hold moisture is increased, but at 

 no place is it so cold that it cannot contain a certain 

 amount of moisture. When water in the solid or liquid 

 condition is exposed to the air, it gradually disappears and 

 is taken up into the air. 



If the water surface is large and the temperature high, 

 there is a large amount of evaporation and the water 

 rapidly rises into the air. In the tropics the evaporation 

 from the water surface amounts to perhaps eight feet per 

 year. This means that the energy of the sun lifts about 

 five hundred pounds of water from every square foot of the 

 surface every year. In the polar latitudes the amount of 

 evaporation is perhaps a tenth of that in the tropics. 



From every water surface on the globe, however, a large 

 amount of water is evaporated each year. In many 

 places much of the water evaporated falls upon the same 



