TEMPERATURE AND EVAPORATION 135 



Such saturation is at the best only local, as, for in- 

 stance, on the seashore during quiet days, when the 

 layer of air over the water may be fully saturated, 

 or in a field containing much water from which, on 

 quiet warm days, enough water may evaporate to 

 saturate the layer of air immediately upon the soil 

 and around the plants. Whenever, in such cases, 

 the air begins to move and the wind blows, the 

 saturated air is mixed with the larger portion of 

 unsaturated air, and evaporation is again increased. 

 Meanwhile, it must be borne in mind that into a layer 

 of saturated air resting upon a field of growing plants 

 very little water evaporates, and that the chief water- 

 dissipating power of winds lies in the removal of this 

 saturated layer. Winds or air movements of any 

 kind, therefore, become enemies of the farmer who 

 depends upon a limited rainfall. 



The amount of water actually found in a given 

 volume of air at a certain temperature, compared 

 with the largest amount it can hold, is called the rela- 

 tive humidity of the air. As shown in Chapter IV, 

 the relative humidity becomes smaller as the rainfall 

 decreases. The lower the relative humidity is at 

 a given temperature, the more rapidly will water 

 evaporate into the air. There is no more striking 

 confirmation of this law than the fact that at a tem- 

 perature of 90° sunstrokes and similar ailments are 

 reported in great number from New York, while 

 the people of Salt Lake City are j^erfectly comfort- 



