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 perfectly comfort- 
