It is of course well understood that the atmosphere as a whole is never saturated with water vapor.  Such saturation is at the best only local, as, for instance, 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 relative 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 temperature of 90 deg sunstrokes and similar ailments are reported in great number from New York, while the people of Salt Lake City are perfectly comfortable.  In New York the relative humidity in summer is about 73 per cent; in Salt Lake City, about 35 per cent.  At a high summer temperature evaporation from the skin goes on slowly in New York and rapidly in Salt Lake City, with the resulting discomfort or comfort.  Similarly, evaporation from soils goes on rapidly under a low and slowly under a high percentage of relative humidity.

Evaporation from water surfaces is hastened, therefore, by (1) an increase in the temperature, (2) an increase in the air movements or winds, and (3) a decrease in the relative humidity.  The temperature is higher; the relative humidity lower, and the winds usually more abundant in arid than in humid regions.  The dry-farmer must consequently use all possible precautions to prevent evaporation from the soil.

Conditions of evaporation from from soils

Evaporation does not alone occur from a surface of free water.  All wet or moist substances lose by evaporation most of the water that they hold, providing the conditions of temperature and relative humidity are favorable.  Thus, from a wet soil, evaporation is continually removing water.  Yet, under ordinary conditions, it is impossible to remove all the water, for a small quantity is attracted so strongly by the soil particles that only a temperature above the boiling point of water will drive it out.  This part of the soil is the hygroscopic moisture spoken of in the last chapter.