17.5
Winnemucca,
Nevada 9.51 80 9.6
St. George, Utah 6.46 90 13.9
Fort Duchesne,
Utah 6.49 75 11.6
Pineville,
Oregon 9.01 70 7.8
Lost River,
Idaho 8.47 70 8.3
Laramie,
Wyoming 9.81 70 7.1
Torres, Mexico 16.97 100 6.0

To understand the methods employed for checking evaporation from the soil, it is necessary to review briefly the conditions that determine the evaporation of water into the air, and the manner in which water moves in the soil.

The formation of water vapor

Whenever water is left freely exposed to the air, it evaporates; that is, it passes into the gaseous state and mixes with the gases of the air.  Even snow and ice give off water vapor, though in very small quantities.  The quantity of water vapor which can enter a given volume of air is definitely limited.  For instance, at the temperature of freezing water 2.126 grains of water vapor can enter one cubic foot of air, but no more.  When air contains all the water possible, it is said to be saturated, and evaporation then ceases.  The practical effect of this is the well-known experience that on the seashore, where the air is often very nearly fully saturated with water vapor, the drying of clothes goes on very slowly, whereas in the interior, like the dry-farming territory, away from the ocean, where the air is far from being saturated, drying goes on very rapidly.

The amount of water necessary to saturate air varies greatly with the temperature.  It is to be noted that as the temperature increases, the amount of water that may be held by the air also increases; and proportionately more rapidly than the increase in temperature.  This is generally well understood in common experience, as in drying clothes rapidly by hanging them before a hot fire.  At a temperature of 100 deg F., which is often reached in portions of the dry-farm territory during the growing season, a given volume of air can hold more than nine times as much water vapor as at the temperature of freezing water.  This is an exceedingly important principle in dry-farm practices, for it explains the relatively easy possibility of storing water during the fall and winter when the temperature is low and the moisture usually abundant, and the greater difficulty of storing the rain that falls largely, as in the Great Plains area, in the summer when water-dissipating forces are very active.  This law also emphasizes the truth that it is in times of warm weather that every precaution must be taken to prevent the evaporation of water from the soil surface.

Temperature Grains of Water held in in Degrees F. One Cubic Foot of Air 32 2.126 40 2.862 50 4.089 60 5.756 70 7.992 80 10.949 90 14.810 100 19.790