The air as a water absorbing medium
Water bodies cover more than four-fifths of the surface of the earth. Water is present as soil moisture and as liquid in plant tissue over most of the remainder of the earth’s surface. Water vapor is present in varying concentration everywhere in the lower atmosphere. The molecules that make up the water on the earth’s surface and the water vapor in the atmosphere are in ceaseless motion. Some of the molecules of the liquid acquire sufficient momentum to break through the surface and escape into the free air. At the same time some of the molecules of water vapor in the air strike the water surface and are unable to leave it. Strictly speaking, the movement of molecules from water to air is evaporation and the reverse movement is condensation. Popularly, evaporation is considered as a net loss of mass from a liquid and condensation as a net gain to a liquid as a result of this exchange (Thornthwaite and Holzman 1942).
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