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Amount Of Water Vapor:
The Formation of Rain.—The atmosphere consists of a mixture of-gases of which several, notably nitrogen and oxygen, exist in constant proportions. Among the gases which vary in proportion is water vapor, the atmospheric constituent having the most direct bearing on rainfall. By weight, the percentage of water vapor in the atmosphere occasionally exceeds 2 per cent; it is also found to be less than %o per cent at times. It has been determined that the capacity of air to hold water vapor decreases as the temperature becomes lower.Relative humidity is the amount of water vapor in the air, stated as a percentage of the maximum possible amount at that temperature. Because this maximum amount decreases with decreasing temperature, relative humidity increases as temperature falls even though the actual amount of moisture is unchanged. When the relative humidity reaches 100%, and the air is further cooled, some moisture condenses into visible forms. The temperature at which this occurs is the dew point.
Water power thus produced is renewable I natural processes. Evaporation takes place fro the oceans and other water surfaces, adding I the water vapor content in the air. The air an water vapor is moved, both horizontally an vertically, over the earth's surface. Under a] propriate conditions the water vapor becomi separated from the air in the form of rain, snov or ice and, under the action of gravity, falls I earth. When this takes place in uplands or in tl:mountains, water collects at elevated locations and proceeds to flow back to the ocean through a network of rivers, using up its potential energy in soil erosion and the production of heat through turbulence and viscous action. By construction of a water power plant or hydroelectric plant, a portion of this potential energy may be made available in controlled form.
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