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Grams Water Vapor: Thus, at normal sea level pressure, a kilogram of air can hold 22 grams water vapor of water vapor when the temperature is 80 °F., but can hold only one gram at a temperature of 0°F. When the air contains all of the water vapor it can hold, it is said to be saturated. For a particular sample of air at a given temperature, the ratio of the water-vapor pressure to that which would prevail if the air were saturated at the temperature is denned as the relative humidity, normally expressed in percentage terms.
When a sample of air rises, its pressure decreases, because above it there is progressively a lighter and lighter layer of air whose weight per unit area constitutes the atmospheric pressure.
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.
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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