Water begins its never-ceasing cycle as vapor in the atmosphere. This vapor in the atmosphere, as well as the water in the lakes and oceans, provides protection against extremes of both heat and cold.

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Hydrologic cycle. This term, or the more common one "water cycle," refers to the complete sky-to-earth-to-sky circuit pursued by water in nature. It includes water's precipitation as rain, snow, hail, or dew; its journey over, around, and through obstacles above, on, and below the earth's surface and its eventual evaporation and return to the atmosphere. It is the largest water purification system known to man.

Scientists estimate that the sun converts matter into energy at the rate of 250 million tons per minute. Even though the earth receives only a token portion of this heat energy (less than one two-billionth part), everything here would burn to a crisp were it not for the fact that the water above and on the earth in large bodies of water absorbs most of the heat.

In a large desert, for example, there is but a small amount of water. Consequently, there are wide extremes in the heat. The Sahara Desert typifies this condition. There under the sun's penetrating rays temperatures rise to 125 during the daytime and fall below freezing at night.

In the atmosphere the various substances do not combine chemically. Instead, each retains its own characteristic properties.

The make-up of the atmosphere. The composition of the troposphere (the layer closest to the earth) has been calculated to be nitrogen, 78.09 percent; oxygen, 20.95 percent; argon, 0.93 percent; carbon dioxide, 0.03 percent; together with minute amounts of neon, krypton, helium, hydrogen, xenon and ozone. In addition to these gases, the atmosphere contains varying percentages of water vapor. About 9/10 of the mass of the atmosphere lies within ten miles of the earth's surface. The above figures are composition by volume.

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