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It is the lowermost atmospheric layer, the troposphere that is of most direct importance to man in his environment at the bottom of the atmosphere. Almost all phenomena of weather and climate that physically affect man take place within the troposphere.
In addition to pure dry air, the troposphere contains water vapour, a colorless, odorless gaseous form of water, which mixes perfectly with the other gases of the air. The degree to which water vapour is present is designated as the humidity and is of primary importance in weather phenomena. Water vapour can condense into clouds and fog. If condensation is excessive, rain, snow, hail, or sleet, collectively termed precipitation, may result. Where water vapour is present only in small proportions, extreme dryness of air typical of the hot deserts results. There is, in addition, a most important function performed by water vapour. Like carbon dioxide, it is capable of absorbing heat, which penetrates the atmosphere in the form of radiant energy from the sun and earth. Water vapour gives to the troposphere the qualities of an insulating blanket, which prevents the rapid escape of heat from the earth's surface.
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The troposphere contains myriads of tiny dust particles, so small and light that the slightest movements of the air keep them aloft. They have been swept into the air from dry desert plains, lakebeds and beaches, or explosive volcanoes. Strong winds blowing over the ocean lift droplets of spray into the air. These may dry out, leaving as residues extremely minute crystals of salt, which are carried high into the air. Forest and brush fires are yet another important source of atmospheric dust particles. Countless meteors, vaporizing from the heat of friction as they enter the upper layers of air, have contributed dust particles.
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Dust in the troposphere contributes to the occurrence of twilight and the red colors of sunrise and sunset, but the most important function of dust particles is not observable and is rarely appreciated. Certain types of dust particles serve as nuclei, or centres, around which water vapour condenses to produce cloud particles.
The stratosphere and higher layers are almost free of water vapour and dust. Clouds are rare and storms are absent in the stratosphere, although winds of high speed are observed.
Atmospheric pressure. Although we are not constantly aware of it, air is a tangible, material substance. At sea level, the atmosphere exerts a pressure of about 15 lb per square inch (about 1 kg per square centimeter) on every solid or liquid surface exposed to it. Because the pressure of air within liquids exactly counterbalances this pressure, hollow objects, its ever-present weight creates no special concern. The pressure on one square inch of surface can be thought of as the actual weight of a column of air one inch in cross section extending upward to the outer limits of the atmosphere. Air is readily compressible. That which lies lowest is most greatly compressed and densest. In an upward direction both density and pressure of the air fall of rapidly. Any instrument that measures atmospheric pressure is a barometer. Standard sea level pressure is 1013.2 mb. The pressure falls with increasing altitude for every 900 ft (275 m) of rise in elevation, the mercury column fall
1/30 of its height.
