Here under Climatology, our object is to examine the atmosphere and oceans with particular reference to air land interfaces, which are so vital to man. The physical geographer seeks to describe and explain the manner in which the environmental elements of weather and climate changes with latitudes and season.
Composition of the atmosphere - The earth's atmosphere consists of a mixture of various gases surrounding the earth to a height of many miles. Held to the earth by gravitational attraction, this envelope of air is densest at sea level and thins rapidly upward. Although almost the entire atmosphere (97 percent) lies within 18 mi (29km) of the earth's surface, the upper limit of the atmosphere can be drawn approximately at a height of 6000 mi (10,000km), a distance approaching the diameter of the earth itself.
From the earth's surface upward to an altitude of about 50 mi (80km) the chemical composition of the atmosphere is highly uniform throughout in terms of the proportions of its component gases. The name homosphere has been applied to this lower, uniform layer, in contrast to the overlying heterosphere, which is non-uniform in an arrangement of spherical shells.
Pure, dry air of the homosphere consists largely of nitrogen (78.084 percent by volume) and oxygen (20.946 percent). Nitrogen does not easily enter into chemical union with other substances, and can be thought of as primarily a neutral filler substance. In contrast, oxygen is highly active chemically and combines readily with other elements in the process of oxidation. Combustion of fuels represents a rapid form of oxidation, whereas certain forms of rock decay (weathering) represent very slow forms of oxidation.
The remaining 0.970 percent of the air is mostly argon (0.934 percent). Carbon Dioxide, although constituting only about 0.033 percent, is a gas of great importance in atmospheric processes because of its ability to absorb heat and thus to allow the lower atmosphere to be warmed by heat radiation coming from the sun and from the earth's surface.
Green plants, in the process of photosynthesis, utilize carbon dioxide from the atmosphere, converting it with water into solid carbohydrate. A pronounced rise in the carbon dioxide content of the atmosphere has been noted since 1900 and is a possible result of Man's combustion of vast quantities of wood, coal, petroleum, and natural gas. In this change, we may find an example of Man's impact upon his environment, for an increase in carbon dioxide, as well as in atmospheric dusts, can result in appreciable increases in average atmospheric temperatures.
The remaining gases of the homosphere are neon, helium, krypton, xenon, hydrogen, methane, and nitrous oxide. They are present in extremely minute percentages.
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