Composition
The earth's atmosphere consists chiefly of nitrogen (78 percent by volume) and oxygen (21 percent). The remainder is largely argon (0.9 percent) and carbon dioxide (0.3 percent), plus traces of a number of other gases. Water vapor is present as well but to a variable extent, ranging from nearly 0 to 4 percent. The lower atmosphere also contains a considerable quantity of small, solid particles of different kinds (such as soot; bits of rock and soil; salt grains from the evaporation of seawater; and spores, pollen, and bacteria); these particles provide nuclei for the condensation of atmospheric water vapor to form clouds, fog, rain, and snow.
Nitrogen, oxygen, and carbon dioxide are important biologically. Nitrogen is a key ingredient of the amino acids of which all proteins consist, and certain bacteria are able to convert atmospheric nitrogen into nitrogen compounds which plants can utilize in manufacturing amino acids. Plants also convert water and atmospheric CO2 into carbohydrates and oxygen in photosynthesis; animals obtain the carbohydrates and amino acids they need by eating plants (and other animals). Plants and animals both derive energy by using atmospheric oxygen to convert carbon in their foods to CO2.
High in the atmospheric solar x- and ultraviolet radiation split N2 and O2 molecules into atoms and into ions and electrons. One result is the formation of a small amount of ozone, O3, from reactions between O2 molecules and O atoms. Ozone is very efficient absorber of solar ultraviolet ar wavelenghts longer than those absorbed by N2 and O2 and so prevents this potentially lethal radiation from reaching the earth's surface. The region of the upper atmosphere that contains ions and electrons is known as the ionosphere, long-range radio communication is possible because radio waves are channeled between the earth's surface and the ionosphere by reflection at both, instead of simply escaping into space.
STRUCTURE
The character of the atmosphere changes more or less abruptly at the tropopause, stratopause, and mesopause, Which occur, respectively, at altitudes that average about 10km (6 mi), 50km (31 mi), and 80km (50 mi). These surfaces divide the atmosphere into four regions, listed in order of increasing altitude (Fig. 34-1):
1. The troposphere is the dense lower part of the atmosphere in which meteorological phenomena such as clouds and storms occur. Air temperature in the troposphere decreases with altitude until it reaches about -55°C at tropopause.
2. In the stratosphere the air is clear and dry. The temperature is constant in the lower part of the stratosphere but then rises because of heating from the absorption of solar energy by the ozone layer. The stratopause is defined by a temperature maximum, which is usually about 10°C.
3. In the mesosphere the temperature falls steadily to about -80°C at the mesopause. Air pressure at the mesopause is only about 3/100,000 of sea-level pressure.
4. In the thermosphere the absorption of solar x- and ultraviolet radiation results in high temperatures (1000°C or more) and considerable ionization. The various layers of the ionosphere are found here. It must be kept in mind that the high temperatures of the thermosphere represent the average molecular energies there; because the number of molecules per m3 is so small, the total energy per m3 is also small despite the high temperature.
