Atmosphere

The mixture of gases that surrounds the solid earth is called the atmosphere. Although it represents a very small fraction of the weight of the earth, it is very important because it is essential to life and is highly reactive, circulates, and plays an important role in the evolution of landscapes. Carbon dioxide plays an important role by trapping heat, causing the greenhouse effect, which keeps the earth warm and livable, for the most part.

The atmosphere is dominated by nitrogen (78.08 percent); oxygen (20.95 percent) and argon (0.93 percent) are next in abundance. Carbon dioxide, which is essential to all plant life, makes up only 0.32 percent of atmosphere.

Oxygen is chemically reactive and important in the weathering of rocks, the decay of organic matter, and combustion. Nitrogen is relatively inert and unavailable to plants directly; therefore it must undergo chemical changes to make it useful to vegetation. Water vapor plays an extremely important role in the hydrological cycle. Unlike most of the gases in the atmosphere, which are fairly constant or vary within limits, water vapor is highly variable. Air in desert areas may contain only 1 percent vapor by volume; in tropical areas, however, where the air is warm and moist, the content can be as high as 4 percent. Water vapor is most abundant near the surface, although it amounts to less than 1 percent of the total atmosphere volume. The source of water on earth has traditionally been explained by degassing from within the planet, released through volcanic eruptions; a few researchers, however, have suggested that at least some water came from cometary impacts.

The temperature of the atmosphere varies considerably, reflecting—like the solid earth— its subdivision into several layers. Vertically, each layer has different properties and is found at specific elevations.

The troposphere is the layer closest to the surface, containing 75 percent of the atmosphere's weight, all of the water vapor, and most of the particulate matter; it is 8 to 18 km high. Its upper boundary, called the tropopause, is closer to the surface at the poles than at the equator. The temperature gradient of the troposphere is about 6.5 degrees centigrade per km of rise from the surface to the tropopause. It is where most of the solar radiation striking the surface is converted to thermal energy and the resulting heat is exchanged by direct contact with surface of the earth. Because the air at the surface expands, it rises and subsequently cools, thus creating a pattern of thermal convection not unlike the process that takes place within the earth at the asthenos-phere. By this process the troposphere is kept in motion and is part of the process that drives the hydrological cycle. It is where almost all the clouds are and where the earth's weather takes place.

The stratosphere, the next higher layer, has a constant lapse rate at lower levels, where it reverses forming a temperature inversion, as cold, denser air is trapped by warm air above, reducing the vertical movement of air and making the stratosphere a cap over the troposphere. The air is almost cloud free and without turbulence—ideal for jet aircraft to fly through—except that occasionally the tops of a few giant thunderstorms extend into the bottom of the stratosphere. Because there is little vertical movement, the air forms horizontal layers and moves within those layers and not across them. Although very dry, the stratosphere does have trace amounts of water, producing faint, pearly clouds at about 30 km. The stratosphere contains 24 percent of the atmosphere by weight, reaches to 45 km above the surface, and contains the ozone layer, which is created when ultraviolet light strikes an oxygen molecule (O2) and breaks it into two individual atoms that then combine with normal oxygen to form ozone (O3). The ozone layer intercepts nearly 100 percent of ultraviolet light from the sun and, as a result, shields the earth's surface against radiation that causes severe sunburn, skin cancer, and genetic mutations.

Above the stratosphere are the mesosphere and the thermosphere, both of which contain the remaining 1 percent of the air by weight. The mesosphere lies between 45 and 92 km, and its temperature decreases with altitude until it reaches the mesopause, where the air is -93 degrees centigrade, the coldest expanse of the atmosphere. The outer layer, the thermosphere, has extremely high temperatures, as high as 1,300 degrees centigrade. But because the air density is so low and the hot molecules are so far apart, an object such as a satellite is unlikely to intersect many of them.

The outermost zone, the magnetosphere, traps particles entering the atmosphere and protects the earth from damaging radiation from the sun. The sun is the source of almost

Earth's atmosphere photographed by astronauts. (NASA)

all of the energy that drives the oceans and atmosphere, the controlling entities that produce weather and climate. Of the solar radiation that the earth receives from the sun, almost 30 percent is reflected back into space by clouds, dust, and the atmosphere itself. The remaining 70 percent heats up the continents, islands, oceans, and atmosphere; drives the winds and ocean currents; evaporates water; and supplies energy for life through photosynthesis. Some of the radiation is scattered and diffused, but about 50 percent of what strikes the surface is absorbed, causing the surface temperature to rise. This heat is then radiated as infrared radiation back into the atmosphere, where water vapor, carbon dioxide, and other greenhouse gases absorb most of it. About 18 percent of the sun's energy evaporates water from the land and oceans; as the vapor condenses, the energy is returned to the air.

When the earth's average temperature remains the same, the amount of energy received equals the amount of energy radiated and reflected back into space. However, if the two do not balance, a change in climate results. Changes can also occur if the sun's output increases or diminishes, or if some parameter on earth changes, such as a period of intense volcanism. On a daily basis, however, the amount of solar radiation that earth receives varies from place to place, day to day, and season to season. Rotation of the earth on its axis causes the change between night and day, and inclination of its axis causes seasonal change. Other factors that influence the amount of solar radiation the atmosphere receives are the presence of clouds and the distance between the earth and the sun.

Earth's orbit around the sun is elliptical (averaging a distance of 150 million km); thus the distance varies throughout the year. The Northern Hemisphere is closest to the sun in winter (January 3) and farthest in summer (July 4), indicating that seasonal temperature changes and distance have only a minor correlation. Because there are more land masses in the northern latitudes, there is rapid seasonal heating and cooling there, making the summers warmer and the winters colder than in the areas south of the equator, where there is much more water.

—Sidney Horenstein

See also: Atmospheric Cycles; Climatology; Hole in the Ozone Layer; Nitrogen Cycle; Oxygen, History of Presence in the Atmosphere

Bibliography

Akin, Wallace. 1990. Global Patterns: Climate, Vegetation, and Soils. Norman: University of Oklahoma Press; Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Lutgens, Frederick K., and Edward J. Tarbuck. 2001. The Atmosphere: An Introduction to Meteorology, 8th ed. Upper Saddle River, NJ: Prentice Hall; Moran, Joseph M. 1991. Meteorology: The Atmosphere and the Science of Weather. New York: Macmillan.

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