Structure of the Atmosphere

The atmosphere is comprised of a sphere around Earth consisting of a mixture of gases, held in place by gravity. The most abundant gas is nitrogen (78 percent), followed by oxygen (21 percent), argon (0.9 percent), carbon dioxide (0.036 percent), and minor amounts of helium, krypton, neon, and xenon. Air pressure, or atmospheric pressure, is the force per unit area (similar to weight) that the air above a certain point exerts on any object below it. Atmospheric pressure causes most of the volume of the atmosphere to be compressed to 3.4 miles (5.5 km) above Earth's surface, even though the entire atmosphere is hundreds of kilometers thick. The atmosphere is divided into several layers, based mainly on the vertical temperature gradients that vary significantly with height. Atmospheric pressure and air density both decrease more uniformly with height, and therefore do not serve as a useful way to differentiate between different atmospheric layers. The lower 36,000 feet (10,972.8 m) of the atmosphere is known as the troposphere, where the temperature generally decreases gradually, at about 70°F per mile (6.4°C per km) with increasing height above the surface. This is because the Sun heats the surface that in turn warms the lower part of the troposphere.

Above the troposphere is a boundary region known as the tropo-pause, marking the transition into the stratosphere, which continues to a height of about 31 miles (50 km). The base of the stratosphere contains a region known as an isothermal, where the temperature remains the same with increasing height. The tropopause is generally at higher elevations in the summer than the winter and is also the region where the jet streams are located. Jet streams are narrow, stream-like channels of air that flow at high velocities, often exceeding 115 miles per hour (100 knots). Above about 12.5 miles (20 km), the isothermal region gives way to the upper stratosphere where temperatures increase with height, back to near-surface temperatures at 31 miles (50 km). The heating of the stratosphere at this level is due to ozone absorbing ultraviolet radiation from the Sun.

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Opposite: Structure of the atmosphere, showing different layers and temperature profile

The mesosphere lies above the stratosphere, extending between 31 and 53 miles (50-85 km). An isothermal region known as the stratopause separates the stratosphere and mesosphere. The air temperature in the mesosphere decreases dramatically above the stratopause, reaching a low of -130°F (-90°C) at the top of the mesosphere. The mesopause separates the mesosphere from the thermosphere, which is a hot layer where temperatures rise to more than 150°F (80°C). The relatively few oxygen atoms at this level absorb solar energy and heat quickly and may change dramatically in response to changing solar activity. The thermosphere continues to thin upward, extending to about 311 miles (500 km) above the surface. Above this level, atoms dissociate and are able to shoot outward and escape the gravitational pull of Earth. This far region of the atmosphere is sometimes referred to as the exosphere.

In addition to the temperature-based division of the atmosphere, it is possible to divide the atmosphere into different regions based on their chemical and other properties. Using such a scheme, the lower 46.5-62 miles (75-100 km) of the atmosphere may be referred to as the homosphere, where the atmosphere is well mixed and has a fairly uniform ratio of gases from base to top. In the overlying heterosphere, the denser gases (oxygen, nitrogen) have settled to the base, whereas lighter gases (hydrogen, helium) have risen to greater heights, resulting in chemical differences with height.

The upper parts of the homosphere and the heterosphere contain a large number of electrically charged particles known as ions. This region is known also as the ionosphere, which strongly influences radio transmission and the formation of the aurora borealis and aurora australis.

Atmospheric gases are being produced at approximately the same rate that they are being destroyed or removed from the atmospheric system, although some gases are gradually increasing or decreasing in abundance as described below. Soil bacteria and other biologic agents remove nitrogen from the atmosphere, whereas decay of organic material releases nitrogen back to the atmosphere. However, decaying organic material removes oxygen from the atmosphere by combining it with other substances to produce oxides. Animals also remove oxygen from the atmosphere by breathing, whereas oxygen is added back to the atmosphere through photosynthesis.

Water vapor is an extremely important gas in the atmosphere, but it varies greatly in concentration (0-4 percent) from place to place and from time to time. Water vapor is invisible; atmospheric water only becomes visible as clouds, fog, ice, and rain when the water molecules coalesce into larger groups. Water forms as gas, liquid, or solid and constitutes the precipitation that falls to Earth and is the basis for the hydrologic cycle. Water vapor is also a major factor in heat transfer in the atmosphere. A kind of heat known as latent heat is released when water vapor turns into solid ice or liquid water. This heat is a major source of atmospheric energy that is a major contributor to the formation of thunderstorms, hurricanes, and other weather phenomena. Water vapor may also play a longer-term role in atmospheric regulation, as it is a greenhouse gas that absorbs a significant portion of the outgoing radiation from Earth, causing the atmosphere to warm.

Carbon dioxide, although small in concentration, is another very important gas in Earth's atmosphere. Carbon dioxide is produced during decay of organic material, from volcanic outgassing, from cow, termite, and other animal emissions, from deforestation, and from the burning of fossil fuels. It is taken up by plants during photosynthesis and is also used by many marine organisms for their shells, made of CaCO3 (calcium carbonate). When these organisms (for instance, phy-toplankton) die, their shells can sink to the bottom of the ocean and be buried, removing carbon dioxide from the atmospheric system. Like water vapor, carbon dioxide is a greenhouse gas that traps some of the outgoing solar radiation that is reflected from Earth, causing the atmosphere to warm up. Because carbon dioxide is released by the burning of fossil fuels, its concentration is increasing in the atmosphere as humans consume more fuel. The concentration of CO2 in the atmosphere has increased by 15 percent since 1958, enough to cause considerable global warming. It is estimated that the concentration of CO2 will increase by another 35 percent by the end of the 21st century, further enhancing global warming. Other gases also contribute to the greenhouse effect, notably methane (CH4), nitrous oxide (NO2), and chlorofluorocarbons (CFC's). Methane is increasing in concentration in the atmosphere. It is produced by the breakdown of organic material by bacteria in rice paddies and other environments, termites, and in the stomachs of cows. NO2, produced by microbes in the soil, is also increasing in concentration by 1 percent every few years, even though it is destroyed by ultraviolet radiation in the atmosphere. Chlorofluorocarbons have received a large amount of attention since they are long-lived greenhouse gases increasing in atmospheric concentration as a result of human activity. Chlorofluorocarbons trap heat like other greenhouse gases and also destroy ozone (O3), the protective blanket that shields Earth from harmful ultraviolet radiation. Chlorofluorocarbons were used widely as refrigerants and as propellants in spray cans. Their use has been largely curtailed, but since they have such a long residence time in the atmosphere, they are still destroying ozone and contributing to global warming and will continue to do so for many years.

Ozone (O3) is found primarily in the upper atmosphere where free oxygen atoms combine with oxygen molecules (O2) in the stratosphere. The loss of ozone has been dramatic in recent years, even leading to the formation of "ozone holes" with virtually no ozone present above the Arctic and Antarctic in the fall. There is currently debate about how much of the ozone loss is due to human-induced ozone loss by chlo-rofluorocarbon production and how much may be related to natural fluctuations in ozone concentration.

Many other gases and particulate matter play important roles in atmospheric phenomena. For instance, small amounts of sulfur dioxide (SO2) produced by the burning of fossil fuels mix with water to form sulfuric acid, the main harmful component of acid rain. Acid rain is killing the biota of many natural lake systems, particularly in the northeastern United States, and it is causing a wide range of other environmental problems across the world. Other pollutants are major causes of respiratory problems and environmental degradation, and the major increase in particulate matter in the atmosphere in the past century has increased the hazards and health effects from these atmospheric particles.

The atmosphere is always moving, because more of the Sun's heat is received per unit area at the equator than at the poles. The heated air expands and rises to where it spreads out, and then it cools and sinks and gradually returns to the equator. This pattern of global air circulation forms Hadley Cells that mix air between the equator and mid-latitudes. Hadley Cells are belts of air that encircle Earth, rising along the equator, dropping moisture as they rise in the tropics. As the air moves away from the equator at high elevations, it cools, becomes drier, and then descends at 15-30°N and S latitude where it either returns to the equator or moves toward the poles. The locations of the Hadley Cells move north and south annually in response to the changing apparent seasonal movement of the Sun. High-pressure systems form where the air descends, characterized by stable clear skies and intense evaporation, km

Thermosphere Mesosphere Stratosphere Troposphere

Doldrums

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Polar front jet

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Major atmospheric circulation patterns on Earth, in map view (top) and cross section (bottom)

because the air is so dry. Another pair of major global circulation belts is formed as air cools at the poles and spreads toward the equator. Cold polar fronts form where the polar air mass meets the warmer air that has circulated around the Hadley Cell from the tropics. In the belts between the polar front and the Hadley Cells, strong westerly winds develop. The position of the polar front and extent of the west-moving wind is controlled by the position of the polar jet stream (formed in the upper troposphere), which is partly fixed in place in the Northern Hemisphere by the high Tibetan Plateau and the Rocky Mountains. Dips and bends in the jet stream path are known as Rossby Waves, and these partly determine the location of high- and low-pressure systems. These Rossby Waves tend to be semi-stable in different seasons and have predictable patterns for summer and winter. If the pattern of Rossby Waves in the jet stream changes significantly for a season or longer, it may cause storm systems to track to different locations than normal, causing local droughts or floods. Changes to this global circulation may also change the locations of regional downwelling, cold dry air. This can cause long-term drought and desertification. Such changes may persist for periods of several weeks, months, or years, and may explain several of the severe droughts that have affected Asia, Africa, North America, and elsewhere.

Circulation cells similar to Hadley Cells mix air in middle to high latitudes and between the poles and high latitudes. The effects of Earth's rotation modify this simple picture of the atmosphere's circulation. The Coriolis effect causes any freely moving body in the Northern Hemisphere to veer to the right, and in the Southern Hemisphere to the left. The combination of these effects forms the familiar trade winds, easterlies and westerlies, and doldrums.

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