atmosphere [Gr.,=sphere of air], the mixture of gases surrounding a celestial body with sufficient gravity to maintain it. Although some details about the atmospheres of other planets and satellites are known, only the earth's atmosphere has been well studied, the science of which is called meteorology.
Components and Characteristics of the Earth's Atmosphere
The first 40 to 50 mi (64–80 km) above the earth contains 99% of the total mass of the earth's atmosphere and is generally of a uniform composition, except for a high concentration of ozone, known as the ozone layer, at 12–30 mi (19–50 km). Calculated according to their relative volumes, the gaseous constituents of the atmosphere are nitrogen, 78.09%; oxygen, 20.95%; argon, 0.93%; carbon dioxide, 0.03%; and minute traces of neon, helium, methane, krypton, hydrogen, xenon, and ozone. The lower atmosphere contains varying amounts of water vapor, which determine its humidity. Condensation and sublimation within the atmosphere cause clouds or fog, and the resulting liquid water droplets or ice crystals may precipitate to the ground as rain, sleet, snow, hail, dew, or frost. The air also carries many kinds of dust, of meteoric as well as terrestrial origin, and microorganisms, pollen, salt particles, and various gaseous and solid impurities resulting from human activity (see pollution). Because of the pull of gravity the density of the atmosphere and the pressure exerted by air molecules are greatest near the earth's surface (about 1 gram per 103 cc and about 106 dynes per sq cm, respectively). The instrument used to measure air pressure is called a barometer. Air pressure decreases quickly with altitude, reaching one half of its sea-level value at about 18,000 ft (5,500 m).
Layers of the Earth's Atmosphere
The earth's atmosphere is composed of distinct layers. The troposphere extends upward from the earth to a height of about 5 mi (8.1 km) at the poles, to about 7 mi (11.3 km) in mid-latitudes, and to about 10 mi (16.1 km) at the equator. The air in the troposphere is in constant motion, with both horizontal and vertical air currents (see wind). Throughout the troposphere temperature decreases with altitude at an average rate of about 3.6°F per 1,000 ft (2°C per 305 m), reaching about -70°F (-57°C) at its apex, the tropopause. Above the troposphere is an atmospheric ozone layer, which is also the lower layer of the stratosphere. Temperature changes little with altitude in the stratosphere, which extends upward to about 30 mi (50 km). Above this layer is the mesosphere which extends to about 50 mi (80 km above the earth); the temperature sharply decreases from around 20°F (10°C) at the base of the mesosphere to -166°F (-110°C) before it begins to rise at the top of the mesosphere. The next layer is the thermosphere, which extends upward from the mesosphere to about 400 mi (640 km); its temperature increases rapidly with altitude because of the absorption of shortwave radiation by ionization processes, although, because of the thinness of the air, little heat energy is available. The final layer is the exosphere, which gradually gets thinner as it reaches into the vacuum of space at around 435 mi (700 km) above the earth's surface; the atmosphere is so attenuated at this altitude that the average distance air molecules travel without colliding is equal to the radius of the earth. Although some gas molecules and particles out to about 40,000 mi (64,400 km) are trapped by the earth's gravitational and magnetic fields, the density of the atmosphere at an altitude of about 6,000 mi (9,700 km) is comparable to that of interplanetary space.
Certain layers of the atmosphere within the main regions exhibit characteristic properties. Aurorae (see aurora borealis), or northern and southern lights, appear in the thermosphere. The ionosphere is in the range (50–400 mi/80–640 km) that contains a high concentration of electrically charged particles (ions); these particles are responsible for reflecting radio signals important to telecommunications.
Role of the Earth's Atmosphere
The earth's atmosphere is the environment for most of its biological activity and exerts a considerable influence on the ocean and lake environment (see biosphere). Weather consists of the day-to-day fluctuations of environmental variables and includes the motion of wind and formation of weather systems such as hurricanes. Climate is the normal or long-term average state of the atmospheric environment (as determined in spans of about 50 years). The atmosphere protects earth's life forms from harmful radiation and cosmic debris. The ozone layer also protects the earth from the sun's harmful ultraviolet rays; seasonal "holes" in the ozone layer, the first detected above Antarctica and the Arctic in the 1980s, have caused considerable alarm about the consequences of air pollution. Meteors strike the thermosphere and mesosphere and burn from the heat generated by air friction.
See also Van Allen radiation belts; global warming.
See O. Allen, Atmosphere, (1983); M. I. Budyko and A. B. Ronov, History of the Earth's Atmosphere, (1987).