atmospheric layersThe gaseous layers into which the Earth's atmosphere is divided, mainly on the basis of the variation of temperature with altitude. The troposphere is the lowest layer, extending from sea level to the tropopause at an altitude of between 8 km at the poles and 18 km within the tropics. It contains three-quarters of the atmosphere by mass and is the layer of clouds and weather systems. It is a region, heated from the ground by infrared radiation and convection, in which the temperature falls with increasing height to reach a minimum of approximately –55 °C at the midlatitude and polar tropopause and maybe –80 °C at the equatorial tropopause.
The stratosphere, lying above the tropopause, is an atmospheric layer in which the temperature is at first steady with altitude before increasing to a maximum of 0°C at about 50 km, which marks the stratopause. This temperature variation inhibits vertical air movements and makes the stratosphere stable. The heating arises from the absorption of high-energy solar ultraviolet radiation by ozone (O3) molecules. These molecules, themselves formed by the action of ultraviolet radiation on oxygen molecules, constitute the region known as the ozone layer. This contains about 90% of the atmosphere's ozone. Decreases in stratospheric ozone, observed especially in high latitudes in winter, are attributable mainly to increasing atmospheric levels of synthetic CFCs (chlorofluorocarbons). The Antarctic ozone hole in particular is allowing increased levels of potentially damaging solar ultraviolet radiation to reach the ground.
Above the stratopause is the mesosphere, in which the temperature falls with height to reach about –90 °C at the mesopause at an altitude of about 85 km. Within the mesosphere the heat contribution from ultraviolet absorption by ozone decreases as the ozone becomes less plentiful. The thermosphere, above the mesopause, has a temperature that rises with height as the Sun's far-ultraviolet radiation is absorbed by oxygen and nitrogen. This process gives rise to ionized atoms and molecules and leads to the formation of the ionosphere layers above altitudes of about 60 km. This is also the domain of meteors and aurorae.
Although the temperature climbs to about 500 °C at 150 km and about 1300 °C at 500 km, these are the kinetic temperatures (i.e. measures of the average random motions) of the few atoms and molecules found at such heights: their combined heating ability is negligible. The atmospheric density decreases to about 0.25 of its sea-level value at the tropopause, 0.0009 at the stratopause, and 0.000 007 at the mesopause. The density at 500 km averages less than one million millionth of its sea level value but is prone to large variations resulting from solar heating and disturbances during periods of enhanced solar activity.
The exosphere, above 500 km, is the region in which atmospheric constituents lose collisional contact with each other because of their rarity and can leak away into space. It contains the Van Allen radiation belts and the geocorona and extends to the magnetopause, where it meets the interplanetary medium.