atmosphere, structure of

atmosphere, structure of The Earth's atmosphere consists of various layers, the nomenclature and altitudes of which depend upon the criteria used for their definition. Based upon the changes of temperature with height (i.e. the lapse rate), four layers may be distinguished. Closest to the surface is the troposphere, in which most of the significant weather occurs, where temperature generally declines with height, and which extends up to the tropopause at c.7 km over the poles and 18–28 km over the Equator. The lowest region of the overlying stratosphere is often isothermal, but the temperature then increases with height to the stratopause at c.50km. In the mesosphere, temperature again falls with height to the mesopause at c.86–100 km. The outermost layer is the thermosphere, in which atoms and molecules may attain extremely high temperatures (velocities), and which extends into interplanetary space. The region above 700 km, at which height atoms may begin to escape into space, is sometimes termed the exosphere.

On the basis of chemical composition, the atmosphere consists of just two layers, the homosphere (largely identical with the troposphere, stratosphere, and mesosphere), in which the composition is essentially constant, and the overlying heterosphere.

Specific ionization and photochemical processes occur in the ionosphere (encompassing part of the upper mesosphere and thermosphere) and the chemosphere and ozonosphere (both part of the upper stratosphere)
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