Any one of a number of strata or “layers” of the earth's atmosphere. Temperature distribution is the most common criterion used for denoting the various shells. The troposphere (the “region of change”) is the lowest 10 or 20 km of the atmosphere, characterized by decreasing temperature with height. The term stratosphere is used to denote both 1) the relatively isothermal region immediately above the tropopause, and 2) the shell extending upward from the tropopause to the minimum temperature level at 70–80 km; the mesosphere is the shell between about 20 and 70 or 80 km that has a broad maximum temperature at about 40 or 50 km; and the thermosphere is the shell above the mesosphere with a more or less steadily increasing temperature with height. The distribution of various physico-chemical processes is another criterion. The ozonosphere, lying roughly between 10 and 50 km, is the general region of the upper atmosphere in which there is an appreciable ozone concentration and in which ozone plays an important part in the radiative balance of the atmosphere. The ionosphere, starting at about 70 or 80 km, is the region in which ionization of one or more of the atmospheric constituents is significant. The neutrosphere, the shell below this, is, by contrast, relatively un-ionized. The chemosphere, with no very definite height limits, is the region in which photochemical reactions take place. Dynamic and kinetic processes are a third criterion. The exosphere is the region at the “top” of the atmosphere, above the critical level of escape, in which atmospheric particles can move in free orbits, subject only to the earth's gravitation. Composition is a fourth criterion. The homosphere is the shell in which there is so little photo-dissociation or gravitational separation that the mean molecular weight of the atmosphere is sensibly constant; the heterosphere is the region above this, where the atmospheric composition and mean molecular weight is not constant. The boundary between the two is probably at the level at which molecular oxygen begins to be dissociated, and this occurs in the vicinity of 80 or 90 km. For further subdivisions, see ionosphere, troposphere.