Atmospheric Shell
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Atmospheric Shell
Any one of a number of strata or layers of the earth's atmosphere.
Also called atmospheric layer, atmospheric region .
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 kilometers of the atmosphere, characterized
by decreasing temperature with height. The top of the troposphere
is called the tropopause. Above the tropopause, the stratosphere,
a region in which the temperature generally increases with altitude,
extends to the
stratopause, the top of the inversion layer, at
about 50 to 55 kilometers. Above the stratosphere, the mesosphere,
a region of generally decreasing temperatures with height extends
to the mesopause, the base of an inversion layer at about 80 to
85 kilometers. The region above the mesopause, in which temperature
generally increases with height, is the thermosphere.
The distribution
of various physicochemical processes is another criterion. The
ozonosphere, lying roughly between 10 and 50 kilometers, 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 kilometers, is the region in which ionization
of one or more of the atmospheric constituents is significant;
the neutrosphere is the shell below this which is, by contrast,
relatively unionized; and 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 photodissociation 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 are
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 kilometers.
The term mesosphere
has been given another definition which does not fit into any
logical set of criteria, i.e., the shell between the exosphere
and the ionosphere. This use of mesosphere has not been widely
accepted.
For further subdivisions, see
ionosphere,
troposphere,
geocorona.
References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use
