DEFINITION OF MESOSPHERE

In solid Earth geophysics, the mesosphere

refers to the solid lower mantle beneath the asthenosphere.

The term is not widely used, however, and this region is more

commonly referred to simply as the lower mantle. The same

term is used by atmospheric scientists for the region that lies

between the stratosphere and the thermosphere, at an average

distance of 31–50 miles (50–80 km) above the surface of the

Earth. It accounts for only less than 0.1 percent of the total

mass of the atmosphere.

There is not much significant weather or heating in the

mesosphere. Temperatures average about 32°F (0°C) at the

base of the mesosphere, with slight heating in the summer

and cooling in the winter. A small amount of heat is transferred

upward from the stratosphere, which becomes heated

by absorption of ultraviolet radiation. The temperature at the

top of the mesosphere is about –230°F (–110°C) in summer,

when the mesosphere is the coldest part of the atmosphere,

and –140°F (–60°C) in winter.

Sometimes very thin clouds called noctilucent clouds form

in the mesosphere, but these are so thin that they can only be

seen at sunset or sunrise, when the lower atmosphere is in

shadow and the mesosphere is still receiving direct sun rays.

See also ASTHENOSPHERE; ATMOSPHERE; MANTLE.

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