Above the stratosphere's temperature peak at 50 km, temperatures
fall precipitously. We are in the mesosphere or 'middle atmosphere'.
At the base of the mesosphere the pressure is only
1/1000 of that at sea level and when the top is reached at
95 km it is a mere millionth. For practical purposes it is
Temperatures there only have significance in influencing the
mesosphere's molecular physics and chemistry rather than in any
ability to heat objects like spacecraft or even dust.
Temperatures decrease with height because heating by ultraviolet
absorption by ozone falls off
and, importantly, traces of carbon dioxide have a significant
cooling effect. In the lower atmosphere CO2 acts
as a so called greenhouse gas by absorbing
infrared radiation radiated by the earth’s
surface. In the rarefied mesosphere CO2 actually
cools the atmosphere by radiating heat into
A temperature minimum of -90°C and lower is reached at ~85 km
- the mesopause. Near here is the realm of the 'night
shining' or noctilucent clouds.
Beyond the mesopause temperatures rise again
because of reduced radiative cooling combined with heating by absorption
of short wavelength,<180 nm, UV radiation by O2,
O atoms and N2.
|The summer mesopause
is getting colder, possibly because of the cooling effect
of increased anthropogenic carbon dioxide and methane