mesosphere

mesosphere The middle layer of Earth's atmosphere, lying above the stratosphere and below the thermosphere, at altitudes of 50–85 km. Temperatures in the mesosphere fall with increasing altitude, reaching a minimum at the mesopause. Measurements from sounding rockets indicate the mesosphere to be extremely dry, but sufficient traces of water vapour are present during the summer months for noctilucent clouds to form near the mesopause.

mesosphere The layer of the atmosphere in the altitude range 50 to 80 km is the mesosphere. Because meso is Greek for ‘middle’, the name is not really appropriate. All significant weather occurs in the troposphere at altitudes below about 15 km, and the mesosphere accounts for less than 0.1 per cent of the mass of the atmosphere.

There are no significant sources of heat in the mesosphere as there are in the stratosphere below and the thermosphere above. At the lower boundary of the mesosphere the temperature is about 0 °C; slightly higher in summer and slightly lower in winter. Absorption of ultraviolet radiation in the stratosphere produces heating below the mesosphere, which leads to this relatively high temperature. Some of this heat is transferred into the lower mesosphere and produces weak vertical motion throughout the mesosphere. As the air rises it cools, and a temperature decrease with height is observed throughout the mesosphere. The decrease in temperature is not, however, as great as would be expected from overturning air and it is about half of the decrease observed in the troposphere. This implies that radiative processes also contribute to the temperature structure of the mesosphere.

At the upper boundary of the mesosphere, the mesopause, the temperature is between −110 °C, in summer, and −60 °C, in winter. In summer the mesopause is the coldest part of the entire atmosphere. The low temperatures combined with weak updraughts are sufficient to form tenuous clouds in this region. These clouds are so thin that under normal circumstances they cannot be seen. At twilight, however, there is a period when the lower atmosphere is in the shadow of the Earth while the upper parts of the atmosphere are still in sunlight. At these times the clouds are visible and are therefore referred to as noctilucent clouds. The best viewing conditions are at middle latitudes near the summer solstice, when the mesospheric temperatures reach their lowest values and the twilight hours are at their longest.

Charles N. Duncan

Bibliography

Wallace, J. M. and and Hobbs, P. V. (1977) Atmospheric science: an introductory survey. Academic Press, London.

mesosphere
1. Upper-atmospheric layer above the stratopause (at 50 km) through which temperature decreases with height up to about 80 km, where temperatures reach a minimum of about −90°C. This level is the mesopause, an inversion above which temperatures rise again. See also ATMOSPHERIC STRUCTURE.

2. That part of the Earth underlying the asthenosphere. The term is no longer in current use in this sense.