Mesosphere, the layer of the Earth's atmosphere between about 50 km and 80 km (31 mi and 50 mi) above the surface. It lies above the stratosphere and below the thermosphere. The stratosphere and mesosphere together are sometimes called the middle atmosphere. The interface between the stratosphere and the mesosphere is called the stratopause, and the interface between the mesosphere and the thermosphere above is called the mesopause.
Despite the fact that the mesosphere contains only about 0.1 per cent of the total mass of the atmosphere below 80 km (50 mi), it is important because of the ionization and chemistry which occur there. The middle atmosphere is made of the same constituents as the troposphere (mostly nitrogen and oxygen), but also includes some minor, but very important, gas constituents-chiefly ozone which, although it reaches a maximum in concentration low in the stratosphere, causes a maximum of solar heating near the stratopause. The mesosphere differs from the stratosphere below. This is mainly because as the ozone heating falls with height from its maximum near the stratopause, so too does the mesospheric temperature. The consequent rapid decrease in temperature with altitude is the major defining characteristic of the mesosphere.
The decrease in temperature coupled with the low density of the air in the mesosphere (about 1 gm-3; at the stratopause, or a thousandth of the density at sea-level, to 100 times less at the mesopause) means that the mesosphere includes both turbulence and atmospheric waves which have a wide range of spatial and temporal scales. Such motions are important not only because of the mixing of chemicals that occur as a result, but also because the mesosphere is the region of the atmosphere where spacecraft on re-entry start to feel the background wind structure, rather than just aerodynamic drag. Some of the small-scale waves drive an average seasonal flow upwards from the lower summer polar mesosphere across the equator and down deep into the winter polar stratosphere.
In summer, the rapidity of the temperature decrease as ozone heating falls away with height is coupled with extra cooling due to the background average rising motion. This means that in local summer the polar mesopause is the coldest place occurring naturally on earth: climatic temperatures are as low as -110° C (-230° F) and, on occasion, temperatures lower than -140° C (-285° F) have been recorded. Sometimes these low temperatures seem to be associated with thin cloud layers, which are best seen in twilight (when the mesosphere is still sunlit, but the surface is in the dark). Such clouds are called noctilucent clouds.
The mesosphere contains the part of the ionosphere known as the D-region, where ionization of molecular (O2) and atomic (O) oxygen releases electrons. This ionization is mainly due to incoming solar radiation, and disappears during the night-one of the reasons why radio reception improves dramatically at night.
Minor constituents in the middle atmosphere, including both natural and anthropogenic chemicals (chemicals produced by human activity), are transported from the regions where they enter the middle atmosphere (often in the tropical lower stratosphere) into the mesosphere, and can then migrate rapidly (in less than six months) to any latitude. Because of the rapid movement and the low natural densities high in the mesosphere, small amounts of some anthropogenic chemicals may first show their effects there.