CT: Recent satellite observations have shown the existence of a persistent layer of light-scattering particles in the vicinity of the polar mesopause during the summer. The suggestion has been made that this layer consists of ice particles, and that noctilucent clouds are a sporadic manifestation of the layer near its low-latitude edge. The consequences of this proposal in terms of the water vapor content of the mesosphere are explored in this paper through the development of a model for such a cloud, in which the water vapor mixing ratio is assumed to be 1 to 10 ppm at 60 km, and the temperature is assumed to drop to values well below 140 K at the mesopause. Water vapor is transported upward by eddy diffusion, and is photodissociated by solar ultraviolet radiation. Sublimation nuclei of radius 10Å are assumed to exist in the vicinity of the mesopause, and the growth of the ice particles and their terminal speed of descent are calculated from simple kinetic theory considerations.

S: AMS – (last access: 14 January 2021)

N: 1. From word-forming element meso- (“middle, intermediate, halfway,” from Greek mesos “middle, in the middle; middling, moderate; between”) and noun pause (“a pause, interruption” (14c.) and directly from Latin pausa “a halt, stop, cessation,” from Greek pausis “stopping, ceasing,” from pauein “to stop (trans.), hold back, arrest, to cause to cease,” a word of uncertain etymology with no certain cognates outside Greek).
2. The mesopause is the transition between the mesosphere and the thermosphere and is the altitude at which the temperature reaches a minimum before increasing with height in the thermosphere.
3. The percentage of oxygen, nitrogen, and carbon dioxide in the air in the mesosphere is essentially the same as that in the levels of the Earth’s atmosphere immediately above the Earth’s surface. The principal differences are that the density of the air is much less, there is very little water vapour in the mesosphere, and the mesosphere contains higher percentages of ozone than the lower levels.
4. The altitude of the mesopause varies with season and latitude. In the polar regions, the altitude of the mesopause ranges between 90 km in summer and 110 km in winter. Atmospheric physicists suspect the mesopause is very sensitive to climate change. Climate change variations are warming the troposphere and this is thought to be causing further cooling in the mesosphere.

S: 1. OED –; (last access: 13 January 2021); Wiktio –,temperature%20reaches%20its%20minimum%20value. (last access: 13 January 2021). 2. Kelley – (last access: 13 January 2021). 3. EncBrit – (last access: 14 January 2021). 4. AAP – (last access: 14 January 2021).


CR: air pollution, biosphere, carbon dioxide, carbon monoxide, ecology, environment, ionosphere, mesospherenitrogen dioxide, nitrogen oxide, nitrous oxide, ozone layer, stratospherestratospheric ozone, tropospheric ozone.