Noctilucent clouds (NLC)
[ also known as Polar Mesospheric Clouds (PMC) ]
These clouds are cirrus-like in appearance but unlike cirrus (which occur at altitudes between 3 to 18 km, depending upon latitude band), NLC occur in the upper atmosphere between 80 and 85 km, preferentially around the mesopause ( which has a mean altitude of 82 km ) and are associated with temperatures colder than -80degC, perhaps with values close to -100degC.
They are normally only visible at night (hence the name) and during the summer months in each hemisphere. The sun must be between 6 and 16 arc-degrees below the local horizon.
Although there has been some debate surrounding the composition, they are now known to be composed of ice crystals (formed on solid particulate debris - see below), which act to reflect light radiation shining from the sun, which is well below the local horizon.
There is still doubt about the origin of the nuclei of such clouds: it was long thought that the catalyst for such displays derived from debris injected high into the atmosphere from the massive eruption of Krakatoa in 1883 (August). Modern thinking tends to believe that the particles (which become coated with water-ice) originate from outside the earth's atmosphere which then descend slowly through the upper atmosphere. Very low temperatures are required as the amount of water vapour at such high altitudes is so low that highly sub-zero temperatures are required to achieve saturation.
NLC are highly variable in frequency from year-to-year & within each season, probably because the temperature is similarly variable at these high altitudes, but there must also be a 'control' depending upon the numbers of available sublimation nuclei available: this in turn points to possible connection with the occurrence, intensity etc., of periodic meteor showers. There is some link with sunspot minima & also a possible coupling with man-made pollution of the atmosphere. However, it should be remembered that NLC observation depends on visual spotting and that depends on the presence or absence of 'normal' (i.e. tropospheric) clouds & the availability of trained observers to take advantage of good observing conditions.
Possible observations of noctilucent clouds: C.J. Butler. 'Weather' (Royal Meteorological Society): May, 2006.
Meteorological glossary: Meteorological Office. London, 1991.
Web site: http://www.nlcnet.co.uk/