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Concept
Noctilucent cloud
Summary
Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. When viewed from space, they are called polar mesospheric clouds (PMCs), detectable as a diffuse scattering layer of water ice crystals near the summer polar mesopause. They consist of ice crystals and from the ground are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between ±50° and ±70°. Too faint to be seen in daylight, they are visible only when the observer and the lower layers of the atmosphere are in Earth's shadow, but while these very high clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds.
They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around .
No confirmed record of their observation exists before 1885, although they may have been observed a few decades earlier by Thomas Romney Robinson in Armagh. There are now doubts concerning Robinson's out-of-season records, partly as a result of observations, from several points around high northern latitudes, of NLC-like phenomena after the Chelyabinsk superbolide entry of February 2013 (outside the NLC season) that were actually stratospheric dust reflections visible after sunset.
Noctilucent clouds are composed of tiny crystals of water ice up to 100 nm in diameter and exist at a height of about , higher than any other clouds in Earth's atmosphere. Clouds in the Earth's lower atmosphere form when water collects on particles, but mesospheric clouds may form directly from water vapour in addition to forming on dust particles.
Data from the Aeronomy of Ice in the Mesosphere satellite suggests that noctilucent clouds require water vapour, dust, and very cold temperatures to form.
Official source
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