The highest clouds in Earth's atmosphere, forming roughly 50 miles up in the mesosphere from ice crystals condensing on meteoric dust in air so cold and thin that ordinary weather clouds cannot exist there. Visible only during deep twilight in summer at high latitudes, they glow with an electric-blue, wispy brilliance. Once a genuine rarity, noctilucent clouds have been observed with increasing frequency and at lower latitudes in recent decades.
Facts
PeriodicitySummer only: mid-May to mid-August in the northern hemisphere, mid-November to mid-February in the southern hemisphere. 1 ScaleHeight of about 76 to 85 km; ice crystals up to 100 nm in diameter. 1 First DescribedFirst known observed in 1885 1 Identification
Distinguishing AppearanceGenerally colourless or pale blue, occasionally red and green. 1 Sources
1. Noctilucent cloud, Wikipedia
Observation
Noctilucent clouds are generally colourless or pale blue, although occasionally other colours including red and green have been observed.
Observation, season
They occur during summer, from mid-May to mid-August in the northern hemisphere and between mid-November and mid-February in the southern hemisphere.
Formation
Noctilucent clouds are composed of tiny crystals of water ice up to 100 nm in diameter and exist at a height of about 76 to 85 km (249,000 to 279,000 ft), higher than any other clouds in Earth's atmosphere.
Discovery and investigation
Noctilucent clouds are first known to have been observed in 1885, two years after the 1883 eruption of Krakatoa.
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