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Glowing, silvery blue clouds that have been spreading around the world and brightening mysteriously in recent years will soon be studied in unprecedented detail by a NASA spacecraft.
Noctilucent" clouds, which glow at night, form in the upper atmosphere, at an altitude of about 80 kilometres, and their glow can be seen just after sunset or just before sunrise.
The clouds were first observed above polar regions in 1885 – suggesting they may have been caused by the eruption of Krakatoa two years before. But they have spread to latitudes as low as 40° in recent years. "They're also getting brighter, and each year there are more of them than in the previous year," Russell told New Scientist.