noctilucent cloud

Definition

Astronomy

  • Cloud seen shining at night, just after dusk or before dawn, high in the Earth’s atmosphere, especially from high latitudes. The name means ‘night-shining.’ Noctilucent clouds have little in common with normal clouds. They consist of ice crystals some 80km above sea level. They commonly have a blue colour and are interesting as clues to the nature of the upper atmosphere and because they probably form by a complex process whereby ice accumulates about nuclei of meteoritic dust.
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