Animated View of Noctilucent Cloud
Polar mesospheric clouds, as they are known to those who study them from satellite observations, are also often called "noctilucent," or night shining, clouds as seen by ground-based observers. Because of their high altitude, near the edge of space, noctilucent clouds shine at night when the Sun's rays hit them from below while the lower atmosphere is bathed in darkness. They typically form in the cold, summer polar mesosphere region and are made of water ice crystals.
Image credit: NASA
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