Noctilucent clouds are actually crystals of ice that hang around 80
kilometers (50 miles) high in the atmosphere. The ice crystals catch the
light of the Sun long after it has set on the horizon. The uppermost
parts of the cloud in this image are iridescent (nacreous), which gives
them the appearance of mother-of- pearl. Natural nacreous clouds occur
at altitudes of 20-25 kilometers. The lower parts of the cloud have a
redder color due to the large amount of dust and water in the lower
atmosphere scattering blue light. The cloud’s shape is due to differing
wind speed at differing altitudes.
The cloud in this image formed from the exhaust of a missile launched from a distant firing range.
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