Noctilucent cloud.
They are the highest clouds in the Earth's atmosphere, located in the mesosphere at altitudes of around 76 to 85 kilometers (47 to 53 mi). They are very faint and tenuous, and can only be observed in twilight around sunrise and sunset when the clouds of the lower atmosphere are in shadow but the noctilucent cloud is illuminated by the sun
There is evidence that the relatively recent appearance of noctilucent clouds, and their gradual increase, may be linked to climate change.
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