PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY
Noctilucent cloud over trees. This type of cloud, also known as polar mesospheric, is visible only in deep twilight and is seen mostly during the summer. It is the highest cloud type in the Earth's atmosphere and occurs only when sunlight illuminates it from below the horizon, while the ground and lower layers of the atmosphere are in the Earth's shadow. Photographed in Turku, Finland, in July.
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