25.5 MB (1.6 MB compressed)
3545 x 2516 pixels
30.0 x 21.3 cm ⏐ 11.8 x 8.4 in (300dpi)
PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY
Noctilucent cloud. These bright displays are seen in the middle of the night in summer at high latitudes. Their composition is unknown, but they are known to exist at 70-90 kilometres altitude - well above normal clouds. They are normally seen when the sun is 4-16 degrees below the observer's horizon, and have a whitish colour against a dark sky. Photographed in Finland.
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