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Noctilucent clouds over Soomaa National Park, Estonia
Noctilucent clouds are the highest clouds in Earth's atmosphere, being bright polar mesospheric clouds illuminated by the Arctic sun from below the horizon, between latitudes of 50° and 70°. They are composed of water ice crystals up to 100 nm across, forming most frequently in the northern summer at altitudes of 76 to 85 km (47 to 53 mi).Photo: Martin Koitmäe

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