Template:POTD/2012-03-22
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