"Night clouds or noctilucent clouds are tenuous cloud-like phenomena that are the \"ragged edge\" of a much brighter and pervasive polar cloud layer called polar mesospheric clouds in the upper atmosphere, visible in a deep twilight. They are made of crystals of water ice. Noctilucent roughly means night shining in Latin. They are most commonly observed in the summer months at latitudes between 50\u00B0 and 70\u00B0 north and south of the equator."@en . . .