Image of noctilucent (mesospheric) clouds taken on June 19, 2010 in Moscow at railway station.
© Igordabari | Dreamstime.com
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. The name means roughly night shining in Latin.