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. The name means roughly night shining in Latin. They are most commonly observed in the summer months at latitudes between 50° and 70° north and south of the equator. They can only be observed when the Sun is below the horizon...

Mammatus clouds (from the Latin for 'udders') are formed when pockets of cold, saturated air sink rapidly from the top of a storm cloud, forming downward bulges like these seen over a sports stadium in Hastings, Nebraska.