What is "noctilucent cloud"

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 ice crystals. Noctilucent roughly means 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. These clouds can be observed only when the Sun is below the horizon for the observer, but while the clouds are still in sunlight.

They are the highest clouds in Earth's atmosphere, located in the mesosphere at altitudes of around . They are normally too faint to be seen, and are visible only when illuminated by sunlight from below the horizon while the lower layers of the atmosphere are in the Earth's shadow. Noctilucent clouds are not fully understood and are a recently discovered meteorological phenomenon; there is no record of their observation before 1885.

Noctilucent clouds can form only under very restricted conditions; their occurrence can be used as a sensitive guide to changes in the upper atmosphere. They are a relatively recent classification. The occurrence of noctilucent clouds appears to be increasing in frequency, brightness and extent.