Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. When viewed from space, they are called polar mesospheric clouds (PMCs), detectable as a diffuse scattering layer of water ice crystals near the summer polar mesopause. They consist of ice crystals and from the ground are only visible during astronomical twilight. Noctilucent roughly means "night shining" in Latin. They are most often observed during the summer months from latitudes between ±50° and ±70°. Too faint to be seen in daylight, they are visible only when the observer and the lower layers of the atmosphere are in Earth's shadow, but while these very high clouds are still in sunlight. Recent studies suggest that increased atmospheric methane emissions produce additional water vapor through chemical reactions once the methane molecules reach the mesosphere – creating, or reinforcing existing noctilucent clouds.
Noctilucent clouds over Kuresoo Bog, Viljandimaa, Estonia
Noctilucent clouds over Uppsala, Sweden
Noctilucent clouds during arctic dawn seen from high altitude
Polar mesospheric clouds over the north pole
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the droplets and crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature.
Cloudscape over Borneo, taken by the International Space Station
Windy evening twilight enhanced by the Sun's angle, can visually mimic a tornado resulting from orographic lift
Cirrus fibratus clouds in March
High cirrus upper-left merging into cirrostratus right and some cirrocumulus far right