Very thin clouds of water ice crystals located in the Mesosphere at a height of 75 km to 90 km.
Noctilucent Clouds are normally only visible in twilight in midsummer, when the sun is below the horizon and the surface and troposphere are in darkness. They resemble Cirrus clouds in appearance.
When the sun is around 6 degrees below the observer's horizon, noctilucent clouds start to become perceptible. The theoretical and observed maximum illuminescence of the clouds is when the sun is 10 degrees below the observer's horizon and the clouds become imperceptible again when the sun is 16 degrees below the observer's horizon.
Noctilucent clouds are generally seen at latitudes between 55 and 65 degrees in the summer, and are mainly seen in the Northern Hemishpere due to topography and population density at these latitudes.
Noctilucent Clouds photographed at twilight by Mika Yrjölä in August 2003 in northern Sweden
Noctilucent Clouds photographed by Marco Quaranta over Tromso, Norway, in January 2017