Noctilucent clouds are extremely rare very high clouds seen in the night sky, usually on clear, summer nights.
Height of base: Above 200,000 ft
Noctilucent clouds become visible about the same time as the brightest stars and are usually bluish or silvery, but sometimes orange or reddish.
They most closely resemble thin streaky cirrus, and are thought to be made of ice crystals. These clouds are usually seen at latitudes between 45°N and 80°N in the Northern Hemisphere.
Noctilucent clouds are the highest clouds in the Earth's atmosphere forming above 200,000 ft. They are typically to faint to be seen, but occasionally become visible during twilight when the sun is below the horizon and the clouds becomes illuminated. They usually take on a blue light, though will sometimes take on a red or even green appearance. It is this illumination that is alluded in the clouds name with nocti the Latin for 'night' and lucent meaning 'shining'.
For more information on cloud spotting and pictures of different types of clouds, view our Cloud types for observers (PDF, 4 MB) guide.
Last updated: 5 August 2014