noctilucent clouds (luminous night clouds) Type of clouds, occurring at 80–85 km altitude, near the upper limit of the stratosphere, characterized by a blue to yellow hue, and similar in appearance to cirrostratus. The clouds are seen on summer nights at latitudes between about 50° and 65° in both northern and southern hemispheres; they move rapidly, at speeds up to 300 knots, often in a wave formation.

noctilucent clouds(luminous night clouds) Clouds, occurring at 80–85 km altitude, near the upper limit of the stratosphere, which are blue to yellow in colour and similar in appearance to cirrostratus. The clouds are seen on summer nights at latitudes between about 50° and 65° in both hemispheres; they move rapidly, at speeds up to 300 knots (556 km/h), often in a wave formation.