rotor cloud

A cloud associated with a turbulent cell or roll, known in aeronautical jargon as a rotor, that forms below the crests of a mountain wave, on the leeward side of the mountain obstacle. Depending on the temperature and moisture content of the air at that low level, close to the Earth's surface, the rotor cloud may or may not manifest. In cases where it does form, it can range from a small fractus cumulus, seemingly harmless, to a large, threatening-looking roll cloud. The most dangerous rotor, where turbulence is most intense, is the one arising beneath the first crest of the wave, about 10-15 kilometers from the ridge line, on the lee side. It is a very dangerous zone for flight, which pilots must avoid.