1 A general term for various forms of rapidly rotating vortex, covering a wide range of strengths and wind speeds, including devils, gustnadoes, landspouts, tornadoes, and waterspouts. Various methods of formation occur. The weakest whirls arise from the funnelling effects of local topography (as in snow devils and water devils); vigorous convection from intense heating that creates a superadiabatic lapse rate initiates some dust devils; strong downdraughts are involved in gustnadoes, landspouts, and waterspouts; and a wide range of factors combine to produce violent tornadoes.
2 A form of noctilucent cloud (classified as Type IV), consisting of arcs or (rarely) rings of cloud with dark centres.
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