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tornado; twister; cyclone; tornado A rapidly rotating column of air developed around a very intense low-pressure centre. It is associated with a dark funnel-shaped cloud and with extremely violent winds (>300km/h) blowing in a counterclockwise spiral, but accompanied by violent downdraughts. The precise mechanisms are not fully understood but the following atmospheric conditions appear to be necessary for tornado development: a layer of warm moist air at low altitude; a layer of dry air at higher altitude with an inversion of temperature at about 1.000 m; a triggering mechanism, usually in the form of an active, intense cold front or solar heating of the ground which will create a vortex |
