Tornado photographed near Caldwell in Kansas, USA, on 13 March 1990. Tornadoes are highly-destructive rotating columns of air, characterized by a funnel-shaped cloud. The strongest surface winds of any weather system may be experienced locally, wind speeds of 370 km per hour have been observed. The precise origins of tornadoes are not yet understood, but the worst seem to occur with warm moist air trapped under an inversion at 2000 feet, a tongue of dry air at around 800 mb pressure, and a triggering mechanism, usually a thunderstorm. Tornadoes occur most frequently in the mid-West USA, although minor tornadoes occur in the UK about once in every two years.