Tornadoes are whirlwinds that are less than 1km across. They are common in the American Midwest, where warm, moist air from the Gulf of Mexico underlies cold dry air from the north, but they also occur in western Europe.
The exact cause of tornadoes is not known, but they form when a long funnel of cloud sinks from a turbulent cumulus cloud. Warm air is sucked upwards around the strong downdraught in a powerful spiral.
Wind speeds are thought to reach 600km/h, although few instruments survive trying prove this a whole culture of 'storm chasers' has sprung up for the very purpose of videoing and recording data from tornadoes.
It is not only the winds that cause devastation, but also the great difference in pressure between the air inside buildings and the extremely low pressures outside; these pressure differences can make buildings collapse.Tornadoes travel up to 500km before dying out. Similar features at sea are called waterspouts.