A tornado develops from a large thunder cloud, or cumulonimbus cloud, in a region where warm, moist air meets with cold air. The warm air is forced upwards while the cold air descends. The thunder cloud that develops can contain air that rotates. Large downdraughts of air, sometimes associated with heavy rainfall, drag the rotating cloud towards the ground. This vortex is classified as a tornado when it is in contact with both the ground and the cloud.
On average the USA experiences about 1,200 tornadoes per year but tornadoes can develop almost anywhere in the world, except in the polar regions. The UK has around 35-40 tornadoes a year and experiences more than any other European country.