Tornado: How the spiralling vortex is formed

Tornadoes form during thunderstorms when warm, humid air collides with colder air to form a swirling vortex that extends down from the clouds and sometimes reaches the ground where it can cause extensive damage.

Warmer air rises rapidly above colder air, forming an updraft that begins to move in a swirling motion and reaches along the length of the column to cause the classic spinning, or twisting.

Just as water leaves a bath more efficiently by spinning down the plughole, so warm, humid air rises more effectively through a storm cloud by twisting in the vortex.

The downward moving column of air in a tornado - the funnel cloud - is caused as water vapour condenses into its outer sheath because of falling pressure and temperature. Eventually, the funnel can reach the ground and, with it, comes the high-speed winds created by the sudden clash of warm and cold air.

Yesterday's tornado in Birmingham, which had winds of up to 130mph, was small by global standards. In America bigger tornadoes with wind speeds of nearly 200mph are regular.

Nick Grahame, chief forecaster at the Meteorological Office in Exeter, said Britain experiences about 33 tornadoes on average a year but it can depend on the number of summer storms.

In 2003, there were just nine tornadoes reported to the Met Office, whereas in 1981 there were 156.