Tornadoes come in many shapes and sizes, but typically form a condensation funnel, with its narrow end touching the ground.
A huge tornado funnel cloud touches down in Orchard, IowaPhoto: AP
12:01AM BST 29 Apr 2011
Most have a wind speed of less than 110 mph and are around 250 feet across, travelling a few miles before dissipating. The most extreme however can attain wind speeds of more than 300mph and stretch more than two miles across. In the case of the Alabama tornado, it stretched a mile across, 20 times larger than the typical tornado. About one in every three is classified as "strong". About 30 per cent of all tornado deaths result from this category, while nearly 70 per cent result from "violent" tornadoes. "Violent" ones however are very rare - only about two per cent.
It was also the deadliest tornado system since the "super outbreak" of April 1974, when 310 people were killed in 148 twisters across 13 states.
Tornadoes can be found on every continent except Antarctica. But the vast majority occur in Tornado Alley in the US.
Tornado Alley covers an area between the Rocky Mountains and the Appalachian Mountains, and consists of Texas, Oklahoma, Kansas, Nebraska, South Dakota and the Colorado Eastern Plains.