Definition: A mobile,destructive vortex of violently rotating winds having the appearance of a funnel-shaped cloud & advancing beneath a large storm system.
Formation is precipitated by warm moist air near the ground meeting cold, dry air. Tornadoes usually form over land but they can form over tropical oceans. When the ground is heated by the sun, warm moist air rises. As it rises it cools forming massive cumulonimbus clouds and releasing heat that sustains the updraft. The updraft meets winds of different directions and speeds. This together with the rotation of the earth causes it to spiral coutnerclockwise, creating a vortex. The strength of the updraft determines how much of the surrounding air is sucked into the bottom of the tornado. As it rises the jet stream takes effect and causes an extra spin and the characteristic anvil cloud.
There are two ways that Tornadoes are measured: 1)The Fujita Scale and 2) Pearson Scale,
The Fujita scale is based on the speed of the tornado, whereas the Pearson Scale is based on the length and width of the tornado path.
Tornado Alley in the USA has the perfect conditions for the formation of tornadoes. This is due to a number of factors. The first being there is a plentiful supply of warm moist air being brought in by the low level winds from the south and the Gulf of Mexico. The second is the very dry air that comes from the Rocky Mountains and the deserts of northern Mexico. The third is the prevailing westerly winds sometimes accompanied by the powerful jet stream, carrying cool air from the Pacific Ocean.
Stormy Weather by Mark Maslin p30-33.
Hazards by Malcolm Skinner p48-49