A storm that produces a tornado generally lasts two to three hours and usually gives birth to one tornado whose life is relatively short. Much of the life of the storm consists of organization and dissipation phases. The maturity period, during which the storm is likely to cause a tornado, sometimes takes a few tens of minutes. During this phase, the storm moves and carries with it a mass constantly moist unstable air renewed. In the most severe cases, the updraft and vortex tornadoes accompanying reach a steady state, and the storm becomes a "supercell". In some supercells, the intensity of mesocyclone increases and decreases rapidly, which causes a series of tornadoes. It was thus observed " families of tornadoes 'comprising up to eight members spread over a distance of 200 to 300 kilometers.
On rarer occasions, the vortex remains active for several hours and gives only one long tornado wreaks havoc in its path. The most destructive tornado that has ever been recorded is the "tornado Three Statements" of 18 March 1925 , that killed 689 people, injured 1980 and made 11,000 homeless . She traveled 352 kilometers, southeast of Missouri southwest of Indiana , through the Illinois , a speed of between 85 and 100 km / h .