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Quick Answer. Tornadoes can last from a few seconds to over an hour, although most last less than 10 minutes. They form from mesocyclones, which are areas of organized rotation within supercell thunderstorms. Scientists still don't know why some mesocyclones become tornadoes and some do not.
How many tornadoes per year occur in tornado alley? It is difficult to say, as Tornado Alley does not have definite  boundaries. If we consider the following states to constitute  Tornado Alley: Texas, Oklahoma, Kansas, Nebraâ¦ska, Iowa, and  Missouri; then our total comes to an average of roughly 450  tornadoes per year.