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.
More tornadoes strike in the part of the United States that lies between the Appalachians and the Rockies than anywhere else on earth, but tornadoes have been observed on every continent except Antarctica. Tornadoes called waterspouts occur over water but do not seem to form from supercells. They are less powerful than tornadoes over land but are still dangerous. Tornadoes can occur singly or as multiple vortexes. Some historical reports of long-lasting tornadoes may be based on a series of tornadoes rather than a single vortex.