http://www.weatherquestions.com/What_causes_tornadoes.htm
this is a good site that explains how tornadoes form.
In America’s mid-west, near Oklahoma City, an area is known as Tornado Alley because it has more tornadoes than any other place on earth. This is because "The relatively flat land in the Great Plains allows cold dry polar air from Canada to meet warm moist tropical air from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet..." http://www.nssl.noaa.gov/primer/tornado/…
