Tornadoes can occur at any time of the year. In the southern states, peak tornado occurrence is in March through May, while peak months in northern states are during the summer.
Tornadoes are most common in spring and least common in winter. Since autumn and spring are transitional periods (warm to cool and vice versa) there are more chances of cooler air meeting with warmer air, resulting in thunderstorms.
Tornado Alley in the Midwest
By Matt Rosenberg, About.com Guide
Jun 18 2008
April through June is tornado season across most of the United States.
While 90% of tornadoes strike in the United States due to storms created by dry cool polar air from Canada which meets warm moist air from the Gulf of Mexico, tornadoes actually strike around the world. About 800-1200 tornadoes occur in the U.S. each year although most occur in uninhabited areas. A large proportion of United States tornadoes strike in an area known as Tornado Alley, which stretches from northwest Texas, across Oklahoma and Kansas.
Tornadoes form a circular column of low pressure (up to about 100 millibars lower than the surrounding air) which has very fast wind circling around it at speeds of 200 to 500 miles per hour. These 450-1800 foot wide columns can drive straw into wood and wood into metal as they create their path of destruction.
Tornadoes, also known as twisters, usually move at about 15 to 30 miles per hour towards the northeast and last from a few minutes to as long as eight hours. Tornadoes kill about 90 people each year in the United States and are the most destructive of any local atmospheric disturbance. The wind, not the low pressure, associated with a tornado is what causes the damage, which can made a city look like it was bombed (experts advise against opening windows when a tornado approaches).