Where do tornadoes happen most frequently?

Tornadoes have hit all fifty states, but this does not occur often.  Most tornadoes occur in what is known to many of us as “Tornado Alley.” 

Answer the following questions: 

• Where is "Tornado Alley"
• Why do tornadoes happen more frequently here?
•  How many tornadoes does the United States average every year?

Thinkquest
Tornadoes by State