Tornadoes happen when warm, moist air masses and cold, dry air masses collide to cause large thunderstorms. Not all thunderstorms produce tornadoes, and scientists are not certain which factors are directly responsible for their formation. They do know that in many cases, horizontal spinning movements in the air can be lifted vertically. Once vertical, its base may span up to three miles from one side to the other.
While tornadoes are theoretically possible in any thunderstorm, they are far more common in the United States and Canada than any other country. They are especially common in a narrow slice of land called Tornado Alley. Tornado alley includes part of Kansas, Nebraska, Texas, South Dakota and Oklahoma. The reason that so many tornadoes occur in this area is that the Gulf of Mexico and Canadian tundra are found at each end. The Gulf of Mexico provides a constant source of warm, wet air, while the air flowing in from Canada is almost always cold and dry. The two air masses often collide over Tornado Alley, causing the frequent tornadoes.
Tornadoes are one of the most destructive forces in nature, and they can cause widespread damage and casualties. Tornadoes are ranked via a scale called the Enhanced Fujita Scale. Some tornadoes have had winds that exceeded 300 miles per hour.