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The central part of the U.S. gets many tornadoes, particularly strong and violent ones, because of the unique geography of North America. The combination of the Gulf of Mexico to the south and the Rocky Mountains to the west provides ideal conditions for tornadoes to develop more often than any other place on earth. The central U.S. experienced a record-breaking week from May 4 through May 10 this year, when close to 300 tornadoes occurred in 19 states, causing 42 deaths, according to NOAA's National Weather Service.

Storms that produce tornadoes start with warm, moist air near the ground. Dry air is aloft (between altitudes of about three to 10 kilometers). Some mechanism, such as a boundary between the two air masses, acts to lift the warm, moist air upward. The boundary can be a front, dryline or outflow from another storm-essentially any kind of difference in the physical properties of two air masses. "Kinks" in the boundary are locations where rotation could occur. An updraft (air going up) traveling over the kink will "stretch" and intensify the rotation, just like an ice skater pulling in her arms.