A geographical quirk makes the United States the most tornado-prone country in the world. Cool dry air descending from Canada gets funneled eastward by the Rockies. Over the Plains, it slams into moist warm air boiling up from the Gulf of Mexico—a recipe for trouble. The peak season in the U.S. South is March through May. Summers are worst for northern states.

Fewer than one percent of the roughly 100,000 thunderstorms that occur each year in the United States spawn tornadoes. They tend to appear on the trailing edge of a storm, beginning high off the ground as intricate combinations of three ingredients: wind, temperature, and moisture.

The process begins with the storm drawing into itself warm, humid air. The warm air rises to a point where the moisture condenses into rain. This sets up an opposing motion: cool downdraft. Changes in wind speed or direction at higher altitudes—called wind shear—can knit together the cool air and warm air in a horizontally spinning tube, like a giant invisible steamroller. If more wind shifts tilt this tube so that one end touches the ground, a tornado is born.