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 altitudescalled wind shearcan 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.