Tornado Alley is the area of the United States in which tornadoes most frequently occur. It encompasses the great lowland region of the Mississippi, Ohio, and lower Missouri River valleys. While no state is entirely free of tornadoes, these storms occur most frequently in the prairie and plains region between the Appalachians and the Rocky Mountains, particularly in Alabama, Mississippi, Arkansas, Florida, Oklahoma, Texas, Kansas, Nebraska, Iowa, and Missouri. The absence of mountains in the middle part of the continent enables masses of warm, moist air to drift northward from the Gulf of Mexico. This air is buoyant and lifts easily as a result of daytime heating of surfaces underneath or along frontal zones associated with the cooler jet stream. Large, active thunderstorms develop, with complex up-and-down drafts. Sometimes circulating wind systems within the thunderstorm clouds result in tornados. Tornadoes are characterized by extremely low air pressure at their vortex—that’s what generates the tremendously high wind speeds. They aren’t very big, measuring on average a hundred yards in diameter, but with winds reaching as high as three hundred miles per hour, their intensity is greater than that of any other type of storm.