The largest concentration of tornadoes occur in Tornado Alley, which encompasses most of the Great Plains of the central United States. One quarter of the most significant tornadoes in the United States occur in Tornado Alley. Oklahoma experiences the highest number of tornadoes and the most severe tornadoes in the United States.
Tornadoes, however, can form any place where the conditions are right, such as during severe thunderstorms when high wind speeds and rotation cause airborne water vapor to form into a funnel cloud.
The flat land of the Great Plains sits between the Appalachian and Rocky mountain ranges, creating a valley that is perfect for cool polar air to mix with warmer, wetter air from the Gulf of Mexico. This mixture of air is a ripe breeding ground for severe tornadoes, which are often spawned by hurricanes coming ashore from the Gulf of Mexico.
Tornadoes are less likely to occur in the mountains, although valleys in mountainous regions are at risk of tornadoes. Locations where tornadoes are rare are often in danger when a tornado occurs since the people are not prepared, and the structures are not built to withstand tornado-force winds. Tornadoes can occur throughout the year but are most common in the spring.