Tornadoes occur all over the world, but conditions in the Great Plains of the United States are particularly favorable for their development. That’s why this part of North America is called Tornado Alley.
South of Tornado Alley, warm, moist air moves northward from the Gulf of Mexico. At the same time, cooler, drier air that has passed over the Rocky Mountains spills eastward. In Tornado Alley, the two air masses clash. The eastward-moving dry air forces the moist air skyward. As it ascends, the moist air cools below its dew point. Water vapor condenses, releasing energy that fuels the formation of supercells.
Fronts usually define these clashes between air masses. However, another boundary can occur and can form violent storms in spring, when the differences between temperatures and humidity are greatest. This smaller scale boundary, called a
dry line, occurs when moist, northward-moving air from the Gulf of Mexico meets dry, eastward-moving air from the Rocky Mountains.
 | Animation
Click to view an animated representation of a Dry Line. |
Explosive development of thunderstorms can take place when the moist air rises rapidly. One or more of these storms can develop into a supercell that produces a tornado. In rare instances, a large thunderstorm or area of severe thunderstorms can spawn a tornado outbreak, an occurrence of multiple tornadoes within the storm area. The largest tornado outbreak on record, which spawned 148 tornadoes across 13 states and Canada, occurred on April 3–4, 1974. This event is known as the “Super Outbreak.”