Tornadoes are formed only under certain types of climatic conditions and that's the reason why tornadoes occur frequently in some parts of United States and not the rest. During spring, the climatic condition between the Rocky and Appalachian Mountains is very conducive for the formation of tornadoes. This is due the fact that moist, warm air from the Gulf of Mexico, collides with the dense, cool air of the mountainous regions. The states around this region are the most likely to face a tornado and therefore are called 'tornado alley'.
Tornadoes occur due to the collision of large volumes of air, having different temperatures. Collision mostly occurs in places where warm and cold air are present together. For a tornado to form, warm air has to be present at the bottom, while the cool air at the top. When warm air tries to rise up, a large updraft is formed, which can lead to a tornado, if the biometric air pressure under the updraft is very low.