Tornado Alley

Why is the favored region for strong tornado formation in the central plains states (often referred to as TORNADO ALLELY)? Severe tornadoes often form when three very different types of air come together in a particular way. Near the ground surface southerly or southeasterly transport warm, humid air from the Gulf of Mexico into the Plains. As you go up to the middle layers of the atmosphere above the humid air, the winds veer to the southwest and transport hot, dry air from the Mexico highlands and deserts over the Plains. This forms a layer of hot, dry air in which the temperature often increases with height. The hot, dry layer is very stable and inhibits any convection that tries to develop. The winds continue to veer as you go up into the higher layers of the atmosphere. At the high layers of the atmosphere, westerly winds transport cool, moist air from the Pacific Ocean over the Rocky Mountains and into the Plains above the convective cap. This sets up a scenario that has warm, humid air near the surface with hot dry air above it and cool, moist air at the higher levels above the dry air. These three air streams also arrive from different directions, which provides the necessary wind shear. This scenario highly favors severe weather, including supercells and tornadoes.

The existence of the mid-level convective cap, which will be referred to as an inversion layer, is a key ingredient. An inversion layer is a layer in the atmosphere where the temperature increases with increasing height. Recall that typically temperature decreases with increasing height. An inversion layer is extremely stable for rising motion. At first an inversion layer will inhibit thunderstorm formation because rising parcels from the surface will not be able to penetrate through this layer. However, as the day progresses and the atmosphere gradually warms from below the inversion layer erodes away, and surface parcels become warm enough to "break through" the old inversion layer and reach the unstable atmosphere above. All day the sun's energy was used to heat the lower atmosphere and increase the water vapor content by evaporation essentially adding more energy to the lower atmosphere. When parcels are finally able to "break through", it is like blowing the lid off of a pressure cooker. This is depicted graphically on this convection cap page from USA Today An illustrative diagram of a convective cap will be drawn in lecture.

Other Tornado Links

A very comprehensive explanation behind the formation of tornadoes is given in the NOAA web page: tornadoes... Nature's Most Violent Storms A PREPAREDNESS GUIDE Including Safety Information for Schools U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Weather Service September 1992 (NOAA, FEMA, The American Red Cross).