What Causes Tornadoes

The first four months of the year brings risk for tornadoes in the southern US. From April through June, the biggest tornado threat shifts to the Plains, Upper Midwest, and Great Lakes. The main tornado risk then stays along the northern tier of the country through much of summer, while tropical storms and hurricanes increase back in the South as they move inland. These are followed in November and December with more chances of tornadoes moving back to the South.

About ninety percent of US twisters occur in a 300-mile wide corridor extending from West Texas to Canada. Warm, moist surface winds blow up from the Gulf of Mexico, while cool high-altitude winds blow over the tops of the Rockies. The cool air wants to sink while warm air wants to rise. However, the mountain air causes a temperature inversion, which prevents the warm surface air from rising. It is like clamping the lid on a pressure cooker. The surface weather systems build up a big head of steam until they break through the inversion and shoot up to towering heights and the violent updrafts and downdrafts lead to form tornadoes. Tornadoes occur most frequently in the central plains of the US. Australia has the second most tornadoes each year.