Why is there less severe weather in the fall?

Dear Tom,
Both spring and fall are transition seasons in the Midwest, but there is less severe weather in the fall. Why?

James Cook

Dear James,
Severe weather can occur at any time of the year, but you are correct to observe that, in the Midwest, less severe weather occurs in the fall than in the spring. It’s because of the temperature structure of the atmosphere from the surface upward to about 20 thousand feet (but this height can vary greatly). Warmer air is lighter and less dense than colder air. In the spring, the atmosphere is beginning to heat up in the lowest elevations but the air aloft still retains winter’s chill; this creates an unstable situation: warm below, cold above. In the autumn, the opposite occurs: the surface layer is cooling but upper levels still retain summer’s heat; it’s a stable situation.