Why the tornado season starts in spring

Dear Tom,
The "change of seasons" is often cited as the explanation for severe weather
in the spring. So why is spring traditionally the tornado season and not the
fall?
Randall George
Dear Randall,
The explanation is not just the "change of seasons"; it's the kind of
change, and it has to do with the stability of the atmosphere.
Powerful currents of warm, rising air -- so-called "updrafts" -- provide the
energy that feeds thunderstorms. When the lowest few thousand feet of the
atmosphere is warm and it is much colder aloft, the warm air gives birth to
updrafts, and the atmosphere is said to be "unstable." That's thunderstorm
weather.
The atmosphere is thunderstorm-prone (unstable) in the spring because the
air aloft is still cold with the lingering chill of winter, but
strengthening spring sun is rapidly warming the surface layer. It's the
opposite (stable) in the fall. Air aloft still retains summer heat and the
surface layer is cooling.