Why does the lengthening of days in the spring seem to produce violent storms?

Dear Tom,
Why does the lengthening of days in the spring seem to produce violent storms as opposed to the relative calm in the fall when the days are getting shorter?
— Ron Winfield, Northbrook
Dear Ron,
Severe thunderstorms (and tornadoes) can occur at any time of the year, but it’s certainly true that spring produces more severe thunderstorms in the US than other seasons. It’s mainly because of the differing temperature structure of the atmosphere at different times of the year. Atmospheric instability (warm air at the surface under much colder air aloft) which leads to thunderstorm development is greatest in the spring, when the surface is warming but it’s still cold aloft. Opposite conditions prevail in the fall: it’s getting cooler at the surface but it is still warm aloft. Other factors produce atmospheric instability (that’s why severe thunderstorms can occur year around) but thermal instability is of extreme importance.