Can tornadoes occur any time of year?

Dear Tom,
When I was a kid, we never heard about tornados until springtime. Larely, we have been seeing them more frequently everywhere. Why is this?
Martha F. Grieashamer Chicago
Dear Martha,
While the spring into early summer period is tornado prime-time, U.S. tornadoes have occurred in every month of the year. May is the nation’s peak tornado month, followed by June, April and July. Atmospheric instability is greatest in spring when the upper atmosphere is still chilled with winter’s lingering cold, but strengthening sunlight is warming the lower atmosphere. Tornadoes target the Gulf states in winter, but the peak occurrence shifts north reaching the northern Plains and upper Midwest in August. Chicago’s peak season runs from March through June, but the area’s deadliest tornado, the F-5 Plainfield tornado, which claimed 29 lives, occurred Aug. 28, 1990.

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