Dear Tom,

How many times has Chicago's temperature dropped below 20 degrees below zero? The minimum temperature for USDA Hardiness Zone 5, which includes Chicago, is 20 below. Lakeshore areas are classified in Hardiness Zone 6, meaning the expected minimum is 10 below.

—Brent Riechers, Chicago

Dear Brent,

Chicago has recorded 12 days on which its official temperature fell to at least 21 degrees below zero. That's the result of a computer sweep of the city's entire 140-year temperature data base (1871-2010). Three of those frigid readings were recorded in downtown Chicago (when the city's official weather observation site was located there) and the remaining nine were logged at either Midway Airport or O'Hare International Airport (when the official site was at those locations). However, the downtown site also registered 18 to 24 below on those nine occasions.