Chicago’s earliest autumn freeze

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Dear Tom,
I know that Chicago’s earliest autumn freeze occurred Sept. 22, 1995, and it happened at O’Hare International Airport. But O’Hare is usually the coldest city location. What were temperatures at other Chicago locations then?
— Millie Frankinson, Chicago
Dear Millie,
On clear, calm nights, cool air often collects in places well removed from buildings and concrete surfaces. Temperatures in such locations will drop a few degrees, or more, lower than in more citylike locations.
That describes the site of the city’s official thermometer at O’Hare, whose overnight temperatures, as a consequence, often register several degrees lower than at Midway Airport.
Chicago logged its earliest autumn freeze, 32 degrees, at O’Hare on Sept. 22, 1995. The low temperature at Midway Airport was 39 degrees and 40 at the lakefront.
On clear, calm nights, cool air often collects in places well removed from buildings and concrete surfaces. Temperatures in such locations will drop a few degrees.