Dear Tom,
You reported that Chicago’s earliest freeze (32(degrees)) in the autumn was on Sept. 22, 1995, at O’Hare Airport, but you also said O’Hare’s thermometer is in a low spot that cools strongly. What was the temperature at a more typical spot in the city on that same night?
Fran Laurek, Chicago
Dear Fran:
It’s true that on clear, calm nights when cool air drains into low spots well removed from buildings and concrete surfaces, temperatures in those places will drop lower than in other more city-like locations.
And that describes the site of the thermometers at O’Hare, whose overnight temperatures, as a consequence, often register several degrees cooler than Midway Airport.
On Sept. 22, 1995, Chicago logged its earliest-ever autumn freeze, 32(degrees) as recorded at O’Hare. At Midway, the low was 39(degrees) and at the lakefront, 40(degrees).
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
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