When is the average date of the first freeze in Chicago?

Dear Tom,
When is the average date of the first freeze in Chicago?
Bill Creshant, Palatine
Dear Bill,
The first freeze, 32 degrees or lower, usually occurs across Chicagoland in October, but that date varies considerably depending on the location. A large metropolitan area, like the Chicago area, is a “heat island,” meaning temperatures within the core of the city are usually higher than in surrounding areas. Across metropolitan Chicago, the urban heat island effect results in a range of temperatures from cooler to warmer as one moves from outlying locations toward the central city.
In the autumn the relatively warm waters of Lake Michigan add an additional modifying influence that exacerbates the city’s heat-island effect. The net result is that the average date of the first autumn freeze ranges from about Oct. 8 in far west suburban locations to Nov. 2 in the Loop.

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