At what rate does the city’s average high temperature decrease this time of year?

Dear Tom,
At what rate does the city’s average high temperature decrease this time of year?
— William Girard, Shorewood
Dear William,
Based on the city’s current 1981-2010 climatological normals, Chicago’s normal high temperature peaks at 85 degrees on July 9, where it remains through July 15. Then it begins a slow decline to its midwinter minimum of 31 degrees in the period from Jan. 2-29. At first that drop is slow, falling only 5 degrees to 80 by the end of August. However, the decline accelerates in September, dropping 11 degrees by the end of the month. October’s total drop increases to 12 degrees, from 68 on Oct. 1 to 56 by Halloween. The month with the greatest drop is November, from 56 to 40 degrees. December’s decline slows to 8 degrees, reaching 32 on New Year’s Eve with the final 1-degree drop to 31 on Jan. 2.

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