How often is December the coldest month?

Dear Tom,
In Chicago, how often is December the coldest month of the winter?
— Charles Kitchak, Berwyn
Dear Charles,
Chicago’s temperature records extend from November 1870 to the present, a period that covers 148 cold seasons (1870-71 through 2017-18). December has been the coldest month 23 times in those 148 cold seasons, or 16 percent of the time. It happened most recently in the winter of 2016-17, when December’s average temperature was 25.0 degrees (November was 46.8 degrees, January 28.8, February 38.0, March 39.5).
January turns out to be the coldest month of the cold season most often, with 80 occurrences (54 percent of the time); February 43 times (29 percent); March two times (1 percent, in the winters of 1889-90 and 1931-32). November has never been the coldest month.
December has been the coldest month during 16 percent of Chicago’s winters since 1870.

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