ASK TOM:Has the summer’s highest temperature ever been in September?

Dear Tom,

I have noticed that Chicago often has hot weather in the first two weeks of September. Has the summer’s highest temperature ever been in September?

– Joseph Schweninger
Dear Joseph,
Chicago has, indeed, logged its highest annual temperature as late as September. It’s happened nine times in 142 years (1871-2012), most recently Sept. 2, 2008, when the afternoon high of 94 degrees was the year’s highest reading. A high of 90 on Sept. 14, 1915, is the city’s most delayed occurrence of a year’s highest temperature.
Compared with mid- and late August, the frequency of 90-degree days increases considerably during the first half of September, an anomaly that occurs because vegetation, which throws a vast amount of temperature-retarding water vapor into the air, is becoming dormant.