What is the record low for the June-August period, and the latest appearance of a 90-degree day?

Dear Tom,
Given the slow warmup this season, I’m wondering about the record low for the June-July-August period, and how about the latest appearance of a 90-degree day?
Tom Gregg, Niles
Dear Tom,
Yes, Chicago (and much of the eastern two-thirds of the nation) has been experiencing a cool start to this year’s “warm” season. Chicago’s official temperature records for the June-July-August period began with the year 1871. The chilliest June-July-August was in 1875 (when the weather station was located in the Loop), with an average temperature of 66.6 degrees.
With regard to 90-degree days, 1875 had none at all. The highest temperature that year was only 89 degrees on June 11. All other years recorded at least a few 90-degree days. The latest first occurrence was in 1915 when the temperature did not hit 90 degrees until Sept. 14.

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