What is the average date of the last 70-degree temperature of the year at Chicago?

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Dear Tom,
What is the average date of the last 70-degree temperature of the year at Chicago?
H
attie Milander, Oak Brook

Dear Hattie,
In 92 years of temperature data (1928-2019) at Midway Airport, the average date of the season’s last-occurring 70-degree temperature is Nov. 2. In 2018, the last 70-degree temperature occurred Oct. 10, 75 degrees — the earliest ever occurrence. Autumn that year was chilly. The latest occurring 70-degree temperatures occurred in two years: 1970 and 2012. It was 71 degrees on Dec. 3, 1970, and in 2012 the temperature hit 72 degrees, also Dec. 3. Midway Airport data were used in this study because they represent the longest temperature record available at a single location. Temperature data from O’Hare Airport are available only from 1959 onward.

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