What are the extremes for time between the last and first 70s?

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Dear Tom,
Chicago’s last official 70 was on Oct. 22, and we finally reached one Thursday. What are the extremes for time between the last and first 70s?
— John Graf, Highland Park
Dear John,
When the mercury reached 70 degrees around noon Thursday, it ended a 171-day period since the city’s last on Oct. 22 last year, nearly a month longer than the average 142 days. For comparison, Chicago climatologist Frank Wachowski checked the record books dating back to the fall of 1942, when the city’s official thermometer was moved inland from its previous lakefront location. Since then, the city has typically logged the year’s last 70 about Nov. 2 and the first around March 25. The current 171-day span marks the city’s ninth longest 70-less period and the longest in four years since the 180 days between Oct. 12, 2013, and April 11, 2014. The shortest was just 90 days, from Dec. 2, 1982, through March 3, 1983.

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