How often each year does the dew point reach at least 70 in Chicago?

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Dear Tom,
My wife and I were in Florida in early October, and daily dew points were between 70 and 80. Is that unusual? How often each year does the dew point reach at least 70 in Chicago?
— Jim Boven, Elmhurst
Dear Jim,
Muggy dew points in the 70s and lower 80s occur frequently in Florida’s warm season, which typically runs from late April through mid-October, so your recent experience was not unusual. The humidity usually breaks by early November as cold fronts usher in eminently more comfortable air. In Chicago, dew points of 70 degrees or higher typically occur about 30 days each summer, but that number can soar to 50 in the city’s most humid summers. Dew points of 80 degrees or higher are rare in Chicago, with only six occurrences since 1871. The city’s highest dew point is 83 logged on a saunalike July 30, 1999.