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How common is a Chicago summer without 100-degree days?

Dear Tom,
It’s been a relatively cool summer here with no 100-degree days. How common is this? Which year logged the most 100-degree days?
— Brian Hayes, Skokie
Dear Brian,
We asked Chicago climatologist Frank Wachowski to research you question, and he reported that in the city’s 147 summers of record dating to 1871, 33 have recorded triple-digit temperatures. This equates to about 22 percent of the summers, with a frequency of at least one 100-degree day every 4.5 summers.
The city’s most recent encounter with 100-degree temperatures was in 2012 when four were recorded, including a torrid three-day holiday string spanning July 4-6. The 1988 drought summer holds the record for the most 100-degree days with seven. The city’s official all-time highest temperature was recorded July 24, 1934, when the mercury reached 105 degrees.