I see that the mercury reached a record high of 100 on September 7, 1939. Is that the date for the city’s latest 100?
—Zachary Martini, Chicago
It is. The mercury has reached 100 degrees twice on September 7, first in 1939 and again in 1960. Those triple-digit highs are the latest in the year for Chicago. It was close, but no cigar, on September 29, 1953, when the mercury just missed triple-digits, peaking at 99 degrees. On the other side of the ledger, the city’s earliest 100-degree day was recorded on June 1, 1934, when the official thermometer, then located near the lake at the University of Chicago, reached 102. Unofficially, Midway Airport logged a high of 102 degrees one day earlier on May 31, while the city’s official thermometer at the University of Chicago peaked at 98.