Dear Tom,

To date this year there have been 17 days of 90º or higher at Midway, but only 10 officially at O’Hare. The stations are not that far apart. Is that the heat island effect?

Greg Kaye, Westchester; Tom Shute, Mt. Prospect

Dear Greg and Tom,

It absolutely is. Midway is located in the heart of Chicago in a decidedly urban environment. Though it has become more urbanized since 1980 when it became the city’s official weather observation site, the O’Hare surroundings are more rural. Chicago climatologist Frank Wachowski compared the annual number of 90-degree days at the two sites since 1980 and Midway has logged the same or more number of days in all but two years. In 1980, O’Hare logged 27 compared to Midway’s 24 and in 2020 O’Hare “out-90ed” Midway 31-25. Since 1980, the largest difference was 10 in 2016, 26 at Midway compared to 16 at O’Hare.