Noticed that the "official" record high for Chicago today is 97, but Midway Airport hit 107 on this day in 1936.
A 10 degree difference is a lot, but there was actually a 15 degree difference between the official high temp and the high temp at Midway on 7/23/1934. Official high was 94, while Midway hit 109, which is the hottest temp ever recorded at a Chicago observation site.
The official observations were taken at the University of Chicago back in the 1930s, which is much closer to the lake than Midway. A number of those really hot days in the Dust Bowl years had a lake breeze.