Temperatures of 100 degrees or more in Chicago are actually fairly rare.
1988 was especially brutal, with seven days of 100 degrees or more.
Here is a list of 100-degree days in Chicago:
July 16, 1887: 100
July 17, 1887: 100
July 10, 1901: 102
July 21, 1901: 103
*July 3, 1911: 100
*July 4, 1911: 102
*July 5, 1911: 102
July 27, 1916: 100
July 30, 1916: 102
August 5, 1918: 102
August 6, 1918: 101
August 12, 1918: 101
July 19, 1930: 101
June 7, 1933: 100
June 27, 1933: 100
June 1, 1934: 102
July 22, 1934: 101
July 24, 1934: 105
August 8, 1934: 100
July 10, 1936: 102
September 7, 1939: 100
July 24, 1940: 101
July 25, 1940: 101
July 17, 1942: 100
June 27, 1944: 100
July 18, 1946: 100
*August 4, 1947: 100
*August 5, 1947: 100
*August 6, 1947: 101
August 24, 1947: 100
July 3, 1949: 102
June 28, 1952: 101
June 19, 1953: 102
June 20, 1953: 104
September 1, 1953: 101
September 2, 1953: 101
June 25, 1954: 100
July 27, 1955: 100
July 1, 1956: 103
September 7, 1960: 100
June 27, 1971: 101
June 28, 1971: 101
July 10, 1976: 100
July 7, 1980: 102
July 20, 1980: 101
July 22, 1983: 100
July 28, 1983: 100
**June 20, 1988: 104
**June 21, 1988: 101
**June 25, 1988: 103
**July 14, 1988: 100
**July 15, 1988: 102
**August 1, 1988: 100
**August 2, 1988: 100
July 10, 1989: 101
July 22, 1991: 101
August 2, 1991: 101
July 13, 1995: 104
July 14, 1995: 100
July 30, 1999: 101
July 24, 2005: 102
June 28, 2012: 100
July 4, 2012: 102*
July 5, 2012: 103*
July 6, 2012: 103*
* Greatest number of consecutive 100 degree days (3)
**Greatest number of 100 degree days in one year (7)