Illinois has a temperate climate, with cold, snowy winters and hot, wet summers—ideal weather for corn and hogs. The seasons are sharply differentiated: mean winter temperatures are 22°F (–6°C) in the north and 37°F (3°C) in the south; mean summer temperatures are 70°F (21°C) in the north and 77°F (25°C) in the south. The record high, 117°F (47°C), was set at East St. Louis on 14 July 1954; the record low, –36°F (–37.8°C), was registered at Congerville on 5 January 1999.

The average farm sees rain one day in three, for a total of 36 in (91 cm) of precipitation a year. An annual snowfall of 37 in (94 cm) is normal for northern Illinois, decreasing to 24 in (61 cm) or less in the central and southern regions. Chicago's record 90 in (229 cm) of snow in the winter of 1978–79 created monumental transportation problems, enormous personal hardship, and even a small political upheaval when incumbent Mayor Michael Bilandic lost a primary election to Jane Byrne in February 1979 partly because of his administration's slowness in snow removal.

Chicago is nicknamed the "Windy City" because in the 1800s New York journalists labeled Chicagoans as "the windy citizenry out west" and called some Chicago leaders "loudmouth and windy"—not because of fierce winds. In fact, the average wind speed, 10.4 mph (16.7 km/hr), is lower than that of Boston, Honolulu, Cleveland, and 16 other major US cities. The flat plains of Illinois are favorable to tornado activity.