The climate of Chicago is classified as hot-summer humid continental (Köppen: Dfa, Trewartha: Dca) with hot humid summers and cold, occasionally snowy winters. Although lakefront areas such as Northerly Island have a Cfa (humid subtropical) climate using Köppen's -3 °C (27 °F) winter isotherm, even those areas are continental (Dca) under Trewartha due to winters averaging below 0 °C (32 °F), and inland areas such as Midway and O'Hare International Airports are continental even under Köppen. All four seasons are distinctly represented: Winters are cold and often see snow with below 0 Celsius temperatures and windchills, while summers are warm and humid with temperatures being hotter inland, spring and fall bring bouts of both cool and warm weather and fairly sunny skies. Annual precipitation in Chicago is moderate and relatively evenly distributed, the driest months being January and February and the wettest July and August. Chicago's weather is influenced during all four seasons by the nearby presence of Lake Michigan.

The National Weather Service office of Chicago has one of the longest periods of official weather records, dating back to 1870, though all the 1870 and 1871 weather records taken at 181 West Washington Street were lost in the Great Chicago Fire. Of the two major airports located in Chicago, Midway Airport began observations in 1928, and O'Hare Airport began them in 1958. Both sites have served in the past as the official observation location, the latter being the current official station. Weather data from Midway Airport before July 1, 1942, and after January 16, 1980, and data from O'Hare Airport before January 17, 1980, are not part of the official climate record of Chicago.

Here is a list of official weather observation locations for the Chicago office:

Note: Some of the addresses prior to 1909 are different from the post-1909 addresses

See or edit raw graph data.

Winter in Chicago is generally cold and snowy. The city typically sees less snow in winter than other major cities near the Great Lakes, such as Cleveland or Buffalo, but more snow than on the East Coast in cities like Washington DC or New York City. Seasonal snowfall in the city has ranged from 9.8 inches (24.9 cm) (in 1920–21) up to 89.7 in (228 cm) (in 1978–79), and the average annual snowfall in Chicago is 36 inches (91 cm). Most winters produce many snow falls during the season in light accumulations of around 2 in (5.1 cm). Cities on the other side of Lake Michigan usually receive more snow than Chicago because of the lake-effect snow that falls on these communities, even though northeasterly winds can sometimes bring lake-effect snow to Chicago area too. However, every three years or so during the winter Chicago experiences a heavier snowstorm that can produce over 10 in (25 cm) of snow over a 1- to 3-day period, a level of snowfall very often seen in cities on the "snowbelt" on other side of the lake such as Grand Rapids, Michigan, Kalamazoo, Michigan, and South Bend, Indiana.

Winter temperatures are generally cold. However, like much of the northern USA, can vary tremendously within the span of one week. The daily average high temperature in January at O'Hare is 31.0 °F (−0.6 °C) with the average daily low of 16.5 °F (−8.6 °C) and the daily mean of 23.6 °F (−4.7 °C). Temperatures drop to or below 0 °F (−18 °C) on 5.5 nights annually at Midway and 8.2 nights at O'Hare and up to 10–14 nights in some far western and northern suburbs, although subzero (°F) readings in the absence of snow cover are rare. There have been streaks of multiple winter seasons without a single subzero reading, yet there have also been winters with 20 or more subzero readings. The highest temperature recorded during the meteorological winter months of December, January, and February is 75 °F (24 °C), set on February 27, 1976. The lowest temperature recorded during meteorological winter is −27 °F (−33 °C), set on January 20, 1985. In addition, the all-time record low maximum temperature of −11 °F (−24 °C) was set on December 24, 1983, and tied on January 18, 1994. However, in late January 2019, a violent polar vortex drifted southward, enveloping the city in new record-breaking temperatures as low as −23 °F (−31 °C) on January 30, though the city just missed out on tying the all-time record low maximum temperature, recording a high of −10 °F (−23 °C) one day during the outbreak. Wind speeds reached at least 20 miles per hour, exacerbating the wind chill effect.

The warming effect of Lake Michigan during the winter makes subzero temperatures somewhat less common on the lakefront than in the more inland parts of the city. Highs reach 50 °F (10 °C) an average of 8.8 days each winter from December to February at Midway.