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 winter isotherm, to even those areas are continental (Dca) under Trewartha due to winters averaging below , 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 or 32 Fahrenheit 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,with 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.
Official locations 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 locations, 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: Dates LocationRemarks October 15, 1870 – October 8, 1871 181 W. Washington St. All records lost due to Great Chicago Fire October 15, 1871 – June 11, 1872 427 W. Randolph St. June 11, 1872 – June 8, 1873 20 N. Wacker Dr. June 8, 1873 – January 1, 1887 Roanoke Building January 1, 1887 – February 1, 1890 Chicago Opera House February 1, 1890 – July 1, 1905 Auditorium TowerAutomatic rain gauge installed in 1897 July 1, 1905 – December 31, 1925 U.S. Court HouseSupplemental observations through 1970 January 1, 1926 – June 30, 1942 University of ChicagoSupplemental observations through 1962 July 1, 1942 – January 16, 1980 Midway AirportContinues to provide observations January 17, 1980 – present O'Hare Airport Note: Some of the addresses prior to 1909 are different from the post-1909 addresses
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Seasons
Winter 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, D.C., or New York City. Seasonal snowfall in the city has ranged from (in 1920–21) up to (in 1978–79), and the average annual snowfall in Chicago is . Most winters produce many snowfalls during the season with light accumulations of around . 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 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, they can vary tremendously within the span of one week. The daily average high temperature in January at O'Hare is ,with the average daily low of and the daily mean of . Temperatures drop to or below 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 , set on February 27, 1976. The lowest temperature recorded during meteorological winter is , set on January 20, 1985. In addition, the all-time record low maximum temperature of 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 on January 30, though the city just missed out on tying the all-time record low maximum temperature, recording a high of one day during the outbreak. Wind speeds reached at least , 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 an average of 8.8 days each winter from December to February at Midway.
Based on 30-year averages obtained from NOAA's National Climatic Data Center for the months of December, January, and February, The Weather Channel ranked Chicago as the sixth-coldest major U.S. city as of 2014.
Although it is extremely rare, temperatures during late (astronomical) winter (generally March 1–20) can reach up to and well over . In 2012, there were eight days in the month of March with temperatures + (with a ninth day occurring in many suburban areas) during the record-breaking March 2012 North American heat wave. The last couple of days in this record-breaking stretch of warmth occurred after the vernal equinox.
Spring Spring in Chicago is perhaps the city's most unpredictable season: Winter weather can persist well into April or even into early May, with the 1953-54 winter in Chicago lasting from November until May, as measured by the timespan between the first and last measurable snows of the season. Thunderstorms can occur any time of the year but are most prevalent in the springtime, as the city's central location within the United States, as well as its lakeside location, makes it a center of conflicts between large volumes of warm and cold air, which can trigger a wide variety of severe weather. The most severe storms can contain large hail, damaging straight-line winds, flooding, and tornadoes. Since 1850, 23 tornadoes have struck the Chicago city limits. During thunderstorms, lightning strikes are seen to frequently hit Chicago's skyscrapers.
Large snowfalls have occurred in late March and in early April. In 1970, over of snow fell in a storm that occurred on April 1–2. Twelve years later, Opening Day for the Chicago White Sox was postponed due to another snowfall that had occurred on April 5. Even more extraordinarily, over of snow fell on March 25–26, 1930, which remains one of the city's five biggest recorded snowstorms despite occurring after the vernal equinox. The average date for the last measurable snowfall (≥) is April 1.
Temperatures vary tremendously in the springtime; at , March is the month with the greatest span between the record high and low. At O'Hare, temperatures as low as have been recorded as late as April 7 and May 21, respectively. Conversely, in official records, the earliest high occurred on June 1, 1934, when official readings were taken closer to Lake Michigan. Though rare, triple-digit heat has occurred in late May at Midway Airport and in outlying suburban locations. Typically, the last freezing low of the season on average occurs on April 13 at Midway and ten days later at O'Hare. The highest temperature recorded during the meteorological spring months of March, April, and May is, officially, on May 31, 1934, when weather records were still taken near Lake Michigan. The lowest temperature recorded in meteorological spring is , set on March 4, 1873.
During the springtime, the effects of Lake Michigan are most prevalent. During this season, the lake is still quite cold, as the effects of much warmer temperatures are slow to affect the large body of water of Lake Michigan. It is common for Lake Michigan shoreline and water temperatures to remain in the 40s even into May. If the winds blow from the east, or from Lake Michigan into the city, a wide discrepancy in temperatures in a matter of miles can be found, especially on particularly warm days. It is not uncommon for high temperatures to be officially recorded in the 80s or lower 90s °F (27–34 °C), particularly in early June, at O'Hare, Midway, and in suburban locations but to have temperatures be cooler along the immediate lakeshore.
Summer On a typical summer day, humidity is usually moderately high, and temperatures ordinarily reach anywhere between . In July, it is not uncommon for the temperature to go around . Overnight temperatures in summer usually drop to around , although even in July and August there can be several nights where the temperature drops below , particularly during the cooler summers, and it is not uncommon to see temperatures plummet lower than . Conversely, on the other extreme, temperatures can on a rare basis remain above overnight, though this level of overnight warmth is generally limited to the city proper with its urban heat island effects along with Lake Michigan nearby. On such warm nights, especially during strong heat waves, most suburban locations drop down to between but quickly rebound in the early morning hours. During such strong heat waves, the outlying suburban areas can record temperatures more than above city and lakeshore locations.
A perfect example of such an occurrence happening, although considered "unofficial", occurred during the Dust Bowl years. Midway Airport recorded a record eight consecutive 100 °F-plus days in July 1936. In that heat wave, temperatures at the lake remained in the middle and upper 90s Fahrenheit (middle 30s Celsius), whereas Midway Airport recorded temperatures over for nearly two weeks, peaking at on July 11. The official record high for Chicago for July 11 is also from 1936—but is recorded as just . Further west in what would today be the near and far suburbs (e.g. DuPage County and westward), temperatures reached a blistering or still higher at points during this massive heat wave. These extreme temperatures are not shown in the official records because daily weather observations were taken at the University of Chicago until 1942. The University of Chicago is close to the lake, which can and does reduce temperatures in the immediate shoreline area in the summer.
The highest temperature recorded in Chicago during the meteorological summer months of June, July, and August, which is also additionally the all-time record high in the city, is , set on July 24, 1934, though at Midway Airport, a future observation site, the temperature reached . The lowest temperature recorded in the meteorological summer months is , set on June 4, 1945. In addition, the all-time record high minimum temperature of was set on July 29, 1916.