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thumb|Downtown Chicago in late winter, showing Lake Michigan in the foreground
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, 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, 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 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:
{| class="wikitable"
|-
! Dates !! Location
|Remarks
|-
| 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 Tower
|Automatic rain gauge installed in 1897
|-
| July 1, 1905 – December 31, 1925 || U.S. Court House
|Supplemental observations through 1970
|-
| January 1, 1926 – June 30, 1942 || University of Chicago
|Supplemental observations through 1962
|-
| July 1, 1942 – January 16, 1980 || Midway Airport
|Continues 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