Dear Tom,

When you and your meteorological team reference Chicago weather records, O’Hare Airport is often mentioned. But where did records come from before that?


Dear James,

Chicago’s official weather records began on November 1, 1870 and have continued ever since then. Records were kept at a variety of downtown loop locations through December  31, 1925. From January 1, 1926, through June 30, 1942, readings were taken at the University of Chicago on the South Side. Midway Airport became the official observation point on July 1, 1942, and official observations continued there through January 16, 1980. Official observations were transferred to O’Hare International Airport on January 17, 1980 and have remained there ever since.

Maximum and minimum temperatures from November 1, 1870 through December 31, 1870, were determined from observations at 7 am, 4 pm and 11 pm. From January 1, 1871, through September 30, 1871, they were determined from observations at 7 am, 2 pm 4 pm, 9 pm and 11pm.

The Chicago Fire of October 8-9, 1871, interrupted weather observations. All original records from October 1-14 were lost. Temperatures at 7 am, 4 pm and 11 pm for the period October 1-8 were obtained from War Department weather maps. From October 9-14 records were from 7am, 2 pm and 9 pm from observations taken by a Smithsonian Institution volunteer weather observer residing at 55 Clark Street. These observations were used to determine the maximum and minimum temperatures.

It was not until June 12, 1872, that maximum and minimum temperatures were used to register temperatures. The readings were not from midnight to midnight because the last observation for the day was at 11 pm and the station did not have a thermograph. Midnight to midnight temperature readings began in June, 1888 when the station was equipped with a thermograph.