On what day of the year has Chicago recorded the biggest swing between the highest and lowest temperatures recorded on that date?
—George Dorinski, Chicago
Chicago’s official temperatures began on Nov. 1, 1870, and have continued in an unbroken string since then. This comprises the data set from which all temperature data for the city is calculated, a period of over 150 years. A search of daily midnight-to-midnight (Central Standard Time) temperature records reveals one standout day: Jan. 20, 1906, when the temperature soared to 63 degrees, establishing a record high for the date, and on the same date in 1985 the mercury plunged to -27 degrees, setting a record low for the date (and also establishing the city’s lowest reading ever). The range between those readings: an incredible 90 degrees.