When is the earliest that a frost has been recorded in the Chicago area?
— Chris Townsend, South Elgin
Because the first occurrence of fall frost is patchy and scattered, the National Weather Service does not keep statistics regarding frost but instead uses the season’s first temperature of 32 (degrees) or lower to define the end of the growing season. Chicago’s “heat island” and its proximity to the warm waters of Lake Michigan delay, by about a month, the average first arrival of freezing temperatures in the core of the city and near the lake. On average, the season’s first freeze occurs about Oct. 3 in the coldest outlying areas, Oct. 13 at O’Hare International Airport, Oct. 24 at Midway Airport and Nov. 5 downtown. In official records since 1871, Chicago’s first fall freezes range from as early as Sept. 22, 1995, to as late as Nov. 24, 1931.