Dear Tom,
When is the earliest that a frost has been recorded in the Chicago area?
Harvey Martin,

Dear Harvey,
Because the season’s first occurrence of fall frost is patchy and scattered and would tend to occur in outlying areas, the National Weather Service does not keep statistics regarding frost but instead uses the season’s first temperature of 32 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 November. 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.