Chicago’s lowest average daily high temperature is 31 degrees from Jan. 2 through Jan. 29. Would it be accurate to calculate Jan. 15 (the half-way point) as the coldest day on average?
It would not. Average daily temperatures are currently calculated from the 30-year period 1981 through 2010. Average daily highs and lows are generated for each day of the year, but those averages bounce up and down by one degree or more from day to day. The daily averages are then smoothed using a complex formula. As you state, Chicago’s lowest daily average high temperature (at O’Hare International Airport) is 31 degrees in the 28-day period from Jan. 2-29. The lowest average daily high is actually on Jan. 18 with 28.0 degrees, followed by Jan. 20 with 28.5, then Jan. 15 with 28.6 degrees.