Is January 15 typically the coldest day on average?

This is an archived article and the information in the article may be outdated. Please look at the time stamp on the story to see when it was last updated.

Dear Tom,

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?

Michael Evans,
Palos Heights

Dear Michael,

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.

Notice: you are using an outdated browser. Microsoft does not recommend using IE as your default browser. Some features on this website, like video and images, might not work properly. For the best experience, please upgrade your browser.