How are Chicago’s “normal temperatures” determined? You mention them frequently.
Jerry Byrne, Elk Grove
The National Weather Service calculates normal temperatures based on a 30-year period, currently 1981 through 2010. Those normals will be used for ten years and will then be updated for the period 1991 through 2020. At a specific location, the average high and low temperatures for a date, say March 1, are obtained by averaging the 30 daily high and low temperatures for all years from 1981 through 2010. The same is done for all days of the year. The daily averages, which bounce up and down a few degrees from day to day, are them smoothed so that they change by no more than one degree from one day to the next. Those smoothed values are adopted as the normal daily high and low temperatures for that location.