Chicago's normal temperatures

Dear Tom,
You often mention Chicago's daily normal high and low temperatures. How are
they arrived at?
Jim Welch
Dear Jim,
The National Weather Service calculates normals for a location (like Midway
Airport) based on temperatures observed there during a 30-year period,
currently the period from 1971 through 2000. Those normals, to be used for
10 years, will next be updated for the period 1981 through 2010.
At a specific location, average high and low temperatures for a given day,
say Aug. 10, are obtained by averaging the 30 daily highs and 30 daily lows
observed on Aug. 10 in all years from 1971 through 2000, and the same is
done for all days of the year. The daily averages, which bounce up and down
a few degrees from one day to the next, are "smoothed," and those smoothed
values are adopted as the normal daily temperatures for that location.