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 (such as Midway Airport) based on temperatures observed there during a 30-year period, currently 1971 through 2000. Those normals, to be used for 10 years, will next be updated for 1981 through 2010.
At a specific location, average high and low temperatures for a given day, say Aug. 10, are obtained by averaging the daily highs and lows observed Aug. 10 in all years from 1971 through 2000, and the same is done 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.
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Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or: asktomwhy@wgntv.com
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