Dear Tom,
How do you determine the daily average high and low temperatures, and do they ever change?
Aaron Million, Chicago
Dear Aaron,
The daily average high and low temperatures are really more than that; they are actually 30-year climatological normals that are recomputed every 10 years. By an international convention defined by the World Meteorological Organization, daily normal temperatures are based on a specific 30-year period, currently 1971-2000. The current set of normals will remain in effect until the year 2011, when new values will be released, based on the 30-year period from 1981-2010. Since the daily temperature averages calculated for only a 30-year period will tend to bounce around a bit, they are statistically smoothed so they rise and fall gradually, changing by only a degree at a time through the annual temperature cycle.
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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.
WGN-TV meteorologists Steve Kahn, Richard Koeneman and Paul Dailey plus weather producer Bill Snyder contribute to this page.
