Dear Tom,
It bothers me when weather forecasters use the term “normal” versus “average”. I think they are confusing the terms.
Jim Peterson
Dear Jim,
Meteorologists (and others in the atmospheric sciences) define “average” and “normal” in ways that are somewhat different from standard usage, and that is an unfortunate source of confusion.
In the atmospheric sciences, an average (such as an average temperature) is just what you think it is–the sum of all the values in a set divided by the number of values.
A normal is different. It is a the specific average over a standard 30-year period, currently 1971-2000. For example, a normal temperature is the average temperature over the 30-year period from 1971 through 2000.
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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.
