Dear Tom:
If daily high and low temperature norms are the average of the previous 30 years, how does the average for 2004 compare with the first year this information was accumulated?
George Berliant, Northbrook
Dear George,
For a specific location, daily normal high/low temperatures are derived from 30 years of data, currently the period from 1971 through 2000. “Normals” are therefore arithmetical averages–but limited to a precise time period.
The National Climatic Data Center in Asheville, N.C., steps the 30-year climatological base period ahead ten years every decade and recalculates the normals for temperature–and many other variables like wind, air pressure, precipitation, cloudiness.
Chicago’s average temp in 2004 was 49.9 (degrees), the normal is 49.1 (degrees), and the average in 1971 (the first year in the base period) was 51.9 (degrees).
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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.
Write to: ASK TOM WHY, 2501 Bradley Pl., Chicago, IL 60618 or asktomwhy@wgntv.com (Mail volume precludes personal response.)
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