Dear Tom,
When you present "normal" spring temperatures, are these really the averages gathered
over all of the years of record, or are they obtained by taking some low point in winter
and a high point summer and plotting a rising line from one to the other?
Ralph Kravis, La Grange, Ill.
Dear Ralph,
Normals for a location are derived from temperatures observed there during a 30-year
period, currently the years from 1971 through 2000.
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. 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.
