30-year Climatological Averages

Dear Tom,
You refer to weather averages based on the 30-year period 1981-2010. Wouldn’t it be better to use a longer period of time?
Donna Mettelford, Chicago
Dear Donna,
Climatological averages are based on a standardized 30-year period, currently 1981-2010, for several reasons. First, for comparative purposes. In order for meaningful comparisons to be made between sets of data and statistics (like averages) derived from them, the “ground rules” must be uniform. Second, we want the statistics to be representative of the current climate. This is an argument for a short (that is, recent) data base. Perhaps the climate was different, say, 100 years ago. We would not want “old” data to bias newer information. Finally, it is necessary to use a large enough data set so that occasional extreme values will not excessively distort the averages. A 30-year period does this.

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