Weather forecasters often make a comparison of the daily high or low temperature to the long-term "average." The frequent use of this term demands an explanation.
First, any reference to "average" weather is never meant to describe the way weather is supposed to be. Weather is usually something warmer, colder, wetter or drier than average. However, the "average" does give us a reference point that can be useful.
Usually, "average" in weather refers to the normalized curve of the 30-year average. The normalization is done so that any particularly anomalous weather during the past 30 years gets smoothed out. And the most recent three decades are used so that one can fairly compare locations with different periods of record.
For example, Grand Forks' weather record-keeping began in 1890 and so does not reflect the terribly cold decade of the 1880s, which is included in the Fargo record, which began in 1881.
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