What’s the difference between the “normal” and “average” temperature

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Dear Tom,

You sometimes refer to “normal” temperatures and at other times refer to “average” temperatures. What is the difference?

— Melissa Kin, Chicago

Dear Melissa,

The definitions meteorologists apply to average and normal temperatures differ in some respects from their everyday meanings. An average temperature is an arithmetical average, but a normal temperature goes beyond that. It is a specific average temperature calculated from a 30-year period, currently 1981-2010. Normals are recalculated every 10 years. Daily highs and lows are averaged, but the results are smoothed so that the average rises and falls by only one degree at a time through the annual cycle of temperatures. In everyday usage, “normal” means “usual” or “ordinary,” but to a meteorologist it’s a specific 30-year average temperature. I occasionally use both meanings.