Dear Tom,
I sometimes hear you talking in your TV program about “normal temperatures”. What exactly are normal temperatures?
Maggie Wentford, Kenosha, Wisconsin

Dear Maggie,
By international agreement, a normal temperature at a location is the average temperature during the 30-year period 1991-2020. The 30-year period is stepped forward every ten years and the next period to be used will be for the period 2001-2030.

To calculate the normal temperature at a given spot, add the daily midnight-to-midnight (local standard time) daily high and low temperatures and divide that sum by the number of entries.

Granted, calculating normal temperatures in this way is arbitrary, but here is the rationale: A 30-year period is used because it is long enough, on the one hand, to include enough data to minimize the effects of occasional extreme temperatures at that location. On the other hand, a 30-year period is short enough to be representative of the current climate and not be influenced by long-term temperature changes that may be occurring (such as a warming trend).

The normals of all other weather variables (such as air pressure, dew point temperatures, wind direction and speed) are calculated in the same way.

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