How, where and when are “normal” temperatures determined?

Dear Tom,

Meteorologists often refer to temperatures as being above or below normal. How, where and when is “normal” determined?

Bill Murray,
Palatine

Dear Bill,

In meteorology, a normal temperature at a specific location is the temperature obtained by averaging the daily midnight-to-midnight (standard time) high and low temperatures at that location during the 30-year period currently defined as Jan. 1, 1981, through Dec. 31, 2010. New temperature normals are calculated every ten years, with the next period to be Jan. 1, 1991 through Dec. 31, 2020.

This recalculation is done to take into account the gradual changes in climate that may be occurring at a location. Normals of precipitation and the normals of all other meteorological variables are calculated using the same time period.

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