How are “normal temperatures” determined?

Dear Tom,

I hear references to “normal temperatures” but I do not know how they are determined.

Ziggy Toadstool
(not my real name)

Dear Ziggy,

In meteorology, what is referred to as “normal” is, by international agreement, a reference to the average of a quantity, such as temperature, over the 30-year period 1981-2010. (That base period will be adjusted upward to 1991-2020 in a couple years.) To calculate a normal temperature, add the daily high and low temperatures at a given spot in the period 1981-2010 and divide by the number of entries.

Calculating meteorological normals is an arbitrary venture. A 30-year period is used because it is long enough to include enough data to downplay occasional extreme values, but on the other hand it is short enough to be representative of changes that might be occurring in the data set (such as a warming trend).

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