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When referring to "normal" temperature meteorologists talk in terms of the 30-year average. Why and when was this protocol instituted and how does the 30-year compare with the longer-term historical average for any given day, week, and month? (My suspicion is that the 30-year average hides the effects of global warming from our view).

Posted March 24, 2007

MIKE MOSS SAYS:      Sal,    The idea behind using 30-year periods ending on the most recent "zero" year as the basis for climate normals is that 30 years serves as a reasonably good compromise between longer time periods that might obscure ongoing trends, whether they be related to global pattern changes or more localized effects (urbanization, deforestation, reforestation, etc) and shorter time periods that might be unduly influenced by short-term variability. Updating the normal every 10 years likewise allows for regular updates while maintaining some stability to the numbers for planning and engineering purposes. For a little more discussion on these points, see the blog entry at


More specifically to your question, this protocol for standardidizing international climate normals was selected at an International Meteorological Conference in Warsaw, Poland in 1935, with the first calculated normals based on the period 1901-1930. The values have been updated every ten years since, with our current normals based on 1971-2000.

I'm not entirely sure if I'm interpreting the remainder of your question correctly, but I gather you'd like to see the current "normals" compared to the "period of record" averages covering the entire history of a given station. Here are a couple of examples, and if I'm on the wrong track feel free to write back for some clarification.

At the Raleigh-Durham airport, the full climate record stretches back to 1948, and also includes data up through 2006. Using January and July as examples, here are some comparisons of lows/highs:

Normals (1971-2000) - Jan 29.6/49.8 Jul 68.5/89.1

POR (1948-2006)         - Jan 30.0/50.6 Jul 68.1/88.8

As a point of comparison, here are similar numbers for a co-op station just southwest of Raleigh with continuous climate records stretching back into the 1800s:

Normals (1971-2000) - Jan 32.0/51.4 Jul 69.3/88.9

POR (1892-2006)        - Jan 32.2/51.2 Jul 68.8/88.4

For this same station, here is a look at daily mean temperature for:

15 Jan - Normal 41.4 POR 40.5

15 Jul -  Normal 78.6 POR 79.0


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