WRAL WeatherCenter Blog

Why would they only use the 3 full decades of temps to come up with the average when it would be more accurate to use all temps ever recorded? Mike Collins

Posted December 27, 2006

MIKE MOSS SAYS:     Mike,   At first blush that would seem to make sense, and for some applications it may actually be a better approach. However, the use of thirty-year averages ending with the most recent "zero" year for climatological "normals" was agreed upon by the World Meteorological Organization as a means of:

1. Establishing comparative climatological values that are standardized from one region or location to another, so that you don't end up trying to compare likely conditions at station A with a 37-year history against station B that has been around for 72 years, for example, and

2. As a compromise between wanting the "normals" to include a significant period of time long enough to be reflective of a full or nearly full range of potential year-to-year variability in weather conditions for the location of interest while at the same time being short enough to be sensitive to real changes in the climatology of a given location, whether those changes might be due to local/regional urbanization and building patterns, natural cycles like the Pacific Decadal Oscillation or multidecadal sea surface temperature variations in the Atlantic, or large-scale variations associated with anthropogenic gas and particulate emissions.

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