Normals - the arithmetic mean of a climatological element computed over three consecutive decades.
Excerpt from NCDC Frequestly Asked Questions:
The term climatic normal has faced a dilemma since its introduction a century and a half ago. A climate normal is defined, by convention, as the arithmetic mean of a climatological element computed over three consecutive decades (WMO, 1989). "... a normal value is usually not the most frequent value nor the value above which half the cases fall." The casual user, however, tends to (erroneously) perceive the normal as what they should expect."
See more FAQ from NCDC Frequently Asked Questions.
Excerpt from NOAA 1981-2010 Climate Normals: Climate Normals are the latest three-decade averages of climatological variables, including temperature and precipitation. This new product replaces the 1971-2000 Normals product. Additional Normals products; such as frost/freeze dates, growing degree days, population-weighting heating and cooling degree days, and climate division and gridded normals; will be provided in a supplemental release.
See more on the NOAA 1981-2010 Climate Normals
Methodology of Temperature-related Normals (pdf)
Computational Procedures for the 1981-2010 Precipitation-related Normals (pdf)
Changes in Normals
When comparing the 1981-2010 Normals to the 1971-2000 Normals computed using the same methodology, both maximum temperatures and minimum temperatures are about 0.5F warmer on average in the new normals. The averaged annual statewide changes in maximum and minimum temperatures are shown in Figures 1 and 2, respectively.
Images courtesy NCDC - click image to see original source
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