Climate normals are 30-year averages that are updated every 10 years in the U.S. The National Climatic Data Center produces the climate normals for the US. By settling on a standard averaging period, users are able to compare climate conditions between two or more locations.
By updating every 10 years, the climate normals can reflect data from newer stations as well as reflect any changes to the climate. One impact of switching from the 1971-2000 to the 1981-2010 climate normals was that the cold, snowy winters of the 1970s fell out of the calculations. As a result, the new normal snowfall has dropped a little in most places. For example, the new normal annual snowfall is 3 inches less in Champaign-Urbana.
The National Climate Data Center has a climate normals page dedicated to the new climate normals and frequently asked questions.