The United States used a standard "30-Year normal" in its official climate data records. The National Climatic Data Center in Asheville (NCDC), NC is the official location where the data are stored and the 30-year normals are calculated. However, this 30-year normal or average is not recalculated at the start of each new year. Instead, the 30-year normals are computed once every 10 years. For example, the last 30 year normals covered the period of 1961-1990 and have just been replaced by the 1971-2000 normals. The next set of 30-year normals will cover the period 1981-2010. Year to year variation in weather in the U.S. can be quite extreme, therefore the normals are not calculated each once every 10 years helping to smooth out the year to year variations.
The new 1971-2000 normals were integrated into the Nebraska Weather and Climate and Lincoln Weather and Climate Web sites in July 2002. Although this is considerably later than the cutoff date of December 2000 for the data going into the current 30-year normals, it took about one full year to quality check all of the data at the National Climatic Data Center. The normals were not released to the climate centers until Spring/Summer 2002.
Is a "Normal" the Climate You Would "Expect"?
Direct Quote from the NCDC: "Climate normals are a useful way to describe the average weather of a location. Several statistical measures are computed as part of the normals, including measures of central tendency (such as the mean or median), of dispersion or how spread out the values are (such as the standard deviation or inter-quartile range), and of frequency or probability of occurrence. Over the decades the term "normal", to the lay person, has come to be most closely associated with the mean or average. In this context, a "climatic normal" is simply the arithmetic average of the values over a 30-year period (generally, three consecutive decades). A person unfamiliar with climate and climate normals may perceive the normal to be the climate that one should expect to happen. It's important to note that the normal may, or may not, be what one would "expect" to happen. This is especially true with precipitation in dry climates, such as the desert southwestern region of the United States, and with temperature at continental locations which frequently experience large swings from cold air masses to warm air masses".
The NCDC official Internet site which has a large amount of background information on the 30-year climate normals can be found at: