In the strictest sense, a "normal" of a particular variable (e.g., temperature) is defined as the 30-year average.

The 30-year interval was selected by international agreement, based on the recommendations of the International Meteorological Conference in Warsaw in 1933. The 30-year interval is sufficiently long to filter out many of the short-term interannual fluctuations and anomalies, but sufficiently short so as to be used to reflect longer term climatic trends. Currently, the 30-year interval for calculating normals extends from 1981 to 2010.

There are a number of precautions undertaken by the NOAA National Climatic Data Center (NCDC) to ensure a homogenous station record. This means that there should be no changes in the station location, instruments, or observation procedures should have been made during the span that would influence the resulting climatic average. In addition, the record should be complete, with no missing values.

Normals can be computed by first computing an average daily value for any given variable, such as minimum temperature or precipitation, over 30 years (1981-2010). Then, a monthly normal value is created by computing an average of all average daily values for a given month. The Normals from NCDC are a large suite of data products that provide users with many tools to understand typical climate conditions for thousands of locations across the United States.

To learn more about the NCDC normals, please refer to the NOAA's 1981-2010 Climate Normals page.