NOAA NCEI’s conventional U.S. Climate Normals summarize a uniform 30-year period of weather-station observations, including temperature and precipitation statistics. NCEI generates these official normals every 10 years. The 30 years describe the observations summarized; the 10 years describe the regeneration cycle. Normals provide a reference for comparisons with typical climate conditions. NCEI also offers supplemental 15-year normals, so the 30-year window applies specifically to the conventional product.

Source: https://www.ncei.noaa.gov/products/land-based-station/us-climate-normals