To reduce the impact of very large or small values on a dataset’s mean value, we smooth out the outlier values by including more observations over a longer period of time.
How long should a period of record be? Thirty years of data are typically used for developing an area’s climatology. For consistency, that length is used for most weather parameters, not just temperature and rainfall. The 30-year period is typically updated every ten years to include the next decade. So the 1997 30-year U.S. climate period of record spanned the years 1961 to 1990, whereas the next reference period went from 1971 to 2000. Note that the data in the table have a longer period of record (34 years).