Several of the recent years have produced above-normal precipitation. When will these wet years be figured into a new “normal” for Chicago?
— Scott Williams, Park Ridge
Precipitation normals, and normals for all other meteorological variables such as temperature and moisture content, are calculated (worldwide) once every 10 years, using a 30-year average.
The current normals are based on 1981-2010. The next recalculation will be done for 1991-2020.
A 30-year period has been selected as the basis for determining normals for meteorological variables because it comes closest to giving an accurate reading of what the average weather is at a location, while realizing that long-range weather is perhaps undergoing changes over time.