A number of of the current years have produced above-normal precipitation. When will these wet years be figured into a new “regular” for Chicago?
— Scott Williams,
Precipitation normals, and normals for all different meteorological variables akin to temperature and moisture content, are calculated (worldwide) as soon as every 10 years, using a 30-yr average.
The present normals are based mostly on 1981-2010. The subsequent recalculation will probably be achieved for 1991-2020.
A 30-yr interval has been chosen as the idea for determining normals for meteorological variables because it comes closest to giving an accurate reading of what the typical weather is at a location, whereas realizing that lengthy-range climate is probably present process modifications over time.