Climatological normal orclimate normal (CN) is a 30-year average of a weather variable for a giventime of year.[1] Most commonly, a CN refers to a particularmonth of year, but it may also refer to a broader scale, such as a specificmeteorological season.[2] More recently, CN have been reported for narrower scales, such asday of year and even hourly scale.[3]
Climatological normals are used as an average or baseline to evaluate climate events and provide context for year-to-year variability. Normals can be calculated for a variety of weather variables including temperature and precipitation and rely on data fromweather stations. Variability from the 30-year averages is typical andclimate variability looks at the magnitude of extremes.[1]Climatological standard normals are overlapping periods updated every decade: 1971–2000, 1981–2010, 1991–2020, etc.
The term "normal" first appeared in the literature byHeinrich Wilhelm Dove in 1840 and the concept was formalized by theInternational Meteorological Committee in 1872.[4] The use of the 30-year period of normals began in 1935 with the 1901-30 period.[5] The continued use of 30 year normals has increasingly been called into question due to substantial evidence that thestationarity of climate statistics can no longer be taken for granted due toclimate change.[4][6] This has led to alternative definitions such as "Optimal Climate Normal" and the "Hinge Fit" approach to supplement the standard 30 year normals which are still commonly used.[7]