Historical practices regarding climate normals, as described in the Guide to Climatological Practices (WMO, 2011), the Technical Regulations (WMO, 2016b) and the Handbook on CLIMAT and CLIMAT TEMP Reporting (WMO, 2009), date from the first half of the twentieth century. The general recommendation is to use 30-year periods of reference. The 30-year period of reference was set as a standard mainly because only 30 years of data were available for summarization when the recommendation was first made. The early intent of normals was to allow comparison among observations from around the world. The use of normals as predictors slowly gained momentum over the course of the twentieth century (WMO, 2011, section 4.8).

Taking into consideration issues identified in The Role of Climatological Normals in a Changing Climate (WMO, 2007) and elsewhere, the Seventeenth World Meteorological Congress (WMO, 2015) endorsed a number of changes, which are reflected in the Technical Regulations, in definitions relating to climate normals. The most significant of these changes was that the definition of a climatological standard normal changed, and it now refers to the most-recent 30-year period finishing in a year ending with 0 (1981–2010 at the time of writing), rather than to nonoverlapping 30-year periods (1901–1930, 1931–1960, 1961–1990, and in the future 1991–2020) as was the case previously. However, the period from 1961 to 1990 has been retained as a standard reference period for long-term climate change assessments.

Source: World Meteorological Organization,WMO Guidelines on the Calculation of Climate Normals, 2017

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