WMO Climatological Normals
Climatological Normals have long filled two major purposes. Firstly, they form a benchmark or reference against which conditions (especially current or recent conditions) can be assessed, and secondly, they are widely used (implicitly or explicitly) as an indicator of the conditions likely to be experienced in a given location. The Technical Regulations and earlier editions of Guide to Climatological Practices contain a number of explicit definitions, as well as terms which are not formally defined but have a clear meaning. These terms are:
Averages: The mean of monthly values of climatological data (which may be monthly means or totals) over any specified period of time (no specific definition).
Period averages: Averages of climatological data computed for any period of at least ten years starting on 1 January of a year ending with the digit 1 (Technical Regulations).
Normals: Period averages computed for a uniform and relatively long period comprising at least three consecutive ten-year periods (Technical Regulations).
Climatological standard normals: Averages of climatological data computed for the following consecutive periods of 30 years: 1 January 1981 to 31 December 2010, 1 January 1991 to 31 December 2020, etc. (Technical Regulations).
Taking into consideration issues identified in The Role of Climatological Normals in a Changing Climate (WMO, 2007) and elswhere, 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 non-overlapping 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.
Collection of the WMO Climatological Standard Normals for 1981 to 2010
Circular letters "Collection of the WMO Climatological Standard Normals for 1981–2010":
Climatological Normals (CLINO) for the period 1961-1990, WMO-No. 847 (1996)
The role of climatological normals in a changing climate, WMO/TD-No. 1377 (2007)
WMO Guidelines on the calculation of Climate Normals, WMO-No. 1203 (2017)