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). These are also referred to in the 2nd edition (1983) of the Guide to Climatological Practices as ‘provisional normals’.
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 1901 to 31 December 1930, 1 January 1931 to 31 December 1960, etc. (Technical Regulations). WMO publishes the climatological standard normals which are computed by the WMO Members for their observing stations. (CLINO), WMO No.847. The latest version of this publication includes the standard normals for the period 1961-1990. Climate normal publication can be requested through the WMO online catalogue
The role of climatological normals in a changing climate
Discussion paper on Climate Normals