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What is Climate Normal ?


    Written by: LI Kin-wai    September 2012


  1. You may have come across some clauses like "the monthly average temperature was above normal" and "the last year's rainfall was less than normal" in weather reports or summaries.  So, what is the meaning of "normal" in these statements?  According to the World Meteorological Organization's regulation, the standard climate normal is derived from the observations of meteorological data calculated from the average over a 30-year period and is updated every 30 years. Under this regulation, the current standard normal covers the period from 1961 to 1990 and the next update carried out in 2021 will cover the period from 1991 to 2020.

    For climate monitoring purposes, a set of climate normal that is stable over a long period is required as a standard.  The 30-year climate normal that is updated every 30 years can fulfill this requirement.  However, when using the 1961-1990 normal in long term planning and engineering design activities, its representativeness may be affected by the significant changes in climate.  Therefore, many  meteorological organizations in the world, including the China Meteorological Administration, the Japan Meteorological Agency, Korea Meteorological Administration, the U.S. National Oceanic and Atmospheric Administration, and the Hong Kong Observatory had widen the choice of data by updating the 30-year climate normal every 10 years in order to meet the needs of different users.  At present, the climate normal for 1981-2010 is used in the Observatory's weather reports and summaries.

    Recently, climatologists are discussing ways to solve the issues related to the standard of climate normal and different purposes in climate normal applications.  One of the proposals is to adopt a dual standard system, viz retaining 1961-1990 normal till next update in 2021 for climate monitoring purpose while adopting 30-year normal updated every 10 years for other climate applications.

    This proposal is now under discussion by relevant stakeholders.  It is envisaged that the dual standard system could help standardize the usage of climate normals in different climate applications in future.


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Last revision date: <27 Dec 2012>