"Dual Standard" of Climatological Normals
When talking about extreme weather events, "climatological normal" is often used as a standard for gauging the anomalies against long-term averages. For instance, in the Monthly Weather Summary [1] issued by the Hong Kong Observatory (HKO), the term "departure from normal" of a meteorological element refers to the difference between the figure of that element and its climatological normal of 1981-2010. However, in recent reports issued by the World Meteorological Organization (WMO), the climatological normal of 1961-1990 is still used as "climatological standard normal" [2] . Also, the 1961-1990 and 1981-2010 climatological normals are shown in parallel in some of the HKO's climatological information. So what is going on?
According to the Technical Regulations of the WMO [2][3] , the "climatological standard normal" of a meteorological element is defined as the average of data for a consecutive periods of 30 years updated every 30 years, which is referred to as the "30/30 model" [4] . For example, the 1901-1930, 1931-1960 and 1961-1990 climatological normals follow this model.
But when considering possible significant climate changes due to global warming and urbanization in recent decades, averages updated every 30 years are no longer suitable as a reference standard for climate applications [5] . As a result, many meteorological organizations, including the China Meteorological Administration, Japan Meteorological Agency, Korea Meteorological Administration, and the U.S. National Oceanic and Atmospheric Administration have started adopting another set of standard - the average of data for 30 consecutive years updated every 10 years, which is referred to as the "30/10 model" [4] [6] . HKO has also made several updates to the period for calculating climatological normals (Table 1).
| Period of climatological normal | Adoption Period |
| 1947-1976 | 1978-1980 |
| 1951-1980 | 1981-1990 |
| 1961-1990 | 1991-2007 |
| 1971-2000 | 2008-2011 |
| 1981-2010 | 2012 to present |
Table 1 List of 30-year climatological normals adopted by the Hong Kong Observatory since the late 1970s.
To resolve the issues arising from different standards, WMO finally adopted a new two-tier approach on climatological normals at the 17th World Meteorological Congress in May - June this year (2015) after several years of discussion [7][8]. For climate monitoring and research purposes, a long-term stable standard for comparison is necessary for climate change assessment, and thus the "30/30 model" is retained, i.e. the currently used climatological normal of 1961-1990. It will be replaced with the climatological normal of 1991-2020 after data update in 2021. For climate application purpose, the more timely and representative "30/10 model" is adopted, i.e. the 1981-2010 climatological normal.
The "dual standard" of climatological normals helps regulate the use of climate normals in various meteorological organizations, and at the same time meets different needs in the aspects of research and services. HKO also adopts the appropriate climatological normals for different applications. The climatological normals of Hong Kong for three most recent periods, viz. 1961-1990, 1971-2000 and 1981-2010, are tabulated side by side on the HKO's Climatological Information Services webpage [9] for reference.
References:
- HKO's Monthly Weather Summary
- IPCC, 2013: "Fifth Assessment Report - Climate Change 2013"
- WMO, 2007: "The role of climatological normals in a changing climate", World Climate Data and Monitoring Programme -No. 61, WMO-TD No. 1377
- Commission for Climatology, 2012: "Discussion paper on the calculation of the standard Climate normals: a proposal for a dual system"
- T.C. Lee, W.M. Leung & K.W. Chan, 2006: "Climatological Normals for Hong Kong 1971- 2000", Hong Kong Observatory Technical Note (Local) No. 83
- "What is Climate Normal ?"
- WMO Media Centre, 2015: New Two-Tier approach on "climate normal"
- 中國氣象報，2015年6月10日，總第3682期：「全新雙重氣候平均值標準推出」
- HKO's Climatological Information Services