In line with the World Meteorological Organisation (WMO) specification that was introduced in 1935, the same 30-year periods are used around the world to determine climatological averages. In order to enable comparisons across national boundaries of climate and climatological reports from the different meteorological services, it is important to have international coordination and standardisation. The averages of these standard 30-year periods are referred to as normals or climate normals.
Normals in a changing climate
In light of long-term climate changes, the normal periods are regularly updated in order to ensure that the normals properly reflect the current climate of a particular region. Whereas the normal period used only to be updated every 30 years, the significant climatological changes of the past decades have led the WMO to recommend that the normal period be updated more regularly - i.e. every 10 years (1981-2010, 1991-2020, etc.). MeteoSwiss has been implementing this recommendation since 1 January 2013, and always uses the most recent complete period for its reports and products. In accordance with WMO recommendations, the period from 1961 to 1990 is used as the fixed reference period for monitoring long-term climate change.