Climate standard normals

Changing the time period for climate monitoring

Weather conditions are described by measured values of meteorological elements such as air temperature, precipitation amount, sunshine duration, etc., while the climate of a place is described by long-term values, the so-called climate standard normals.

Climate normals represent mean values in 30-year periods defined by the World Meteorological Organization (WMO, 2017). Starting with the 1901-1930 normal, the standard normals 1931-1960 and 1961-1990 followed, i.e. the standard normals were usually updated every 30 years.

However, due to observed climate changes caused to the greatest extent by human caused greenhouse gas emissions, it is recommended to refresh the normals every ten years. Thus, from January 2023, the new climate standard normal 1991-2020 is used for most climate monitoring products instead of the previous normal 1981-2010.

For long-term climate change assessment, it is still recommended to use the normal 1961-1990 which is sometimes called a reference normal.

Although changing the standard period does not affect the calculation of the trend or the absolute ranking, the average values of the period will change, as well as anomalies, depending on the normal used. For example, the average annual air temperature at the Zagreb-Maksimir station increased by 0.7 °C in the period 1991-2020 compared to the 1981-2010 average (that was 11.2 °C) Thus, the anomaly of the mean annual temperature in 2022 (13.9 °C) to the first and second normal was 2.7 °C and 2.0 °C, respectively which is a consequence of the warmer new standard period compared to the previous one. This means that the maps will show more regions with below-average temperature anomalies and fewer with above-average when using the new, warmer normal. On the other hand, precipitation anomalies regarding the new normal will not show such a clear direction toward dryness or wetness since the trends in precipitation amounts in Croatia are not spatially coherent in their signs.

WMO (2017) WMO Guidelines on the Calculation of Climate Normals