Homogenization

The measurement conditions, under which the meteorological data are collected, can change in the course of time. The principal causes are site relocation, use of new instruments or change of the environmental conditions. A site relocation accompanied by a change in altitude clearly demonstrates the problem that can occur. As certain meteorological elements such as air pressure or temperature decrease with changing altitude site relocations lead to abrupt shifts in the measurements. The data series do not reflect the natural trends of the measurements in time. The scope of homogenization is to remove non-climatical influences from the time series and to adapt historical measurements to current measurement conditions. For this task, MeteoSwiss has established a methodology (Begert et al. 2003, 2005) and applied systematically to data series of its observations stations.

Homogenized climatological series reveal an unbiased picture of climatic developments in the past. Only such climate series permit to make accurate statements about climate change, because the discrepancies between the inhomogeneous/original and the homogenous climate series can be substantial.

The following document shows a comparison of original and homogeneous long-term temperature series of Switzerland:

As an example, the comparison between the number of ice days (fig.1) and the number of summer days (fig.2) per year at the Swiss NBCN station Zürich/Fluntern is presented here. It is clearly recognizable that the number of ice days decreases when the homogeneous datasets are used, while this number increases when inhomogeneous datasets are used. Similar discrepancies can be observed when looking at the number of summer days. The conclusion must be that ONLY homogeneous time series can serve as a basis for making climatological statements. Therefore, MeteoSwiss only provides homogeneous long-term data series on its webpage. However, original data series can be obtained from the MeteoSwiss data service at any time.

Consultancy and ordering

The methodology of homogenization as well as the analyses of the actual inhomogenities and evaluations of the development of temperature and precipitation in Switzerland since 1864 have been presented in detail in Begert et al. (2003, 2005) as well as in Bader and Bantle (2004). Homogeneous monthly values of temperature and precipitation dating back to 1864 of 12 observation stations are available on the internet.

• Begert M, Schlegel T, Kirchhofer W. 2005. Homogeneous Temperature and Precipitation Series of Switzerland from 1864 to 2000. International Journal of Climatology 25: 65-80.
• Begert M, Seiz G, Schlegel T, Musa M, Baudraz G, Moesch M. 2003. Homogenisierung von Klimamessreihen der Schweiz und Bestimmung der Normwerte 1961-1990. Schlussbericht des Projektes NORM90.
  Veröffentlichung der MeteoSchweiz, Nr. 67. Zürich
• Bader S, Bantle H. 2004. Das Schweizer Klima im Trend. Temperatur- und Niederschlagsentwicklung 1864-2001. 
  Veröffentlichung der MeteoSchweiz, Nr. 68. Zürich