The measuring conditions under which meteorological data are collected may change over time. Statements about the past, long-time climatic development can only be made if the influence of such changes is removed from the data. This is achieved by homogenisation of the data.
Almost all available series of climatic measurements have been established under measuring conditions that have changed over time. The most common causes of such changes include the relocation of stations, the use of new measuring tools or changes in the surroundings. This problem is illustrated in more detail in connection with the territorial shift of the temperature measurement associated with a difference in altitude. As the temperature on average decreases with increasing altitude, such a shift results in an abrupt change in the series of measurements, which does not in any way correspond to the actual and natural development.
The homogenisation removes these artificial changes in the series of measurements. In so doing, historical measured values are adapted to current measuring conditions and non-climatic influences are hence removed from the series of measurements. The weather service MeteoSchweiz has worked many years to develop a proven method for this task and is applying it systematically to data series of its ground-based stations.
Homogenised climatic series reveal an unaltered picture of the past climatic development. Such climatic series are the only option allowing us to make accurate statements about the climatic development, because the discrepancies between inhomogeneous original and homogeneous climatic series can be considerable.
These differences are clearly recognisable based on the series of temperature measurements from Zurich / Fluntern. While the number of summer days per year calculated from the original series since 1901 is first considerably higher than later on, the homogenous series of measurements for the same period reveals a more consistent course, with a tendency toward higher values approaching the end of the depicted measuring period. The reason for the abrupt decrease in the original data series in the middle of the 20th century can be found in the relocation of the measuring station by about 80 meters in altitude in 1949. The second noticeable decrease of summer days in the original data series around 1971 is connected to the change in the temperature shelter type (Wild to Stevenson). The relocation of the station and the change in shelter type led to slightly lower temperatures and analogously fewer summer days starting from this time. This bias is corrected by homogenisation.
Accurate climatological statements can only be made on the basis of homogenous data series. This is the reason why MeteoSwiss only publishes homogeneous long-term data series on the internet. However, the original data can likewise be ordered any time under Service & Publications.
The homogeneous data series do not always cover the same period as the original measurement series of the station in question. There can be a number of reasons for this:
The method of homogenisation as well as analyses of the existing inhomogeneities and evaluations concerning the temperature and precipitation development in Switzerland since 1864 are described in detail in a variety of publications and work reports from the weather service MeteoSchweiz.
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.