Meteorological measured parameters such as temperature or precipitation vary greatly month over month and year over year. To describe the climate of a location, these measurements are averaged over a prolonged period of time. The calculated averages are known as normal values.
In central Switzerland, winter months such as January are often characterised by hazy and fairly cool weather conditions with north easterly winds and high fog. Yet, the same month in the following year may just as well be mild and sunny, because the weather fluctuations in Switzerland are relatively large year over year. But what is a typical January like in the central part and how best to describe an average month of April in Lugano? These questions can be answered by means of averages calculated from long-time measuring periods. Unless the climate is changing, it is possible to use these averages to capture the natural fluctuations and to describe the average conditions of a location.
According to the World Meteorological Organisation (WMO) definition that has been issued in 1935, the same 30-year periods are used around the world to determine climatological averages, namely the standard periods from 1901-1930, 1931-1960 and from 1961-1990. The international coordination is important to be able to compare the climatological statements from different weather services beyond countries‘ borders. The averages of these 30-year periods are referred to as normal values or climate normals.
Normal values in a changing climate
In view of the progressive climate change in recent decades, the WMO suggested in autumn of 2011 to provide other normal values whose period is adjusted every ten years in addition to the standard periods (for example 19712000, 19812010). The purpose of this is to provide normal values that adequately describe the current climate and can be used as expected values. The standard period of 1961-1990 is kept so as not to change the anchor for the observation of the long-time climate development. The weather service MeteoSchweiz has been implementing this recommendation since 1 January 2013, in each case using the most recent periods for its statements and products. The normal values of the WMO standard period continue to be available.
Adjustments in connection with changes in the measuring conditions
Normal values are often used to compare the current weather conditions of a region with the average expected climate. This allows us to determine whether a period was too wet, too warm or too sunny. For these comparisons to be relevant, the normal values must be comparable to the current measured values. This is achieved with the homogenisation of the data. In so doing, non-climatic influences are eliminated and the normal values adjusted, if the measuring conditions at a station have changed, for example because the station was relocated. The weather service MeteoSchweiz will make these kinds of adjustments as needed per 1 January.
Available data and maps pertaining to normal values
The weather service MeteoSchweiz is providing annual and monthly data and maps of the valid normal periods for a variety of measured parameters and stations: