Maps of the long-term averages
For the parameters hailstone size and number of hail days, the long-term annual and monthly averages per km2 are calculated throughout Switzerland. The maps show how many hail days occur on average each year and how often events with large specific hailstone sizes, for example with diameters from 2 cm, or from 4 cm respectively, are expected.
The hail frequency of hail days shows a distinct spatial pattern in Switzerland. Especially in southern Ticino, Emmental, Entlebuch and the Napf region, as well as along the Jura, there are regions where hail is to be expected frequently. In southern Ticino, hail occurs on average up to three times a year in some places. Hail occurs much less frequently in the mountainous regions - often a hail signal was only measured here in individual cases during the entire measurement series. The frequency of hail days is slightly higher in northern Graubünden, where in the past one hail day occurred every 2-3 years.
Due to the rare occurrence of hail, averaging of hailstone sizes over many years is not meaningful. The number of days where maximum hailstone sizes of at least 2 cm (or 4 cm) occur is provided as climatology. These allow for a robust statement about the frequency of events of certain hailstone sizes.
The evaluations take into account all available radar hail data since 2002. The long-term averages can be selected for the entire period (2002 to the present) and for the period of the latest weather radar generation (2013 to the present). In the years 2002 to 2012, which are based on an older radar generation, there are still isolated regions of increased uncertainties in the data, which show up in radar-typical patterns such as wave- or beam-shaped signatures around the La Dôle radar station. These have been almost completely eliminated with the latest measurement technology.