de fr it

Hail risk in Switzerland

General

Some of the areas in Europe most under threat from hail are located in Switzerland. The danger is greatest in the foothills of the Alps from the canton of Bern into eastern Switzerland. On average, inhabitants here have to contend with damage caused by hailstones 1 cm in diameter once a year. Further danger zones are the western Bernese Oberland, southern Ticino, the Lake Geneva region and the central and eastern Jura Mountains.

In order to extend the Zurich natural hazards radar (http://www.zurich.ch/naturalhazards), Zurich Insurance Company Ltd has appointed the company Meteotest AG to create a hailstorm map for Switzerland with the latest data.

The Zurich Natural Hazard Radar was developed for Switzerland in collaboration with the engineers and geologists at GEOTEST AG, and draws upon the hazard and risk advisory maps of the Cantons and maps of the Federal Office for the Environment (FOEN).

Interpretation of the hailstorm risk

With a spatial resolution of 1 km x 1 km, the interpretation of the hailstorm risk for Switzerland makes statements on the maximum hailstone size for a recurrence period of 50 years. For the respective hazard zones, hailstones of the specified size can be expected at least once every 50 years at any building location.

Data base and methodology

Data records from the period 1999 to 2018 were evaluated for the analysis and interpretation of the hailstorm risk. The following three records were used for this purpose:

  1. Radar images with vertical sections from 2008 to 2018 in 5-minute intervals and a spatial resolution of 1 km x 1 km.
  2. Radar images without vertical sections from 1999 to 2018 in 5-minute intervals and a spatial resolution of 1 km x 1 km.
  3. Hailstorm observations from the Skywarn Switzerland database since 2011, www.skywarn.ch

The cloud height can be derived from the radar images with vertical sections and the maximum hailstone size can be deduced from the zero degree level (record 1). Record 2 comprises 20 years of radar images with which hailstone sizes were estimated on the basis of the precipitation intensity of 100 mm/h (55 dBZ). Record 3 contains 500 documented soil observations of hailstone size, location and time since 2011. This source provides important information for the calibration of hailstone sizes derived from records 1 and 2.

During a hailstorm event, the largest hailstone sizes only account for a small proportion within the low percentage range. As a result of this, it cannot be assumed that the largest hailstone size is always found and reported during an observation. Nevertheless, a correlation between observation and radar image information of the cloud height was able to be derived for calculating the maximum hailstone sizes.

The spatial distribution of the maximum hailstone size as well as the range of values are clearly comparable with the results of Nisi (2016)[1], which means the present interpretation of the hailstorm risk for Switzerland can be classified as realistic.

The project produced five analyses with the frequencies for different hailstone sizes from 0 to 5 cm (number of days per km2 and year with the corresponding hailstone size). The interpretation of the hailstorm risk for Switzerland was compiled from this for the recurrence period of 50 years, as the logic is comparable with the hazard maps for other natural hazards.

Disclaimer

© Meteotest AG, Fabrikstrasse 14, 3012 Bern, 2018


[1] Nisi, Luca et al., 2016: Spatial and temporal distribution of hailstorms in the Alpine region: a long-term, high resolution, radar-based analysis, Quarterly Journal of the Royal Meteorological Society