Hail is a common phenomenon in most parts of Switzerland. Factors which determine the extent of damage caused are the grain size and the density of the ice. Hailstorms with hail stones larger than 3 cm occur rarely, but cause severe damage not only to agricultural crops but also to vehicles and buildings. Large ice grains can pose a direct risk for people and animals.

The shape and size of the ice grains as well as the speed and intensity of the falling hailstones are determined by climate, terrain and wind conditions. The structure and location of the building, the characteristics of its surface and the incline of the roof also influence the effect of hail.  


Every occurrence of hail is caused by a thunderstorm. On the other hand, thunderstorms often occur without hail, for example when the atmospheric conditions are not suitable for formation of hail or when hail stones or ice pellets are so small that they melt during their fall and reach the ground as a rain shower instead.

As a general rule, only one in ten thunderstorms includes the occurrence of hail. Areas where dry, cold air and moist, warm air collide, or alpine regions where mountain masses further intensify convection, are most at risk of hailstorms.

Thunderstorms are caused by sudden and powerful vertical shifts in the atmosphere. These shifts of air masses are created either by strong insolation over hot land masses or by clashes of air masses, particularly when a summer heat wave is terminated by an advancing cold front. Such shifts can occur only if the atmospheric layers are unstable.

Strong updrafts inside the thundercloud are needed to form large hail stones. When ice grains are kept suspended, they are able to extract water droplets and ice crystals from the surrounding air in the cloud and so become larger.

In most cases, there is a concentration of updrafts in narrow funnels inside the storm. When the updraft in such an air funnel suddenly eases, the hovering water or hail mass suddenly falls to the earth.

The same thunderstorm can cause several hailstorms in different locations. Zones where hail accumulates are also known as hail streaks or hail swaths and are normally several kilometres long and less than a kilometre wide.


Head office St. Gallen

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