Storms

Storms can happen in any part of Switzerland. Their effect is strongest in exposed locations. The extent of the damage is determined not by the average wind speed but by the maximal speed of the wind gusts, which are often most harmful.

Depending on the strength of the storm, it can damage roofs, break trees or bring down scaffolding. People are normally only at risk outdoors.

Winter storms

Winter storms occur only in the transition area between the subtropical and the polar climate zones, i.e. 35-70 degrees of geographical latitude (Switzerland is situated at approximately 47 degrees). Cold polar air collides with masses of subtropical hot air and this leads to the formation of low-pressure turbulence over large areas.

The intensity of the storm areas is proportional to the temperature difference between the two air masses and is therefore highest in late autumn and winter, when the oceans are still warm, but the polar air masses are already very cold.

Wind gusts of up to 39-56 m/s (140-200 km/h) are possible. In extreme cases, wind gusts of 70 m/s (250 km/h) or more can occur in the Alps.

The storm area (low-pressure area) can reach a diameter of 1000-1500 km. Therefore, winter storms are the storm type responsible for the highest loss amounts per event in Switzerland.

Thunderstorms

The most common storm events in Switzerland are locally occurring strong winds caused by thunderstorms. The formation of large convective clouds, created by upward-movement of warm, moist air, is a necessary condition for the occurrence of thunderstorms. It requires thermal instability, the presence of mountains and the occurrence of a weather front.

Thunderstorms can be classified as single-cell, multi-cell or super-cell storms. This classification expresses the increase in duration and intensity.

The following two factors and their relationship to one another determine the type, intensity and duration of a thunderstorm: 1. the thermal layering of the atmosphere (stable/unstable) which determines the updraft energy of the air in the clouds, and 2. the vertical increase and turning of the wind (shear forces) which determine the kinetic energy of the air in the clouds. The relationship between these energy values determines the type of thunderstorm.

When the updraft energy is high (unstable layering) and the shear force small, a single-cell thunderstorm is formed. This creates heat thunderstorms, typically in summer, which last for 30 to 60 minutes.

When the updraft energy is high and the shear forces very high, multi-cell thunderstorms are formed. They last 1 to 3 hours and are often accompanied by strong wind gusts and hail.

Super-cell thunderstorms are created in cases of high updraft energy and medium shear forces. They last from 1 to 6 hours and have an internally rotating cell. Hail, wind gusts, vertical downbursts and occasionally tornados are accompanying phenomena. This is a progressing type of thunderstorm.

Foehnstorms

“Foehn” is a warm, dry, mostly strong downslope wind, which occurs on the north side of the Alps. It can also happen on the south side of the Alps, as a so-called “north foehn”, when cold air masses cross the Alps from the north or northwest.

Warm, moist air on the south side of the Alps is subjected to moist-adiabatic lifting (temperature decrease of 0.6 °C/100 m) and rains out part of the water (relief precipitation), so that during the subsequent dry-adiabatic sinking (temperature increase of 1 °C/100 m), the air is drier and warmer when it reaches the same height. This causes the high temperatures and the dryness of the foehn winds.

Foehn conditions occur frequently during the colder months of the year. Very good visibility in the extremely dry air is characteristic of foehn conditions. Cloud banks on the windward side reach over the mountain ridge as a massive cloud wall and thus the characteristic “foehn wall” can be seen from the downwind side. The cloudless area on the downwind side is recognisable as a so-called “foehn window” on satellite images.

Foehn winds can reach hurricane speed (e.g. during the storms on 16/11/2002 with maximum speeds of 60 m/s (215 km/h) in the eastern Alps).

Tornados

Tornados do not only occur in the Midwest of the United States, but everywhere in the temperate zones and therefore also in Switzerland.

They develop alongside storm fronts in thunderstorm cells and can occur together with hail storms. The average diameter of the “tornado funnel” is about 100 m, the average length several kilometres. The maximum speed at the edge of the funnel is estimated at more than 139 m/s (500 km/h) for extremely powerful tornados. However, for most tornados, gusts of slightly more than 27.8 m/s (100 km/h) are measured.

In Switzerland, there are an average number of 1-5 tornados per year. They mainly occur in the Jura region and in the north of the country, but not in the Alps. Despite the fact that most tornado events in Switzerland only cause minor damage or none because of their small size, there is a possibility of potentially larger damage (e.g. in urban areas).

Areas where tornados cause damage are generally small. However, they can cause considerable damage to affected objects.

Contact

Head office St. Gallen

Egli Engineering AG
Bogenstrasse 14, 9000 St.Gallen
T +41 (0)71 274 09 09
F +41 (0)71 274 09 08
info(at)naturgefahr.ch

Branch office Bern

Egli Engineering AG
Schwarztorstrasse 87, 3007 Bern
T +41 (0)31 381 52 90
F +41 (0)31 381 52 91

Contact point Leysin (VD)

Egli Engineering SA
Avenue du Censuy 5b, 1020 Renens
T +41 (0)24 545 12 00

Contact point Fribourg

Egli Engineering AG
Route de la Poudrière 41,
1700 Fribourg
T +41 (0)26 526 10 09

More informations

www.planat.ch