Storm

Storms can strike anywhere in Switzerland. They are particularly severe in exposed locations. The determining factor is not the average wind speeds, but the maximum strength of the gusts. These usually cause the greatest damage. Depending on their strength, house roofs can be ripped off, trees torn up and scaffolding hoisted from its anchors. People are normally only at risk outdoors.

Winter storms

Winter storms occur in the transition zone between the subtropical and polar climate zones, i.e. at about 35–70 degrees geographical latitude (Switzerland is about 47 degrees). This is where cold polar air collides with warm subtropical air masses and forms extensive low-pressure systems.

The intensity of the storm fields is directly related to the temperature difference between the two air masses. As a result, they are at their greatest in late autumn and winter, when the oceans are still warm but the polar air masses are already very cold.

Wind gusts of 39–56 m/s (140–200 km/h) are reached. In extreme cases, wind speeds of 70 m/s (250 km/h) may be recorded in the Alps.

Given that the storm field (low-pressure system) can reach a diameter of 1000–1500 km, this is the type of storm that causes the greatest amounts of damage per event in Switzerland.

Thunderstorms

In Switzerland, the most frequent storm events are regional strong winds caused by thunderstorms. For thunderstorms to occur, large convective clouds created by the upward movement of warm, humid air are necessary. This is due to thermal instability, the presence of mountains or the appearance of a front.

There are single cell, multi-cell and supercell thunderstorms. The duration and intensity of the thunderstorm increases from single to supercell.

Two variables and their relationship to each other are key to the type, strength and durability of a thunderstorm: 1) The thermal layers of the atmosphere (stable/unstable). This determines the buoyancy energy of the air in the clouds. 2) The vertical increase and rotation of the wind (shear force). This determines the kinetic energy of the air in the clouds. The relationship between the two energies is what determines the type of thunderstorm.

If the buoyancy energy is high (unstable layering) and the shear force is low, single cell thunderstorms occur. They form the typical heat thunderstorms in summer that last from half an hour to one hour.

If the buoyancy energy is high and the shear force is very high, multicellular thunderstorms are created. They last one to three hours and are often accompanied by squalls and hail.

Supercell thunderstorms occur when the buoyancy energy is high and the shear force is moderate. These thunderstorms last for a long time, from one to six hours, with one cell rotating within itself.

Hail, squalls, vertical downbursts and occasional tornadoes are associated phenomena. This class of thunderstorm is progressive.

Foehn storms

The Foehn is a warm, dry, mostly strong downdraft wind that occurs on the northern side of the Alps. It also occurs on the south side of the Alps as the so-called ‘Nordfoehn’, when cold air masses cross the Alps from the north or north-west.

Warm humid air on the southern side of the Alps is forced to rise in a humid-adiabatic way (temperature drop of 0.6°C/100m) and some of the water precipitates in the process (stagnant precipitation), so that during the subsequent dry-adiabatic descent (temperature increase of 1°C/100m) the air is warmer and drier at the same altitude. This is what causes the high temperature and dryness of the Foehn.

The conditions for Foehn often occur during the winter months. One of its most characteristic features is the exceptional visibility in the extremely dry air. The bank of clouds on the windward side reaches over the mountain ridge as a massive wall of clouds and creates the ‘Foehn wall’ visible from the leeward side. The cloudless leeward area is clearly identifiable as a ‘Foehn window’ on satellite images.

The Foehn wind can attain the force of a hurricane (e.g. an event on 16.11.2002 with a top speed of 60 m/s (215 km/h) in the Eastern Alps).

Tornadoes

The midwestern United States is not the only place where tornadoes occur; they also occur in temperate latitudes all over the world and consequently also in Switzerland.

They often develop along storm fronts in thunderstorm cells and are sometimes accompanied by hailstorms. The average diameter of the ‘tornado funnel’ is about 100 m, the average length several kilometres. Maximum speeds at the edge of the funnel are estimated to exceed 139 m/s (500 km/h) during extreme tornadoes. However, the majority of tornadoes only exhibit gusts of just over 27.8 m/s (100 km/h).

Switzerland can expect an average of 1–5 tornadoes per year. Most of these occur in the Jura and northern Switzerland, but not in the Alpine region. Even though most tornado events in Switzerland cause little or no damage as they do not cover a large area, the possibility of major damage cannot be ruled out (e.g. in urban areas).

Tornadoes have a very limited geographical damage zone. High levels of damage are possible per affected site.

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