In Central Europe, each year severe winter storms cause considerable damage. A storm is defined for wind speeds in excess of 75 km/h (Beaufort 9).
In Central Europe, and particularly in Germany, a large amount of the total damage is caused by winter storms. Particularly the series of storms with Vivian and Wiebke in 1990 or with Lothar and Martin in 1999 and Kyrill in 2007 caused very high damage on a large scale. However, the total damage caused by severe whirlwinds is subject to large annual fluctuations, as they have a low probability of occurrence of about once every ten years.
According to the Beaufort Wind Scale, a storm begins at wind force 9 (75 km/h). The scale is named after Sir Francis Beaufort and is used to classify wind speeds. It ranges from force 0 (calm) to force 12 (hurricane, 118 km/h). The wind forces generally relate to a 10-minute mean value. If this value is exceeded within a few seconds, squalls are referred to.
The wind fields of extratropical storms extend more than 1,000 km. Storms usually develop in a zone with a large temperature gradient (spatial temperature difference between the equator and the poles) above the North Atlantic. As this temperature gradient is most pronounced in the winter months, severe storms occur almost exclusively at this time of the year and are therefore referred to as winter storms.
Formation of winter storms
The cause of all atmospheric movement processes is different amounts of solar radiation. This results in a different extent of warming of the Earth’s surface and the atmosphere at the equator and the pole. This gives rise to large-scale movements in the atmosphere and the ocean, which compensate the energy deficit at the pole. Due to the prevailing Coriolis force, the diverting force caused by the earth’s rotation (apparent force), this direct energy transport weakens more and more towards the higher latitudes. The consequence is a pronounced horizontal temperature gradient in the mid-latitudes (50° – 60°N). Warm, moist subtropical air meets cold polar air in these latitudes. This range provides optimum conditions for the development of a storm. The more pronounced the horizontal temperature gradient, the more intensive is the development of the storm.
In the upper troposphere at an altitude of about 10 km is a band of strong winds, the so-called jet stream, which can have speeds in excess of 300 m/s. In the mid-latitudes the jet stream controls the shifting of extratropical storms and is of major importance for the formation of storm cyclones. This causes a strong divergence (flowing apart of air masses) at height, resulting in a large pressure drop on the ground.