According to the Beaufort scale, wind speeds in excess of 75 or 118 km/h more (equal to Bf 9/12) are referred to as storm and gale, respectively. This scale named after Sir Francis Beaufort is used for the classification of wind speeds according to the impacts on land or on the sea. Usually the indicated wind speeds refer to 10-minute means. Short-term fluctuations of wind speed and direction lasting only for a few seconds (3 – 20s) are referred to as (wind) gusts. The gusts usually exceed the mean wind speed by a factor between 1.5 and 3. This is the reason why the gusts are most relevant for the damage to buildings, critical infrastructure or forests. The ratio between gust and mean wind speed, referred to as gust factor, depends on surface roughness (i.e., flat surface vs. orography or build-up areas) and static stability in the lower troposphere. According to the German Weather Service (Deutscher Wetterdienst, DWD), the highest gust speed occurred on June 12th, 1985 on the Zugspitze, where a wind speed of 335 km/h was measured.
Depending on the season, different types of storms occur across Europe. Extra-tropical storms or gales are low pressure systems with a large spatial extent of the wind fields, reaching 1000 km or more. They usually develop in a zone with a large horizontal temperature gradient (Baroclinity) over the northern Atlantic. Since the temperature gradient is highest during the winter months and located more to the south, severe storms that may enter Europe occur almost exclusively during the winter months, thus referred to as winter storms. The strongest winds are usually observed at the southern flank of the low pressure system. As severe winter storms hitting most parts of Germany usually occur once in 10 years related losses show strong annual fluctuations. Especially the storm series 1990 with Daria, Judith, Vivian and Wiebke or 1999 with Anatol, Lothar and Martin, but also single events affecting the whole area of Germany such as Kyrill 2007 caused considerable damage in the order of several billion Euros.
Severe convective storms (thunderstorms, summer storms) in Germany occur only during the summer months, as their energy results from the evaporation and subsequent condensation of water vapor, which is a function of temperature. With a spatial extent from a few kilometers (single cells) to several 100 km (Mesoscale convective systems MCS or Squall lines), these weather systems are much smaller compared to extratropical storms. Likewise, related phenomena such as heavy rainfall, hail, severe gusts or tornadoes are limited to a local scale. Compared to winter storms, severe thunderstorms occur much more often, but with a high temporal variability on a certain location.
Text: Dr. Susanna Mohr & PD Dr. Michael Kunz, Karlsruhe Institute of Technology