Storms

Thunderstorms over Germany

Tornadoes in Germany are more common than you might think, although they rarely cause any great damage. However, two storms in 2015 were so strong that not even solid stone walls were able to withstand their power.

09.06.2016

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In spring and summer, it is not unusual for Germany to experience an inflow of warm, humid air masses from the southwest. When these air masses encounter colder air from the north and there are suitable wind shear conditions in place, they trigger a selfamplifying convection process: the warm, humid air rises to colder layers where it condenses and releases energy, causing this air to rise higher still and form thunderclouds. The enormous amounts of energy released into the atmosphere create thunderstorms, or severe convective storms as meteorologists prefer to call them. In their most extreme form, known as supercells, these storms not only bring lightning, hail, torrential rain and violent gusts, but can even spawn tornadoes.

Europe also affected

Tornadoes are most commonly associated with the US Midwest: in the world-famous Tornado Alley. This region extends from Texas in the south through Oklahoma, Kansas and Nebraska to South Dakota and Iowa in the north, running more or less parallel to the Rocky Mountains. In total, an average of 1,300 to 1,400 such whirlwinds are registered each year in the United States.

In Germany too, tornadoes are a more prevalent phenomenon than is commonly thought, with 20 to 60 occurrences a year based on figures from the German Meteorological Service. According to the European Severe Weather Database (ESWD), between 300 and 400 confirmed tornadoes are recorded each year for the whole of Europe. Considering that the above-mentioned tornado region in the US is almost 20 times the size of Germany, the frequency per unit area is almost as high in Germany as in the United States. However, in Germany most of these local tornadoes do not achieve anything like the destructive force needed to cause the same sort of devastation witnessed each year in the US (thanks to the frequently more robust construction found in Germany). Besides which, many of these storms are confined to undeveloped areas, such as fields and forests.

Tornado in Germany, 5 May 2015

The remains of a house after the wall and roof were torn off following the hurricane in Bützow, Germany, on 5 May 2015.

Series of tornadoes in May

In 2015, however, at least two tornadoes were so strong that even solid stone walls were unable to withstand them. A series of at least five tornadoes occurred on the evening of 5 May, one of which, in Bützow near Rostock in Mecklenburg-Western Pomerania, developed such force that it caused two deaths and left 30 injured by flying debris, destroyed 16 houses and damaged more than 100 cars. All in all, it caused losses totalling more than €30m. At least three tornadoes occurred in southern Germany on 13 May, one in the Waldshut district in the southern Black Forest, another near Constance in Baden-Württemberg and the third in Affing near Augsburg in Bavaria. The latter cut a swathe of destruction over 16 km long and about 150 metres wide, injuring nine people and damaging or partly destroying some 230 buildings, as well as causing losses assumed to be around €50m. The two strongest tornadoes (Bützow and Affing) reached wind speeds of more than 250 km/h, which places them as category F3 on the Fujita or F Scale (a wind speed-based system developed by and named after T. Theodore Fujita in 1971).

However, classifying tornadoes according to categories is not without its snags, as it is very difficult to precisely measure the actual wind speeds involved. To achieve this, a so-called dual polarisation radar would have to be installed at or at least near the location of the storm. This special Doppler radar uses the polarised microwaves reflected by the raindrops, ice and dust particles circulating in the tornado vortex to determine its forward speed. During the season for severe thunderstorms, professional US storm chasers – often with the financial backing of local weather TV stations – use such modern and extremely expensive mobile equipment (Doppler On Wheels, known as DOW for short) to chase after supercell thunderstorms in their specially equipped vehicles on the lookout for potential tornadoes. Despite this, however, their pickings are fairly lean: only between 20 and 30 of the more than a thousand US tornadoes each year can be analysed in this way.

Enhanced Fujita Scale in the US

As most European storm chasers do not have access to such sophisticated equipment, it would be sheer luck for a mobile Doppler radar to be in the right place when a tornado strikes. Classifying a gale-force wind according to the F Scale is therefore a highly imprecise – even arbitrary – affair and can in most cases only be achieved by analysing damage data to establish the wind speeds involved.

This is where the Enhanced Fujita Scale (EF Scale) comes into play, according to which windstorms have been classified in the US since 2007. It is based on a formula which assesses 28 loss indicators for the building stock and vegetation, and thus provides an indirect indication of the wind speed. Structural engineering experience has shown that certain damage patterns can occur at lower wind speeds than indicated in the F Scale. More pronounced differences between the two scales (see page 26) are particularly evident for extreme values (EF3 and upwards). Clearly, the indicators based on building stock damage depend on the local building codes and materials in use. The masonry commonly used for building construction in Germany is less vulnerable to the destructive forces of a tornado than the timber constructions frequently found in the US. In order to transfer the EF Scale to Germany or central Europe, all 28 parameters relating to the building stock would have to be adjusted or redefined. In the meantime, it has been suggested that tornadoes should not be classified purely on the basis of wind speed but rather on more loss-relevant factors such as kinetic energy and impact force. Engineers and scientists are working on this subject, but a final consensus has not yet been reached.

Difficult to establish pure tornado losses

Another problem with classifying tornadoes is that they almost always occur together with other destructive forms of severe thunderstorms such as hail or gale-force winds. It is therefore rarely possible to determine the loss portion solely attributable to the tornado. However, one thing holds true: the stronger a tornado is, the greater its share of the overall loss caused by the severe thunderstorm. The costliest tornado losses in the United States were caused by two supercell outbreaks in spring 2011, with several hundred tornadoes per storm event, including several EF4s and even a few EF5s. Together with hail and flash floods, these two events caused overall losses amounting to over US$ 21bn, including insured losses of more than US$ 14bn. Despite improved alerts, 528 people lost their lives in these two storms. The most severe tornado loss in Germany occurred in Pforzheim in 1968. Within the space of three minutes, the F4 tornado damaged around 2,300 buildings and destroyed hundreds of cars. More than 200 people were injured and three killed. The property loss was estimated at DM 125m, which could well be equivalent to a billion-dollar loss in today’s values. Although the mean probability of a tornado rated as category F3 or higher occurring in Germany is only about once every two years (there are around 30 per year in the US), the loss potential must be considered high on account of the greater building density (as compared to the US).

Outlook

Climate change causes the atmosphere to absorb more water vapour, fuelling severe convective storms. Yet the number of severe thunderstorms in the US has not increased in recent decades. The number of tornadoes also appears to have remained constant. What has been observed, however is an increase in the severity of the thunderstorms, especially severe thunderstorms with tornado potential in the form of supercells. There is not yet any statistical evidence of such a development in central Europe. However, there is reason to believe that climate change is more likely to lead to an increase in the severity rather than in the frequency of thunderstorms in Europe as well. To shed further light on the subject, Munich Re has launched a cooperation initiative with the European Severe Storm Laboratory (ESSL) to help improve our understanding of present and future exposure to severe thunderstorms in Germany and central Europe.