Europe also affected
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.
Series of tornadoes in May
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.