Frequency of severe hailstorms in Europe
Generally we talk about hail from a hailstone size larger than 0.5 cm. Smaller sizes are denoted as graupel or soft hail. However, the probability that hailstone sizes smaller than 2cm cause damages to solar thermal collectors or PV-modules can be assessed as marginal. On this account the following consideration about the trend of the frequency of severe hailstorms in Europe and their potential for economical losses will consider basically hailstones equal or larger than 2 cm. Primarily the following data presented here are based on observations of the 'competence centre for local thunderstorms (Tordach)' in Germany, Austria and Switzerland. This competence centre was founded in 1997 as a network of more than 30 scientists. They collect information about local thunderstorm in Europe and their associated climatologically secondary phenomena like hail in a period of 10 years. The main objective was to obtain reliable and complete climatologically records on these severe local storm phenomena in each of the three countries. The collected information was implemented into the European Severe Weather Database (ESWD) of the European Severe Weather Storm Laboratory (ESSL) which was under construction since 2002. Additionally, since 2006 the climatologically record of thunderstorms all over Europe takes place within the European Severe Weather Database ESWD. Fig. 2 shows the increase of severe hailstorms in Europe during the last 10 years. Each picture shows all in a time period of 4 years registered Hailstorms. Thereby each point represents one event. Additional shown is the geographical distribution of severe Hailstorms.
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01.01.1997-31.12.2000 01.01.2001 -31.12.2004 01.01.2005-02.08.2008
However this Fig. 2 also shows one problem in the approach of an overall European presentation of the development of the frequency of severe Hailstorms. The illustrated pictures were generated by the European thunderstorm database ESWD. As mentioned above, up to 2006 basically the data of the competence centre for local thunderstorms (Tordach), whose data acquisitions was limited on the countries Germany, Austria and Switzerland, are available for this database. Thus it must be assumed that the recognised unbalance of the frequency of hailstorms between South/Western respectively Eastern Europe and Central Europe displays not the real circumstances, but rather results in the not - continuous detection of severe hailstorms in South/Western - and Eastern Europe. This proposition is also backed up by the illustration of the period from the 01.01.2005 to 02.08.2008 which shows a more homogeneous distribution. The reason therefore is the climatologically registration of thunderstorms all over Europe within the ESWD which is performed since 2006. The development of the annual frequency of severe hailstorms during the last 10 years is given in Fig. 3. Apart from the given total number of hailstorms in Europe per year, the percentage of hailstorms registered in Germany, Austria and Switzerland based on the total number is given. This indicates two trends which are acting in opposite directions. On the one hand, the total number of hailstorms is dramatically increasing. On the other hand, the percentage of severe hailstorms in the countries Germany, Austria and Switzerland is decreasing in relation to the total number. Once more this fact makes clear the above mentioned major problem of the currently defective standardisation in the local systems of monitoring of the European countries and thus the different way of data collection. It also shows the importance and the
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mandatory provision of the further development of an overall European thunderstorm database to backup the assessment of the economical loss potential of severe hailstorms by an established database in the near future.
