Severe thunderstorms in Europe

Severe thunderstorms can occur almost everywhere in Europe. A particularly high level of severe thunderstorm activity has been observed in regions of southwestern, central, southern and southeastern regions of the continent. The strongest activity has been recorded in northern Italy’s Po Valley directly south of the Alps. There is also a high level of activity directly north of the Alps along an arc stretching from the northern half of Switzerland over southern Germany and into parts of Austria. Further high-incidence regions are at the foot of the Pyrenees, in southeastern Spain, in the area close to the Massif Central in France, and in southeastern Europe in the vicinity of the mountain ranges there. Severe thunderstorm activity reduces directly above the high mountain regions because, on average, there is less convection due to lower surface temperatures and moisture. Thunderstorm activity progressively declines towards the coastlines in the northern and northwestern regions of Europe. It is true that autumnal flooding losses on the French Mediterranean coast or in northern Italy in the course of a northward atmospheric flow from the Mediterranean are often conditioned by a low pressure system in the western Mediterranean, but these are triggered locally under the influence of thunderstorm cells.

Over the last few years, severe thunderstorms in Europe have frequently resulted in insured losses of more than a billion euros, mainly from hail and strong gusts, but also in connection with flash flood events. For example, the severe thunderstorms on 27/28 July 2013 in the north and southwest of Germany cost the insurance industry as much as US$ 3.8bn. In many cases, building losses are sustained because the fall direction of hailstones is pushed away from the vertical by wind, so that they impact on building walls with external thermal insulation, with the result that the thin plaster finish is chipped off down to the reinforcement fabric. Other vertical surfaces, such as façade elements, illuminated advertising and external sun protection systems on the windows, are also damaged in this way. As a general rule, it has been found that roofs and walls, or façade elements in buildings, usually dominate hailstorm loss patterns. Losses to the roof and interior can increase dramatically if rain then penetrates the building through broken roof tiles, invariably in older building stock.

Needless to say, as well as damage to commercial and residential buildings, losses in marine and motor insurance are also major contributors to the overall loss, especially where these involve car storage yards or traffic on the roads during busy periods. It is clear that the use of more expensive construction materials and rising repair costs are a major factor in the increasing losses in Europe from severe thunderstorms, and in particular from hailstorms and storm gusts.

Insurance data, such as the number of days with hail damage and loss figures in excess of specific thresholds, actually show increases in the number of events for the southwest of Germany, on top of the increase in available thunderstorm energy in the region, and other thunderstorm-related variables (Kunz et al., 2009). Observations in France (Atlantic/Pyrenees) using hail pads, which can measure the kinetic energy of hailstones, found substantial increases in the region of 70% in the annual mean value for kinetic energy per hailstorm during the period 1989 to 2009 (Berthet et al., 2011), although there was no trend for the annual frequency of the hail events. Similarly, in northern Italy in the period 1975 to 2009, significant increases of almost 60% were observed in kinetic energy for severe events, i.e. the top 10% (Eccel et al., 2012). An interesting observation in this context is that the height of the freezing level above ground plays a significant role in the distribution of hailstone size in a hailstorm event, and thus also influences the kinetic energy: the height of the freezing level rises as the temperature increases. Under these conditions, smaller hailstones (approx. <1 cm in diameter) in a storm would melt faster during their descent; for this reason, evaluations of the hail pads for a higher freezing level show corresponding decreases. On the other hand, because of the thicker layer beneath the freezing level in warmer conditions, a pronounced updraught region results in which larger hailstones can form. Consequently, in this scenario a greater number of large hailstones (approx. >1 cm in diameter) reach the ground. The fact that the average height of the freezing level has increased over the last few years suggests that this process has already contributed to the observed increases in the kinetic energy of hailstorm events and will continue to contribute in the future (Dessens et al., 2015).

On the question of future changes in thunderstorm activity due to climate change, the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, published in 2013, stated the following: “Overall, for all parts of the world studied, the results are suggestive of a trend toward environments favouring more severe thunderstorms, but the small number of analyses precludes any likelihood estimate of this change.” (IPCC, 2013, WG I, p. 1087). Reference is made to two studies on the estimation of insured losses: for agricultural insurance in the Netherlands, hail claims covered under outdoor farming insurance are projected to increase by between 25% and 29%, and claims under greenhouse horticulture insurance by between 116% and 134%, assuming a temperature increase of +1°C (Botzen et al., 2010). According to a joint project undertaken by the German Insurance Association (GDV) and climate research institutions, a 15% increase in the annual claims rate of homeowners’ comprehensive insurance due to hail-dominated summer storms has been projected for the period 2011 to 2040, compared with the reference period 1984 to 2008, and an increase of as much as 47% for the period 2041 to 2070. The assumed emission scenario (SRES A1B) and the global warming resulting from it will remain roughly consistent until the 2040s with the path to meet the two-degree limit (Gerstengarbe et al., 2013). The statement on the sign of the change is more important than the actual percentage figures, which are subject to many uncertainties relating to the models and the greenhouse gas concentration scenarios. Even if humankind manages to meet the two-degree limit, substantial increases should be expected over the next few decades.

For risk carriers, this means that ever greater importance must be attached to efforts to ensure construction materials are more resistant to hailstones, and to promote the use of hail nets and loss prevention efforts across the board. This is because the volume of destructible assets will also increase further along with the potential changes in the hazard. With this in mind, the insurance industry fully supports efforts to improve the strength and resistance of buildings. The Swiss Cantonal Fire Insurance Association runs the “Elementary Safety Register Hailstorm”, which establishes the hail resistance of various materials used in building exteriors. Companies can have their products tested by means of a hail impacter, which fires hailstones of defined properties at the surfaces used on buildings. Products that pass this test are then listed in the hail register. Initiatives of this kind can help make loss prevention an integral part of competition among the manufacturers of such materials. In this way, the aspect of loss prevention can be incorporated in the building planning phase and help to ensure that costly repairs become less likely.