Wetenschap - 22 februari 2001

Thunderstorms strike hardest in Italy

Thunderstorms strike hardest in Italy

Lightning strikes and kills over one hundred people in Europe each year. Where do most thunderstorms occur and how are they formed? Guest speaker Dr Aarnout van Delden shared his views on the matter on 20 February in the Meteorology and Air Quality building of Wageningen University.

The heavy rains and lightning that thunderstorms bring are among some of the most spectacular and dangerous forces of nature. Lightning reaches temperatures of 14,000 degrees Celsius in a split second, killing all life in its path. According to Van Delden thunderstorms have favourite areas in which they develop and others where they then move to: "These areas are all found in the vicinity of the Alps."

Van Delden works at the Institute for Marine and Atmospheric Sciences at Utrecht University and specialises in thunderstorms. He has studied thousands of thunder and lightning reports from more than 220 weather stations all over western Europe in order to develop an understanding of the origin and occurrence of these powerful phenomena: "The reports are not complete; we have to rely on subjective observations from many observers during a limited period of time. Nevertheless we are quite certain that thunderstorms prefer specific regions in Europe."

The Alps

Roughly speaking, the highest frequency of thunderstorms in western Europe occurs in three regions near the Alps. The first of these regions lies to the north of the Alps, and spans the French Massif Central, the Saone valley, the Jura mountains, the Swiss plateau and the higher ground of Bavaria in southern Germany. The second region is the Po valley in Italy to the south of the Alps and further east near the southeastern side of the Alps. The third region preferred by thunderstorms spans the Gulf of Genoa in the northern part of the Italian Mediterranean coastal area, including Corsica.

Thus it appears that the Italians bear the brunt of the thunderstorm activity in western Europe, and to make matters worse peak activity is during the holiday season. The cities of the Po valley, such as Bologna, Modena and Verona have the most thunderstorms during the summer; the coast near Genoa is most dangerous from August to October. It is perhaps comforting to note that most thunderstorms occur in the evening, as Van Delden found out.

Loaded gun

The reason behind the high frequency of thunderstorms in northern Italy is the warm and extremely humid conditions, believes Van Delden. The moist air comes from the Mediterranean and drifts over the gulf of Genoa and the Po Valley, where it remains for a long time. When cold air from the Alps meets the warm moist air the atmosphere becomes instable, leading to thunderstorms. The thunderstorms occurring to the north of the Alps have their origin further away, in the zone referred to as the 'Spanish plume' or the 'loaded gun': a thunderstorm track stretching from the Iberian peninsula over the Bay of Biscay and western France. "The Basque country and the southwest of France is the seat of the formation of many severe thunderstorms. These drift towards the northeast and reach maximum intensity to the north of the Alps."

According to Van Delden the area where people are least likely to be struck by lightning is northern Europe, including Scandinavia, Ireland, Wales, southwestern England and Brittany. Despite the high activity surrounding the Alps, the central part of this mountain range is also low in thunderstorm activity: moisture levels are so low that heavy rains do not occur.

Hugo Bouter

Lightning strikes over the centre of Wageningen. While the town is not located in a high activity area for thunderstorms, they do occur, particularly in the summer months when humidity and temperature levels rise. Photo Guy Acermans

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