by HansPeter Roesli and Jochen Kerkmann (EUMETSAT)Jump to images
On 18 July 2005 a cold front approached Switzerland from France. In the pre-frontal air mass of tropical-maritime origin, lines of severe thunderstorms were forming. Over Switzerland the day started with sunny conditions and temperatures rose to 28°C-30°C. In the inner Alpine valleys weak south-foehn flow persisted. Around noon a first thunderstorm line crossed the eastern part of Switzerland while at its extreme western border (Geneva) a second line was forming. One of its cells developed into a major thunderstorm that in the early afternoon hit the northern shores of the Lake of Geneva with increasing intensity, moving from from west to east. Due to a forward speed of some 60 km/h locally it produced only short downpours, but they were accompanied by wind-driven hail (see light yellow to white colours in the Weather Radar Animation, 12:45-14:30 UTC, AVI, 5297 KB, source: MeteoSwiss).
The wind gusts were considerable, increasing from around 60 km/h to 161 km/h at the eastern end of the lake (see maximum wind gusts over Switzerland, JPG, 321 KB, and time plot from Le Bouveret and Glarus, JPG, 37 KB, source: MeteoSwiss). This cell then rapidly lost intensity when leaving the Lake of Geneva and crossing the Alpine regions of Fribourg and Berne. On the satellite images from Meteosat-8 one can derive many details of the thunderstorm cell. In particular, as evidenced by the IR window channel (IR10.8) after 12:45 UTC a cold arc was developing on top of the cell with temperatures around -60°C indicating a possible penetration into the stratosphere (see red-black to white colours in the upper left image and in the corresponding animation). The arc is not seen on the radar images due to the particular data processing (projection of maximum radar reflectivity of the volume scans). The thunderstorm cell produced considerable wind and hail damage. There were at least 8 people injured, but fortunately nobody was killed. It is estimated that about 15,000 cars were damaged or destroyed. The northern shore of Lake of Geneva (east of Lausanne), being one of the major wine growing regions of Switzerland, is particularly vulnerable to hail damage. Some 500 hectares of vineyards have been destroyed to a level that will severely reduce the yield of the excellent red and white wines of 2006 vintage.
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