Although summer is already a while back, we take a look at the thunderstorm activity of the months June to August. Compared with the relatively short measurement series of modern lightning recording, there were slightly more than average discharges this year, with around 360,000. This year, lightning activity was particularly high in the north and east, but the greatest lightning density at the community level was in the south.
Slightly more flashes than average
In summer 2023, a total of 362,256 lightning discharges were registered in Switzerland, including both cloud and ground lightning. This value is slightly above the average value since the beginning of modern lightning recording in Switzerland in 2009 and in sixth place overall. The year with the highest number of lightning discharges had about 140'000 more (2017).
While thunderstorm activity in June and August was relatively average, July presented itself comparatively lightning-rich with around 187,000 flashes – only twice before there were more in this month. Across Switzerland, thunderstorms occurred on 73 of 92 days from June to August this year. The regional differences are quite large, however, there were only 12 lightning days in Geneva and 60 in Bern.
Most lightning in Graubünden
With exactly 46,355 lightning discharges, Graubünden, which is large in terms of area, clearly leads the cantonal ranking, as is so often the case, followed in second place by Bern with 39,238 lightning discharges. St. Gallen completes the podium with 29,263 discharges. Most lightning occurred in the north and east of the country, while significantly fewer thunderstorms were registered in the French-speaking part of Switzerland last summer.
Fig. 1: Number of flashes per canton in summer 2023; Source: MeteoNews/Ubimet
High lightning density in the northeast
In terms of lightning density, Basel-Stadt is in first place with 30.7 lightning bolts per km², closely followed by Schaffhausen with a good 23. In last place are the Valais with about 3.8 and Geneva with about 2.1 lightning bolts per km². On average, about 9 lightning flashes per km² were registered in Switzerland in summer 2023. The following map visualizes this, however, individual locations and not cantonal figures are shown here.
Fig. 2: Lightning density in summer 2023; Source: MeteoNews/Ubimet
On the municipal level, Comunanza Caprisaca (TI) had by far the highest lightning density with 176 discharges per square kilometer, followed by Bottmingen (43.1 discharges/km², BL) and Witterswil (37.9 discharges/km², SO).
Fig. 3: Highest lightning density by municipality in summer 2023; Source: MeteoNews/Ubimet
Strongest lightning in Ilanz
The power of lightning is indicated by the current strength in the unit ampere. The strongest lightning was measured in Ilanz (GR) with a discharge of about 323 kA (July 4). In a very short time, a current of about 20,000 times higher was reached than is available in a household socket with 16 amperes.
Fig. 4: Strongest lightning in summer 2023; Source: MeteoNews/Ubimet
Lightning spectacle on July 12
The most thundery day of the summer was July 12, when about 60,000 lightning discharges were recorded, more than 8,000 of them in Zurich alone. However, the previous day was also exceptionally thunderstorm-rich, with around 35,000 lightning strikes, meaning that on both days together more than a quarter of the lightning strikes of the entire summer were recorded.
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