Today around noon a record gust of 217.4 km/h occurred in La Chaux-de-Fonds in connection with a supercell thunderstorm. If this value is confirmed, it would be the strongest gust in connection with a thunderstorm in Switzerland. But how did this extreme gust come about?
A supercell thunderstorm moved from France over the Jura today at noon. Shortly before half past midday, an extreme wind gust of 217.4 km/h occurred at the measuring station La-Chaux-de-Fonds. This value is only preliminary, but should it be confirmed, it would be the strongest thunderstorm gust measured in Switzerland. Stronger gusts have only been measured at six other stations in the Swiss measuring network, but these are exclusively exposed mountain stations (more on this below). Exactly how this potential record gust came about is not fully understood at this time. The two most likely explanations are briefly explained below:
Downburst
The most likely explanation for this wind gust, based on current knowledge, is a downburst. This term refers to a severe downburst, which can occur in the run-up to a supercell thunderstorm. For such downbursts to occur, a somewhat drier layer in the middle cloud layers is required. In this intermediate layer the precipitation falling from further above evaporates. During the evaporation process, latent (sensible) heat is used and extracted for the phase change from the liquid to the gaseous phase. The air cools, compresses, and further accelerates the downdraft caused by the falling drops. This effect can be further enhanced by melting hail, because a lot of energy is also extracted from the environment from the solid to the liquid phase. According to the definition, a downburst is spoken of as soon as the gusts have a value of 119 km/h and greater. This also corresponds to the upper end of the Beaufort scale or the definition of a hurricane gust.
Tornado
A tornado is also not excluded, although this scenario is less likely than a downburst. Tornadoes also often form during so-called supercells. For supercells to form, sufficient moisture, a trigger mechanism, and wind shear are necessary. The technical term of wind shear means the different wind directions and speeds in different altitudes of the atmosphere. This leads to a rotating thunderstorm system (supercell) and in certain, but in Switzerland extremely rare, cases to the formation of tornadoes.
Damage pattern
Typically, a downburst shows a two-dimensional damage pattern, for example in forests, where the direction of rupture is often on the same side or slightly fan-shaped. Slight variations can be explained especially by topography or turbulence. In the case of tornadoes, the characteristic feature is the slice-like devastation, with destruction occurring from different directions due to the rotating nature of the tornado.
Final assessment
For a final assessment and classification of the extreme event, therefore, the damage pattern must be investigated in the coming days on the one hand, but radar and measurement data must also be evaluated and verified. With the so-called Doppler radar it is possible to detect typical tornado signatures. If these signatures are missing, a tornado does not seem to be the right explanation.
Wind records in Switzerland
If the 217.4 km/h are confirmed, then the previous thunderstorm record gust of Glarus would be clearly surpassed. By the way, the previous station record in La Chaux-de-Fonds is 134 km/h and dates back to 26.12.1999 (hurricane Lothar).
| Measuring station | Wind record | Height of the station |
| Great St. Bernard | 268.2 km/h | 2472 m |
| Jungfraujoch | 267.5 km/h | 3571 m |
| Weissfluhjoch | 232.2 km/h | 2691 m |
| Bernina Pass | 231.5 km/h | 2260 m |
| Säntis | 229.7 km/h | 2501 m |
| Gütsch above Andermatt | 226.1 km/h | 2286 m |
| *La Chaux-de-Fonds | 217.4 km/h | 1017 m |
| .... | ||
| Glarus | 190.4 km/h | 517 m |
