Weather situation

The storms of 24 to 27 July were related to a weak pressure pattern over western Europe. In such a situation, the pressure is relatively uniform (around 1013 hPa) and thus the pressure gradient is low, resulting in light and variable winds at ground level. This allows the formation of warm bubbles of air at ground level produced by the heating of the Earth’s surface by the sun. This warm air, which is lighter than the surrounding air, ascends, giving rise to convection and to the formation of convective clouds, which can, as they develop and grow larger, generate thunderstorms.

In such a situation, storm-generation cumulonimbus clouds can appear over any region depending on the heating of the ground by the sun, and the storms usually affect only a small area. They typically occur over regions with rough topography such as the Jura, the Prealps or the Alps. In the affected region, thunderstorms occurred especially over the upper Emmental and the upper Entlebuch, where the topography allowed the formation of warm bubbles of air, which led to the formation of large convective clouds.

The thunderstorms of 28 July were caused by another meteorological phenomenon: a cold front. A small low pressure system formed on 28 July above northern France, accompanied by a cold front that passed over Switzerland during that day. The confrontation of different air masses along the front caused the ascent of the air and the formation of cumulonimbus clouds which produced thunderstorms. The precipitation amounts related to the storms that day were larger and were spread over most of Switzerland, in comparison to those of the previous days that were rather limited to areas with rough topography.