In order for thunderstorms to develop, warm, humid air is needed. These conditions are fulfilled on the south side of the Alps on many summer days. The Alpine Divide also contributes significantly to the formation of thunderstorms: when warm, humid Mediterranean air flows over the Alps, the rapid uplift of the warm air into a much cooler environment causes an additional vertical upward movement.This often triggers violent thunderstorms.
In the dry interior of the Alps, such as in the Valais, northern and central Graubünden and the Engadine, there is much less humidity, and thunderstorms are a relatively rare occurrence in those areas.
On the north side of the Alps, in central Switzerland, there is slightly more thunderstorm activity, which may be attributable to the Napf region. In typical thunderstorm conditions, when warm, humid air is carried from the south-west over the Lake Geneva basin to the Swiss Plateau, the Napf area is the first major obstacle in the way of the flow. Here, too, the rapid uplift of air leads to condensation, which then releases heat. This causes the upward movement of the air to be further accelerated. As a topographic obstacle, the Napf thus serves as the ideal starting point for thunderstorm formation.