Thunderstorms are a common weather phenomenon in spring and summer in Switzerland. They arise when the air is warm and moist, and can have a severe impact. As well as being associated with lightning, they often bring heavy rain, hail and strong gusts of wind.
By definition, a thunderstorm is an atmospheric disturbance. The phenomenon is a very chaotic one and is caused by the vertical movement of air masses, typically accompanied by very clearly visible phenomena, namely heavy showers, lightning, wind gusts and hailstones. Technically speaking, a thunderstorm starts from the moment the first electrical discharge is registered, even if this has only taken place inside the cloud.
Thunderstorm clouds are known as cumulonimbus clouds. They have a pronounced vertical development and can be distinguished from other clouds by the fact that they are accompanied by thunder and lightning. If lightning is seen to light up a cloud from a long distance, this is called sheet lightning.
Thunderstorms can occur locally or across a whole region and can be severe. They are often associated with precipitation in the form of rain showers that can become very heavy within a short space of time and can also be accompanied by hail. They can also produce strong gusts of wind.
Thunderstorms mainly occur in summer, when there is sufficient heat and humidity forthe air to rise and for cumulonimbus clouds to form. Thunderstorms are relatively uncommon in autumn and winter. When they do occur at that time, they are associated with a cold front, and they either develop when the cold front passes through, because of the strong updraft zone and rapid cooling, or after the front has passed, when their formation is caused by the unstable cold air.
Thunderstorm and lightning frequency in Switzerland
Weather forecasts mention the distribution and intensity of thunderstorms, and sometimes also the strength of the wind gusts. In such cases, the following terms are used (in ascending order according to the number and distribution of the thunderstorms):
If the terms ‘strong’, ‘violent’ and ‘intense’ are used, thunderstorms can be expected to cause damage.
In fact, heavy rainfall or hail can cause flooding in towns and cities, as well as landslides and mudslides. Strong winds can cause a great deal of damage to buildings and vegetation, including uprooting plants and trees. Moreover, lightning can injure people and animals, and it is often the cause of forest fires and damage to buildings.
In Switzerland, it is the regions south of the Alps, especially southern Ticino (Malcantone and Lugano), as well as the Entlebuch and the Jura, that are the most prone to thunderstorms. On the other hand, the heaviest thunderstorms usually occur on the northern side of the Alps, in the Alpine foothills, and on Switzerland’s Central Plateau.
Some of the strongest gusts recorded in the automatic monitoring network of MeteoSwiss were measured during the passage of thunderstorms over the stations. In addition, thunderstorms are also responsible for the heaviest precipitation measured within a short space of time. The record amount of precipitation within a 10-minute timeframe was 41 mm, which was registered in Lausanne in 2018. The location of Locarno Monti holds the record for the most precipitation within one hour, with 91.2 millimetres, measured in 1987. The strongest gust was recorded in July 1985 in Glarus, with a wind speed of 190 km/h. The largest hailstones ever were observed on the Central Plateau and in the districts of Mendrisio and Locarno, with a maximum diameter of 6-7 centimetres. The most severe damage from hail in Switzerland was recorded in the Entlebuch/Lucerne region and in an area between Berne and Zurich. Significant hail damage also occurs in Ticino from time to time. One of the most devastating thunderstorms was the violent hailstorm that hit the Locarno region on 25 August 2023.
Thunderstormsare an integral part of our climate. They play an important role in maintaining the water balance, and they exert a significant influence on nature as well as on our culture. Thunderstorms are an impressive natural phenomenon, but attempting to predict them even a few hours ahead can sometimes prove to be a challenge. Even when deploying state-of-the-art technology to analyse the structure and dynamics of thunderstorms with the help of weather data, the prediction of this locally developing phenomenon nevertheless requires a high level of temporal and spatial precision. This is where meteorology is pushed to its limits and meteorologists have to content themselves with highly generalised statements, which are often unsatisfactory to both the experts and the users.
Because it is difficult to make accurate forecasts, advance warnings of severe thunderstorms at Level 3 or 4 are issued for large regions when the conditions are right for thunderstorm development.
With such warnings, the probability of one or more thunderstorms occurring in these regions is between 40% and 70%. These warnings are generally issued between 24 and 48 hours ahead. As soon as a thunderstorm develops, a Level 3 or 4 warning is issued at very short notice for regions that are in the area of origin or along its path. These warnings are usually issued 30 minutes to 2 hours in advance. Nevertheless, there are occasions when warnings are not issued soon enough, such as when a thunderstorm forms rapidly over a region without any prior indications.
The advance and immediate warnings also contain information about the dangers associated with wind gusts, hail and heavy rain. If an advance warning of severe thunderstorms has already been issued, additional warnings of heavy rain and stormy winds will not be issued. When these warnings are issued, it is important to follow the the recommendations for behaviour.
As the temperature rises, the amount of water vapour in the air increases, although this is not the only factor that influences the formation and intensification of thunderstorms. One important factor is the vertical temperature profile, i.e. the way in which the temperature decreases with altitude. How climate change will affect temperatures in the upper layers of the atmosphere is not yet known with certainty and is still being researched. Climate scenarios indicate that there could be periods of heavy rainfall despite increasingly dry summers with frequent heatwaves. Extreme events could increase by 10-20%. It is difficult to predict whether the frequency of thunderstorms will increase or decrease, but it is likely that we will be faced with more extreme weather phenomena: long periods of drought, interspersed with very heavy thunderstorms. The last few summers, such as those of 2021, 2022, 2023 and 2024, give us an indication of what we can expect in the future.