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.
A thunderstorm is an atmospheric disturbance caused by the vertical movement of air. Cumulonimbus clouds are typical for thunderstorms. They form towering shapes and are distinguished from other clouds by the presence of thunder and lightning. If lightning is seen to light up a cloud from a large distance, this is called sheet lightning.
As soon as a thundercloud is close enough for thunder to be heard, this is termed a thunderstorm in meteorology. Thunderstorms can be local or regional, 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 be responsible for strong gusts of wind.
Thunderstorms mainly occur in summer, when there is sufficient heat and humidity for the air to rise and for Cumulonimbus clouds to form. Thunderstorms are relatively uncommon in autumn and winter. At that time of year, they are associated with a cold front, and occur either with a strong updraft zone and rapid cooling as the cold front is passing through, or behind the cold front, where small thunderstorms can sometimes develop.
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.
Some of the strongest gusts of wind recorded at weather stations in the MeteoSwiss automatic monitoring network have occurred during thunderstorms. Thunderstorms are also responsible for the heaviest precipitation measured within a short space of time. The highest amount of rainfall to be measured in a 10-minute period is 41 mm, recorded in Lausanne in 2018.
Thunderstorms are 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%. When thunderstorms develop, Level 3 or 4 warnings are issued at very short notice for regions that are in the area of origin or along their 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 warnings and warnings of severe thunderstorms also draw people’s attention to the dangers associated with wind gusts, hail and heavy rainfall. If an advance warning of severe thunderstorms has already been given, additional warnings of heavy rain and strong winds will not be issued. When these warnings are issued, it is important to follow the recommendations for behaviour.