Overview of all examined climate variables and phenomena and their changes

This overview shows if and how climate variables such as temperature, precipitation and wind, as well as phenomena such as hail and fog, have changed in the past. In addition, possible future trends and reasons for existing uncertainties are explained.

Sunshine duration and global radiation

Since the mid-1980s, global radiation in Europe has been increasing. This development can be attributed to changes in aerosol concentration and cloud cover. In the 1980s and 1990s, aerosol concentrations in the atmosphere decreased, as a result of measures to reduce human-induced emissions. This made the atmosphere more permeable and global radiation increased. From the 2000s onwards, the aerosol concentration hardly changed. Since the turn of the Millennium, further increase in global radiation can be attributed to a decrease in cloud cover, or cloud thickness.

A similar picture can be seen in terms of sunshine duration. MeteoSwiss's series of sunshine duration measurements, which date back to the 19th century, show a marked decline in the period from 1960 to 1980. After that, the values rose again to their original level (southern Ticino and Valais) or even exceeded it (northern side of the Alps and Lake Geneva region). Some of the sunniest years occurred after 2000. This trend is less pronounced in the Alps. The observed change in sunshine duration can also be attributed to the decline in aerosol particles in the 1980s and 1990s and the decrease in cloud cover since the 2000s.

Climate scenarios show that global radiation will increase slightly in summer in the future, especially north of the Alps. In contrast, a slight decrease is expected for winter and spring. However, the extent to which global radiation actually changes also depends on whether and how weather patterns change in terms of their frequency and duration. However, there are currently no clear indications as to future development of these weather conditions.

Fog and low stratus

Fog days have become less frequent since the 1970s. The reasons for this are not yet fully understood. One possible explanation is the general improvement in air quality and in particular, the decline in aerosol emissions, which promotes condensation of water vapour and thus fog formation. Over-development of land could also be a possible reason for the decrease in fog frequency, as more sealed surfaces means that less moisture is available, than on otherwise “natural” ground. However, it is important to note that prevailing weather conditions are an important factor in formation of fog or low stratus. As the frequency of weather conditions varies greatly over the years, frequency of fog is also subject to these fluctuations.

Due to the fluctuation in the frequency of weather conditions that promote fog and low stratus formation, it is also difficult to estimate how frequency of fog and low stratus will change in future. Furthermore, there is currently insufficient research on the extent to which progressive global warming influences frequency of fog-relevant weather conditions. Furthermore, future frequency of fog depends, among other things, on how air quality will change. Therefore, it is not yet possible to make any statements about future changes in fog and low stratus.

Thunderstorms

Thunderstorms occur frequently during the summer months and are typically accompanied by heavy precipitation. Analyses of measurement data show that short summer heavy precipitation events lasting 10 minutes to one hour have become more intense in recent decades. This indicates an intensification of thunderstorm precipitation. Little is known about the details of the changes in thunderstorms themselves.

Initial analyses show that supercells (very strong thunderstorms) will occur more frequently in Switzerland, especially on the northern side of the Alps, as global warming progresses (MeteoSwiss & ETH Zurich, 2025; Chapter 4). A warmer atmosphere can absorb more water vapour and thus store more energy, which is then released during thunderstorms.

Hail

Radar data are used to observe frequency of hail events. However, the recorded time series are still too short for a trend analysis. Reanalyses can be used to reconstruct atmospheric conditions that favour hail formation. Reanalyses are reconstructions of past weather and climate, in which historical measurement data is fed into models. Wilhelm et al. (2024) were able to show that frequency of hail in Switzerland increased significantly between 1959 and 2022.

Initial studies for Switzerland also show that both the frequency and the size of hailstorms could increase significantly in future, particularly on the northern side of the Alps, as global warming progresses (MeteoSwiss & ETH Zurich, 2025; Chapter 4). Damage simulations indicate that this could lead to an increase in hail damage to buildings.

Wind gusts during thunderstorms

Based on observations, no change in the frequency or the intensity of wind gusts associated with thunderstorms has been detected in Switzerland, to date.

However, there are indications that supercells (very strong thunderstorms), which can potentially lead to strong wind gusts and violent downbursts (Downbursts), could occur more frequently in future (see section "Thunderstorms"). There is still uncertainty as to whether and how the frequency and the duration of weather conditions which favour thunderstorms (due to unstable stratification of air masses and warm, humid air at lower altitudes), will change.

Large-scale circulation patterns

Large-scale circulation patterns influence the weather over several days. Examples include high-pressure systems which bring heat in summer, and low-pressure systems, which cause heavy precipitation in winter. The frequency of these patterns is part of natural variability and depends on the season. This makes it difficult to estimate how the frequency of weather conditions has changed in the past. For example, since the 1990s, a slight increase in the frequency of high-pressure systems over northern Europe has been observed, especially in summer and autumn. However, due to the natural fluctuations mentioned above, it is not possible to say conclusively whether this is a systematic trend or a random accumulation.

Currently, there are no clear indications for Switzerland as to whether the frequency of circulation patterns will change in the future, as natural fluctuations remain significant even in a warmer climate. At the same time, the intensity of weather phenomena associated with certain conditions is expected to increase due to climate change. Even if the frequency of these patterns does not change, high-pressure systems in summer, for example, will produce more intense heat waves, and low-pressure systems will bring heavier precipitation.

Wind phenomena and winter storms

At present, it is not possible to make any robust statements about past or future changes in small-scale phenomena that depend on local factors, such as average wind speed, Foehn, Bise or tornadoes. As the frequency and the intensity of winter storms varies too greatly from year to year, there are currently no clear indications of trends for this phenomenon either.