Climate is more than just the average temperature, and precipitation values over a long period of time. Additional indices can be derived from these primary observations that shed light on the changes in Swiss climate over recent decades. For example, climate change is causing an increase in the frequency of heatwaves as well as heavy precipitation, and a decrease in snowfall.
The temperature in Switzerland has risen sharply since measurements began. The current climate mean is already 2.8 °C above the pre-industrial average of 1871-1900 (as of 2024). It is expected to rise further by a similar magnitude by 2060, unless greenhouse gas emissions are drastically reduced. What these average temperature rises in Switzerland mean in practical terms for humans, animals and plants can be made understandable with the help of so-called climate indices.
Climate indices are parameters derived from measurements such as temperature, precipitation and sunshine duration. Each one focusses on a specific aspect of climate such as heatwaves, frost, drought or heavy rainfall. Climate indices thus portray the climate in a very clear and comprehensible way. Furthermore, historic and future developments of the climate can be analysed in an easily understandable way. This makes it possible to answer very practical questions, such as whether there are more hot days now than there were 50 years ago, whether winters these days are actually not as cold as they used to be, or on how many days in the year will fresh snow fall in 2060.
MeteoSwiss compiles numerous climate indices for the main weather stations in the Swiss National Basic Climatological Network.
Many people seek out the swimming pools or lakes to cool down on days during hot days when the temperature hits 30°C or more. However, such high temperatures pose a very serious risk, particularly older people and those who are sick are affected by intense heat. Changing heat event patterns are taken into consideration by the building and civil engineering industries to ensure that targeted measures are adopted for achieving comfortable living and working environments, for example through improved insulation and appropriate air conditioning.
The number of hot days has increased significantly in Switzerland over recent decades, as demonstrated by the weather station in Lucerne. Up until the beginning of the 1980s, there were a maximum of 10 hot days per year, whereas nowadays this is about average. In Lucerne, there has not been a single year without a hot day since 1981. The number of summer days has also increased significantly. A summer day is defined as a day when a maximum temperature of at least 25°C is reached.
A tropical night is defined as a night in which the temperature does not fall below 20°C. Tropical nights occur predominantly in lower-lying areas and especially on the southern side of the Alps. However, a clear uptrend can be seen on both sides of the Alps. Longer periods with tropical nights can impair sleep, which in turn can have a detrimental effect on human health.
Dry spells are when there is no precipitation for an extended period of time. Combined with hot days and high levels of evaporation, this dryness leads in summer to drought. This can put pressure on agriculture and forests. Dry winters lack the snow and rain needed to fill the reservoirs for spring and summer, as well as presenting a challenge for snow sports.
There are various indices for dryness. One commonly used indicator is the maximum number of consecutive dry days per year on which less than 1 mm of precipitation is measured. There is no clear trend for this indicator across Switzerland from 1961 onwards. The reason for this is that the stable weather conditions that are necessary for long dry spells occur very unpredictably in terms of their geographical and temporal distribution. Only in Geneva has the maximum number of consecutive dry days seen a statistically significant increase from 1961 to the present day.
Heavy precipitation events bring very large amounts of rain in a short period of time. This can lead to flooding and destruction of infrastructure and can endanger lives. On the north side of the Alps and in the Alps themselves, heavy precipitation is predominantly a summer phenomenon, whereas on the south side of the Alps, several consecutive days of heavy precipitation often occur in the autumn as well.
Heavy precipitation events are becoming more intense throughout Switzerland. The maximum amounts of precipitation measured in a single day (one-day maximum precipitation) have been going up at most weather stations since 1901. The reasons for this are based in physics: Warmer air can absorb more water vapour. An increase by 1°C means 6-7% more water vapour in the air. If the air then releases this excess water in the form of rain, the heavy precipitation events become more intense.
A day in which the temperature sinks below 0°C is termed a frost day. Frost presents a risk for agriculture and can also lead to road traffic accidents. At the same time, minimum temperatures below 0°C are important for winter sports regions.
The number of frost days across Switzerland as a whole is steadily decreasing due to climate change. In Davos, the number of frost days has decreased by around 20% over the last 40 years.
Less cold also means that there is less cost involved in heating, due to a reduction in the number of days per year on which heating is normally required. Here a “heating day” is defined as one in which the average temperature is below 12°C. This trend can be seen very clearly in the example of the weather station in Geneva: In the 1960s, heating was required on an average of 220 days of the year in Geneva, while these days, it is only required on 180 days on average.
Ice days are another climate indicator that clearly shows how warming is affecting the winter months. On ice days, the temperature never rises above 0°C, and a general decline in those days can also be seen throughout Switzerland. High-lying areas are particularly affected by this, as shown by the data series taken on the Jungfraujoch (3,571 m above sea level).
Since the 1960s, there has been a statistically significant drop in the number of days with fresh snow as well as in the fresh snow heights per year in many areas. The result is that total snow cover is less frequently recorded at many weather stations nowadays than it was during the period from 1960 to 1985.
The winter sports resorts at medium altitude, in the Alpine foothills for example, are particularly affected by this change. The example of Einsiedeln shows how the number of days with at least 1 cm of snow is decreasing. The reduced snowfall also enhances the melting of glaciers.
The CH2018 Climate Change Scenarios clearly indicate that the trends elucidated here will continue in the future. Looking towards the middle of the 21st century, if greenhouse gas emissions continue unabated, we can expect even more heatwaves and drier summers, along with heavier precipitation and less snowy winters. However, a consistent reduction of greenhouse gas concentrations in the atmosphere can mitigate the consequences of climate change.