Summer is here It's getting hot now - but are the lakes warm enough for your first dip?
The coming weekend promises bathing weather - although only the air is already at summer level. In the long term, Swiss waters are getting warmer and warmer - which can be a problem even for swimmers.
No time? blue News summarizes for you
- Why are there precise forecasts for almost every weather phenomenon, but not for the temperature of lakes and rivers? Why is Lake Walen so cold, while Lake Biel is often much warmer?
- blue News had these and many other questions answered by the Federal Office for the Environment FOEN and other experts.
- Swiss waters are getting warmer on average. But this also has disadvantages for swimmers.
When are lakes at bathing temperature and what does it mean when they get warmer and warmer in midsummer? blue News asked experts what the temperatures of Swiss waters are all about.
What role do Swiss water temperatures play for the federal government?
When asked by blue News, the Federal Office for the Environment (FOEN) explained that it records water temperatures because of their "environmentally relevant aspects". The experts analyze the measured values in order to understand what influences cause the water temperatures to change and to what extent.
However, this area does not have a long tradition. The FOEN has been systematically recording how cold or warm Swiss rivers are since the 1970s. The office has only been recording lake temperatures since 2022.
"The 'bathing water temperature' is an additional use that is very popular," says the FOEN. It is not clear from the answers whether it intends to expand this offer.
Why is there no forecast for water temperatures?
While studying the weather forecast before every hike is a must, the same is hardly possible for a swimming trip or canoe tour.
The FOEN is currently examining the possibilities of water temperature forecasts. The interrelationships depend on many factors and are very complex, the communications office points out. This means that reliable forecasts such as those for air temperature and precipitation are difficult.
For research purposes, the FOEN calculates a temperature forecast for certain measuring stations together with the Swiss Federal Institute for Forest, Snow and Landscape Research WSL. It is clear from the graphs that these forecasts are primarily produced for scientists.
How warm will the lakes be at the weekend?
As there are no forecasts for standing bodies of water, the FOEN hydrologists refer to the long-term averages. The water temperatures are currently within this range and are between 10 and 14 degrees. This is also likely to be the range at the weekend.
How warm the preferred lake will be over the next few days can also be roughly determined from the current temperatures. Water naturally warms up or cools down much less than the air from day to day.
One exception is strong winds, while the upper layers of a body of water have only just warmed up, especially in early summer. The wind pushes the warm surface water on one side of the lake to the shore, so that the cold water from the depths reaches the surface on the other side. This can greatly reduce the overall temperature in a short time.
However, strong sunlight and warm air can also raise the temperature of a small and shallow body of water by several degrees within a day, explains the FOEN. Because strong winds are also expected at the weekend, it is still likely to get chilly at Swiss bathing waters, both in and out of the water.
What factors determine the water temperature?
According to the FOEN, three factors in particular currently determine water temperatures: air temperature, solar radiation and snowmelt. Rivers and lakes that are fed directly by streams carrying meltwater are therefore cooler than midland waters, for example.
In general, precipitation, currents and water flow also determine how warm or cold a body of water is.
Ultimately, climate change can also be determined by the average temperature, writes the FOEN. For example, the Rhine near Basel is now over 2 degrees warmer than in the 1960s.
In certain rivers, the temperature is also determined by the discharge of cooling water from nuclear power plants or industry or purified water from sewage treatment plants.
Why is Lake Walen so cold and Lake Biel usually warmer?
Small, shallow lakes and narrow, shallow rivers warm up more quickly than deep, large lakes and rivers. Although Lake Walen is not particularly large, it is very deep. It is also surrounded by steep terrain and fed by streams that have little time and distance to absorb heat before they flow into the lake.
Another unfavorable factor for Gfrörli: Lake Walen has hardly any shallow shore areas where the water warms up more quickly in the sun. The picturesque body of water at the foot of the Churfirsten mountains remains one for the hardy.
Lake Biel, for example, is completely different: the sun has an easy time on its shallow shores. Small lakes and ponds also react more quickly to the summer weather due to their small amount of standing water.
Do bathers additionally heat up the water with their body temperature?
The FOEN answers dryly: "We have no data on this." It is not known whether the expert who formulated this answer was smiling. At least she adds that it can be assumed that body heat has no measurable influence on the temperature of "the usually large masses of water". The heat energy of even so many bathers is too small compared to the millions of liters of water contained in a medium-sized lake.
The chain of effects runs in reverse: the lakes and rivers cool the people who swim in them. That's why many people get into them in summer.
What are the long-term effects of warming waters?
According to forecasts published by the FOEN, watercourses will warm by 1.6 degrees in summer by the end of the 21st century. The surface water of lakes is likely to rise by 3 degrees.
Warm water is a problem for creatures that prefer cold water unless they can dive into colder areas. This is not possible in shallow rivers, for example. Masses of dying grayling are the result. The responsible authorities repeatedly carry out emergency fishing operations: They stun fish and transport them to areas with colder water.
David Bittner, President of the Swiss Fishing Association, explained to blue News 2024 the consequences for brown trout, which are particularly sensitive to heat: Their preferred water temperature is 15 degrees C. "At 18 degrees, they are stressed and eat less, even though they need more energy. At 22 degrees they stop feeding, which they don't survive for long, and at 25 degrees they die directly in water that is too warm for them."
For swimming fans, the warmer lakes are a blessing, aren't they?
Although it is easier to get in, the rising temperatures also have disadvantages. The FOEN explains: "High water temperatures affect the water quality and also the reproduction of duck fleas, for example."
During hot spells, blue-green algae also occur in large numbers. The water is then cloudy green, in extreme cases covered with a green film. Cyanobacteria, as the blue-green algae are technically called, are only dangerous if they are ingested. For this reason, the rule is to keep children and dogs out of the water in the affected areas when blue-green algae is present.
Should we expect more bathing bans to protect fish during hot periods?
The FOEN is certain: "Due to climate change, a further increase in water temperatures in surface waters is to be expected in the coming decades." For sensitive aquatic organisms, the stress will therefore increase in certain stretches of water and survival conditions will deteriorate. "It is therefore quite possible that bathing bans will have to be issued more frequently during periods of extreme heat." The cantonal authorities are responsible for this.
A further restriction on swimming could occur with warmer water temperatures: The catfish likes it warm and is spreading in Switzerland. Although it is naturally shy, catfish up to two meters long have attacked humans on several occasions in recent years.
