Lake Constance partially frozen over Is the Seegfrörni now coming to us?
In cold winters, Swiss lakes can freeze over completely. This rare phenomenon is called Seegfrörni. The decisive factor is not a single day of frost, but a prolonged period of cold weather.
No time? blue News summarizes for you
- Seegfrörni only occurs when there are significant sub-zero temperatures over a longer period of time.
- The decisive factor is the so-called cold sum, i.e. the sum of all negative daily mean temperatures.
- Deep and large lakes require significantly more cold than small, shallow bodies of water.
A lake freeze is when a closed ice cover forms on a lake. Depending on the definition, it is only considered a "real" lake freeze-up when the ice is at least partially passable. Small, shallow lakes or wind-protected bays usually freeze over first - and only when the cold continues can larger bodies of water freeze over completely.
blue News gives you an overview of what you need to know now.
What exactly is a Seegfrörni?
A lake freeze is when a closed ice cover forms on a lake. Depending on the definition, the ice is only considered a lake freeze-up when it is at least partially passable. Initially, small, shallow lakes or sheltered bays usually freeze over, later - if the cold persists - larger bodies of water also freeze over.
Why doesn't every lake freeze over at the same rate?
Whether and how quickly a lake freezes over depends on several factors. The ratio of water depth to surface area is key: shallow lakes with a large surface area lose heat faster than deep lakes. Wind, sunshine, cloud cover and flow also play a role. Wind, for example, keeps the water moving and significantly delays the formation of ice.
What is the decisive value?
The most important criterion is the duration and intensity of the cold. To measure this, meteorologists use the cold sum (also known as negative degree days). All days with a negative average temperature are counted and their values added up. A lake can only freeze over when a certain threshold is reached.
If the average daily temperature falls below 0 degrees, this day contributes to the so-called cold sum. For example, if the daily average is minus three degrees, three cold points are included in the calculation; at minus five degrees, five cold points are included.
If the temperature drops to an average of minus two degrees on the following day, a further two points are added. After three such days, a cold total of ten has already been built up.
These values are added up day by day. The longer a cold spell lasts and the lower the temperatures are, the faster the cold total increases.
When do which lakes freeze?
According to MeteoNews, the lakes reach the average point required for a lake freeze from the following cold sums:
- Lake Baldegg: 170
- Lake Biel: 170
- Lake Constance: 370
- Lake Constance/Untersee: 160
- Greifensee: 130
- Lake Hallwil: 180
- Lake Murten: 170
- Lake Neuchâtel: 420
- Lake Pfäffikon: 120
- Lake Sarnen: 230
- Lake Sempach: 230
- Lake Thun: 450
- Lake Zurich: 320
- Lake Zurich/Upper Lake: 170
- Lake Zug: 340
On request, "Meteonews" states: "Lake Pfäffikon would be one of the first bodies of water to freeze." If the table were used as a guide, a cold total of minus 55 would be required for the lake to freeze. It goes on to say: "Perfect conditions must also prevail. These include continuous cold, temperatures below zero and no wind."
What happens under the ice?
Before a lake freezes, the surface water first cools down to four degrees and sinks - a special feature of water. Only when the entire body of water has reached this temperature can the surface continue to cool and form ice. If it remains cold, the ice contracts and cracks (known as whales) form. If it gets milder, the ice expands and piles up in places.
