In harsh winters, ice can form on Swiss lowland lakes, attracting public attention when they become safe to walk on. We outline below various aspects of the climate as they relate to the freezing over of lakes.
No systematic record keeping
There is no systematic record-keeping relating to ice on Swiss lakes. Nevertheless, there are groups and individuals who have been collecting data over a long period relating to ice on their lakes. Media archives also provide important information on the subject. In 2008, Hendricks Franssen and Scherrer published their data on ice in Swiss lowland lakes, and data are now available up to 2020, which affords deeper insights into the climatology of the phenomenon.
Definition of a frozen lake, or “Seegfrörni”
“Seegfrörni” is the Swiss word for a frozen lake that was used by Hendricks Franssen and Scherrer in their 2008 publication. We will also use this term here. According to the authors’ definition, a Seegfrörni is a lake that is completely or almost completely covered in ice for more than a day. In contrast to other definitions, this one does not require the ice to be thick enough to carry a person’s weight.
When does a lake freeze over?
Lakes react in different ways to cold. Simply put, the deeper a lake, the more coldness is needed before it freezes over. This is because the entire water mass of the lake has to be cooled to around 4°C before the water masses at or near the surface can be cooled further and make freezing over possible. So-called freezing degree days (FDD) have proven to be a simple and effective way of ascertaining the level of coldness that was needed for a lake to freeze over. For the calculation, the degrees Celsius of the days with a mean temperature below freezing are added together. So, one day with a mean temperature of -5°C gives a value of 5 FDDs. Every lake has its own FDD number that is reached before it freezes over. Figure 1 provides an overview of several lakes on the Swiss plateau. Important: The number of FDDs required for a Seegfrörni can fluctuate widely. The Greifensee, for instance, froze over one year at around 100 FDD, whereas in another year, not even 200 FDD was enough to achieve a Seegfrörni. An important factor here is the weather pattern from the previous summer through to the winter in question.