Currently an ice age reigns at Lake Pfäffikon and other Swiss lakes: since the end of January thick ice has covered this lake in the Zurich Oberland. This and other frozen spots attract tens of thousands of pleasure-seekers, who, on foot or on skates, make the most of them. And they should -- because the freezing of lakes in central Switzerland is now a rare event.

Frozen lakes more seldom

Such events, as a new study from ETH researcher Harrie-Jan Hendricks Franssen shows, are indeed becoming ever rarer. His statistical assessment of the ice data for 10 lakes in central Switzerland over the past 100 years shows that people have wait longer to don their skates. Over the past 40 years, and particularly in the past 20, these lakes have frozen over fewer times. This trend is especially striking in the lakes of cantons Argovia and Lucerne. With an average depth of 25 to 45 metres they are deeper than, e. g., Lake Pfäffikon, and need colder temperatures to freeze over.

Lake Sempach froze eight times between 1916 and 1945. In the next 30 years it was covered with ice a further six times, but since 1976 the lake’s surface has never entirely frozen over. Lake Pfäffikon is an exception. Here the frequency of freezing actually increased between 1965 and 1985, a period when the lake was covered with ice 18 times. Since then, however, the occurrence of freezing has sunk by a third and now, at 15 times, lies below pre-1965 levels.

A frozen Lake Zurich in the winter of 1962/63. Thousands romped on the ice. (Picture: ETH Zurich Library Archive) large

Wind delays freezing

Hendricks Franssen also discovered that the linear correlation of air temperature and the freezing of lakes postulated by some researchers is not as straightforward as has been assumed up to now. "Even if a lake’s water is actually cold enough to build ice, wind can considerably delay the freezing process," says the Dutch scientist. Wind churns up the cold water on the surface and mixes it with underlying layers of warm water, preventing ice formation .

These results could also cast doubt on climate data which deduces historical winter temperatures from records of lake freezing. The well-known University of Berne climate historian Christian Pfister used, among other things, data on the freezing-over of bigger lakes such as Constance and Thun to calculate average winter temperatures all the way back to 1525. The freezing of bodies of water did correlate more strongly with average winter temperatures than other factors, such as the length of time snow lay on the ground, admits Hendricks Franssen. But he maintains that the uncertainty of the wind factor should be taken into account in historical climate data.

Data collected outside working hours

This researcher from the ETH Institute of Environmental Engineering collected and analysed the data in his free time. His main sources of data were the Swiss Meteorological Office, reports of fishermen and lake rescue services, private diaries and newspaper articles. He was particularly interested in data on the ten central Swiss lakes, which sometimes freeze, though not every winter: Pfäffikon, Greifen, Hallwyl, Sempach, Baldegg, Biel, Murten, Sarnen, Aegeri and the “Untersee”. Hendricks did not study the 22 lowest-lying lakes, which have either never frozen in the past 100 years (Lake Geneva) or freeze regularly every winter (Lake Lauerz).