When the winter days and nights get colder and colder, fans of black ice start to hope. Although the Engadin is famous for its cross-country ski trails over the frozen lakes, the phenomenon of "black ice" is by no means a foregone conclusion, even in the relatively dry high valley.
It is common knowledge that water freezes at 0 degrees. Consulting a physics book, we learn that water has its highest density at 4 degrees. In other words, water is heaviest at 4 degrees. As soon as autumn cools the top layer of water in the Engadin lakes down to 4 degrees, it sinks and pushes warmer water from the depths to the surface. For a lake to freeze, the first step is for the entire water volume to settle at a temperature of 4 degrees. Only then can the top surface of the water continue to cool without sinking. If air temperatures continue to remain around freezing, the water at the surface slowly begins to turn into a layer of ice. However, the actual duration of the freezing process depends not only on the number of cold days in autumn and winter, but also on how deep a lake is. The deeper the lake, the longer it takes for all the lake water to circulate and reach 4 degrees.
For the Engadin lakes, this means that the Lago Bianco reservoir on the Bernina Pass always freezes over first. This is followed by Lake St. Moritz and the smaller Lake Champfèr (Lej Suot) - usually as early as December. In the case of Lake Sils, it takes a little longer due to its size and somewhat more exposed location - the wind can also stop a rapid "Seegfrörni" and literally blow away the freezing process. The last to freeze is usually Lake Silvaplana, which is significantly deeper than the other lakes in the Upper Engadin.
Complete freezing of the lakes in the Engadin can (still) be expected every year, even in comparatively milder winters. For the formation of the mysterious black ice, however, an additional condition must be fulfilled: It must not snow during the days of freezing. Only in this way does the frozen lake surface remain transparent and lend the Engadin plateau a particularly magical glow for a few days or weeks.
If you want to venture onto the black ice on skates this year, you need to keep your fingers crossed: Lago Bianco officially opens a defined zone for skaters for the first time. Alternatively, there are various prepared lake ice rinks in the Upper Engadin for ice skating, such as the two round ice rinks on Lej da Staz or on Lake St. Moritz.
Black ice offers an unforgettable experience not only for the eye but also for the ear as the dark beauty also attracts attention with mysterious sounds. The ice concert from the depths of the frozen lakes includes loud pops and cracks, strange bubbling and crackling, and supernatural sounds reminiscent of whale songs.
Science has a simple explanation for the singing lakes. The reason for this natural phenomenon is the changing air temperature. If it gets warmer, the ice on the surface expands; if the temperature drops, it contracts. This causes cracks to appear on the underside of the ice layer lying on the water. These movements of the ice surface and its cracking are responsible for the special acoustic effects of the black ice. When it begins to snow, the snow absorbs the sounds and slowly silences the mysterious music.