With some brief interruptions, the ice sheet on Lake Zurich basically remained safe to walk on until 7th March 1963, when the daily minimum temperature finally rose above freezing in the Zurich region. A strong southerly wind that blew across the lower half of the lake on 26th and 27th March 1963 ensured that the lake was free of ice within two days.
Smaller lakes frozen first
The smaller lakes on the north side of the Alps were the first to freeze over, with Lake Zurich following later. Lake Biel, for example, was being walked on by thousands of visitors on 27th January 1963. On the other hand, Lake Constance did not fully freeze over until some time in February, although the lower part of Lake Constance, known as the “Untersee”, had frozen over by the second half of January.
Lakes that did not freeze over
However, the low winter temperatures were not sufficient for Lakes Geneva, Neuenburg, Thun, Brienz, Lucerne and Walen to freeze over. There are few, if any, historical accounts of these lakes freezing over. When it comes to the lakes on the south side of the Alps, there are no instances of complete freezing over in recorded history.
Icy polar winds
The catalyst for the lakes freezing over was the persistent, icy polar winds against a background of an extensive wave motion in the Atlantic atmosphere. This gave rise, on the American side of the Atlantic, to a strong flux of warm wind from the West Atlantic to Greenland. Over the Arctic, the direction of the flow shifted towards the south. This allowed the influx of cold polar air towards Central Europe and as far as the Mediterranean region. With a further change in direction over the Mediterranean, mild air was then carried towards the East, to Siberia. This substantial shift of air masses over a very large geographical area caused temperatures in Greenland and Siberia to rise to around 5 and 8 degrees higher, respectively, than the norms for that time of year.