Analysis: part one of a series on the impact of the avalanche winter of 1951, from protection forests to avalanche barriers and hazard maps.
The winter of 1950/51 saw Switzerland in a state of emergency, with 234 people caught in avalanches and 98 people killed. Some 235 head of livestock died under the huge volumes of snow, and around 1,500 buildings were destroyed. Adjusted for inflation, the financial loss ran into hundreds of millions of Swiss francs. The winter came to be known as the 'Winter of Terror', and reached two sorry climaxes, one in late January and another, just three weeks later, in mid-February. Vals (canton of Grisons), Andermatt (canton of Uri) and Airolo (canton of Ticino) were particularly hard hit. "It was Switzerland's most catastrophic avalanche winter of the 20th century," says Jürg Schweizer, head of the WSL Institute for Snow and Avalanche Research (SLF).
It was also to pose major challenges for the institute, which was then still in its infancy. In the years that followed, the SLF launched a raft of research projects, covering everything from analysis to protective measures. The avalanche warning service was expanded. The avalanche bulletin was published more frequently. The first guidelines on avalanche defence structures were drawn up and SLF researchers developed a blueprint for the hazard map. In their essential features, both of these remain relevant today. In this series, the SLF will explain how the disaster unfolded and explore the key impacts of that avalanche winter of 1950/51.
There is no hard-and-fast definition of what constitutes an avalanche winter. There are no threshold values, for example regarding the size and number of damaging avalanches that occur, the number of fatalities or the surface area affected. "The impact on society, or quite simply the extent of the damage, is certainly a key factor," explains Schweizer. Some avalanche winters have seen hundreds of damaging avalanches, including that of 1887/88. Notable examples over the past hundred years include 1968, 1975, 1984, 1999 and 2018 – not forgetting, of course, the winter of 1950/51.
88 hours of continuous snowfall
One of the first people to analyse how the Winter of Terror came about was SLF meteorologist Theodor Zingg. He compared the amounts of precipitation from November 1950 to February 1951 with long-term averages. A very snowy November was followed by a below-average December. Then things really got going. "Precipitation in the disaster areas in January and February exceeded 300% of normal levels," wrote Zingg. Further investigations confirmed this finding. In January 1951 alone, over 200% of the usual January precipitation fell south-east of the line linking Zermatt, Simplon, Furka, Erstfeld, Glarus and Sargans. In much of Ticino, central Grisons, Engadine and the southern valleys of Grisons, it was up to 300% or even 400%. Earlier in the month, the snowfall level was still at 1,500 metres above sea level in some places, but during the night of 15–16 January cold air flowed into Switzerland and snow fell right down into the valley bottoms. After a short break, the precipitation resumed on 18 January – and it then snowed uninterruptedly for 88 hours. That meant three and a half days of continuous snowfall. This coincided with the onset of a storm. "In total, there was 100 to 250 cm of new fallen snow north of the Main Alpine Ridge. Even on the northern flank of the Alps, which sees a lot of snowfall, that volume of freshly fallen snow is rare, typically only occur every ten to fifty years," says Schweizer.
That snow then started to move downhill, making its way even into residential areas. In the space of just four days, from 19 to 22 January 1951, there were almost 1,000 avalanches that caused damage. In Vals (canton of Grisons), a 300-metre-wide avalanche destroyed 23 buildings between the village bridge and the Kurhaus Therme spa hotel. It buried 30 people, of whom 19 died. In Andermatt (canton of Uri), an avalanche destroyed a house, killing the eight occupants and a worker on the road. A man shovelling snow on a roof was thrown 60 metres to the other side of the valley, where he landed unhurt. Just a few hours later, another four people died in the same area. These three avalanches alone destroyed or damaged 28 buildings in the village.
People and livestock also died in other villages in Switzerland, and houses, stables and infrastructure were buried under the huge volumes of snow. Many settlements were cut off from the outside world. Aircraft dropped medicine, food, wood and fuel, bringing 30,000 kg of supplies to those in need, with a total flight time of 167 hours.
A brief lull
The situation calmed down after that, but only briefly. While the north side of the Main Alpine Ridge bore the brunt in January, now it was mainly the south. "Around 400% of the usual precipitation fell in February, rising to as much as 600% in the Onsernone Valley," says Schweizer. It had been looking like a quiet winter for avalanches in these areas. Until early February, the snowpack in central and northern Ticino was pretty much ideal for a winter with few avalanches, explains Schweizer. "High snowfall at the start of the winter had created a firm base in the snowpack."
The new fallen snow in the first half of February therefore landed on a stable foundation. Nevertheless, it started to slide, taking deeper layers of the snowpack with it. From a snow research perspective, this was interesting. "This example shows that, in very specific circumstances, a thick, solid snowpack can also have unfavourable effects," says Schweizer.
The second extreme situation of the winter began on 11 February with heavy snowfall. By 15 February, 300 avalanches had claimed 16 lives. Ten of these fatalities occurred in Airolo (canton of Ticino) on 12 February, when an avalanche buried 30 buildings shortly after midnight. Five people had already lost their lives the previous day in Frasco in the Valle Verzasca (canton of Ticino), when two avalanches damaged or destroyed more than 40 buildings.
South of the Main Alpine Ridge too, the snow cut people off from the outside world. The residents of Bosco/Gurin had a particularly long wait: it was not until 28 May that the authorities reopened the access roads to Ticino's highest municipality.
In the next instalment, available from 27. January, we'll learn how the Winter of Terror changed avalanche barrier technology.
Additional information
Catapulted out of the cradle: the avalanche bulletin
During that severe winter of 1950/51, getting avalanche bulletins to the Swiss population remained a technical challenge. The SLF building still stood almost 2,700 metres above sea level on the Weissfluhjoch near Davos, far from any urban infrastructure. Fax was not yet available, let alone the internet. The short reports were sent to the radio and press by teletype, and that was only once a week. In the entire 1950/51 season, there were just 35 avalanche bulletins. By way of comparison, the avalanche warning service publishes that number in two and a half weeks today. That said, the SLF did issue a special edition twice during the extraordinary avalanche conditions in January. Learning lessons from that winter, the institute increased the number of reports to three per week in the years that followed. It was not until the winter of 1997/98 that the avalanche bulletin started being published daily. The number of monitoring stations also increased from 28 to 50 over the subsequent years. Today, up to 200 observers are out and about every day for the avalanche bulletin, reporting on the snow situation on the ground. Data from greatly improved weather models, as well as snow and wind measurements from 180 automated stations and reports from ski tourers provide further input for the reports.
The Winter of Terror in Davos
The treacherous winter of 1951 also caused damage and claimed lives in the municipality of Davos, where the SLF is based. On 21 January 1951, an avalanche started in the Büelenwald forest above the Dischma Valley, destroying a dwelling and, just above it, a stable with living accommodation. The 87-year-old occupant of the upper building, the owner of the lower house and his nine-month-old son were killed. A five-person rescue team from the Swiss Armed Forces, who had made the arduous journey to the scene of the accident, found a cradle with an infant inside on top of the debris. The child was unharmed. His sister was also found safe and sound a short time later. After several hours, the search team rescued the children's injured mother from the remains of the house. A day earlier, an avalanche had already destroyed the Rhaetian Railway station in the village of Monstein, damaging the railway line and road. Six people were buried by the snow, of whom two were dead when the rescuers reached them.