Sarah Stirling talks to British, French and Swiss avalanche experts to find out why winter has not got off to a good start in the Alps, and how you can stay as safe as possible.

Across a widespread area of the Alps, the first substantial snowfall of the season was followed by three weeks of cold, blue skies and light winds. When snowfall was finally predicted the week before Christmas, instead of celebrating, mountain gurus prophesied ‘avalanche carnage’ and a dangerous winter season ahead. While I was writing this article, avalanches killed six people in the Alps, and severely injured several more. So what’s going on?

The combination of cold and a shallow snowpack in the Alps resulted in a steep temperature gradient within the snow. Put simply, this is the rate at which the temperature changes as you travel down through the snowpack. Glassy, straight-edged and fragile faceted snow crystals commonly form under these conditions. It’s the kind of snow that doesn’t easily form snow balls.

“Faceted snow crystals don’t bond to anything easily,” explained Director of Chamonix’s Avalanche Academy, Stuart MacDonald, who regularly digs snow pits to examine the different layers forming. “Recently there were vast amounts of facets right through the snowpack in Chamonix.”

The Swiss Institute for Snow and Avalanche Research are Europe’s leading experts in the complex field of snow science. Dr Kurt Winkler, an avalanche forecaster on their team, commented: “The thin snowpack and good weather without appreciable snowfall for a long time resulted in a weak snowpack, containing lots of large, faceted crystals. This weak, old snowpack will be a persistent unstable base for following snowfalls.”

One of the greater dangers here is public perception. Most average off-piste skiers know that it’s dangerous right after a heavy snowfall, but time stabilises most snow packs if there’s not much wind or new snow. However, this year the weak snow layer could persist to the end of the season in some areas of the Alps, making snow conditions much more difficult to assess accurately and safe terrain less predictable.

British Mountain Guide Andy Perkins raised another concern: wind-loading, which occurs when wind lifts snow from one side of the mountain and dumps it on the other, where it doesn't necessarily bond to the new snowpack right away.

Speaking on Boxing Day, Andy said: “The snowpack is very varied, from none at all on south-facing slopes through to hard windpack deposits and faceted snow, especially on north-facing slopes. There have just been two days of strong southerly winds, followed by moderate northerlies, so pretty much all aspects could be wind-loaded.” The next day, five people died in avalanches in the Alps.

French avalanche expert Alain Duclos commented: “We’ve not seen these kind of conditions for a number of years across the Savoie and Hautes-Alpes.” Looking on the brighter side, he added: “The conditions have improved on sunny slopes recently, due to both general warming and sun effect from time to time.”

A return to no snow on some slopes is, of course, better than a weak layer. And, while avalanches are a complicated, inexact and often unpredictable science, Alain noted: “The storm we had recently has shown that the well-known formula ‘new snow over a weak layer on slopes more than 30 degrees’ really works very well [i.e. is avalanche prone].”  Many avalanches released as a result.

So what do the experts at the Swiss Institute for Snow Research predict for the season ahead?

“The starting conditions are not favourable, but a lot depends on the coming snowfalls. In the southern region, we had 1.5m of fresh snow recently,” Dr Winkler explained. “So, after a short time span with great avalanche danger, the conditions quickly got better.” Larger amounts of snow can make it harder for a skier to affect a weak layer and trigger an avalanche.

“In other regions, where only about 50cm of fresh snow fell, the unfavourable avalanche conditions will remain,” continued Dr Winkler, then added a final warning: the soft base of the snowpack could lead to big, wet avalanches during spring thaws.

Current conditions in Chamonix

The week before Christmas, the ‘world capital of Alpine sports’ was predicted 25-40cm of snow, arriving on a strong wind. This was the nightmare scenario many had feared. A strong layer of slab, sat precariously on a fragile layer of facets. However, Chamonix received nearer 10-15cm at 2000m. Stuart MacDonald headed out to dig a snow pit at Le Tour.

The facets he had noticed in the snow the previous week were almost completely gone, below 2100m. “An increase in temperature, combined with reduced snowfall has stabilised certain aspects around Chamonix pretty well,” he told me. “Plus, the wind wasn't that strong, so a hard slab was less likely to form. Between 2100 and 3000m the weak sub layer seems to have remained, however.”

Of course, other areas may well be very different. Take extreme care, especially on shady slopes and any northern aspects where these faceted crystals have not yet had a chance to bond due to the lack of the warming effect of the sun. If in any doubt about your off-piste and avalanche awareness skills, consider hiring a Mountain Guide.

Tips from the experts

Stuart Macdonald: Always go off-piste fully equipped with shovel, probe, transceiver and know how to use them. Always read the avalanche bulletin and learn how to plan your day properly to minimise avalanche risks. As the level of risk increases, reduce the angle you ski.

Alain Ducros: Be especially cautious to avoid remote triggering. i.e. as well as avoiding slopes more than 30° as much as possible, be wary of the bottom of those slopes. Learn how to recognise a typical weak layer and how to measure 30° in the field.

Andy Perkins: Be particularly wary of slopes steeper than 30°, slopes facing north through east when it’s cold, slopes facing south through west when it’s warm and lee slopes after wind-loading effect.

Kurt Winkler: Consult the most up-to-date avalanche forecast at www.slf.ch or with the phone app ‘Whit Risk’ (also available in English). Take extra caution on steep, shady slopes in little-frequented backcountry terrain, especially for several days after the last really big snowfall.

Recommended reading for off-piste skiers: Staying Alive in Avalanche Terrain by Bruce Tremper.

The BMC's Off-Piste Essentials DVD in association with The National Mountain Centre at Plas y Brenin contains essential skills and techniques for back country skiing, ski touring and ski mountaineering - inlcuding avalanche awareness.

Watch the trailer for BMC Off-Piste Essentials here on BMC TV.  

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