In European ski resorts, pistes whether green, blue, red or black are made safe from avalanches by the 'Securité des Pistes' (piste rescue services).
As such, there is very little danger from avalanches on the marked runs in Europe, but as soon as you leave the piste the area is no longer secured and skiers and snowboarders find themselves in potential avalanche terrain.
It is the decision of the individual to enter this terrain and unlike in Canada and North America, you are completely free to do so. With the growing popularity of skiing and snowboarding and the introduction in recent years of fat skis and snowboards more and more people are able to venture into the off piste terrain. Sometimes before they fully understand the potential danger of the terrain.
Most natural avalanches occur during the spring time on South facing slopes when the sun warms the snow up and causes 'Point Release' avalanches. In general these are not a significant danger as there are usually no people involved; the snow is wet and heavy and skiers/snowboarders tend not to ride it. These avalanches also move quite slowly and start from an individual point, growing in size as they move down the hill.
Most avalanche accidents by contrast happen in the cold months of December, January and February on North facing slopes. This is when the powder is best and the traffic in the off piste is heaviest. It is also when most slab avalanches occur.
In 90% of avalanche accidents the victim or someone in the victim's party triggers the avalanche and 90% of these accidents involve a slab avalanche.
The snow we ski and snowboard on is made up of lots of different layers created by different weather conditions, temperature fluctuations, and snowfalls. Slab avalanches occur when a layer of snow that is well bonded breaks free from the layer below and the well bonded layer slides away breaking up into large blocks of snow and ice as it makes is way down the hill.
Avalanche accidents can occur without any obvious warning and even the most practised back country experts can get caught out. To help skiers and snowboarders make an informed decision about the risk of avalanches there is an "Avalanche Hazard Rating" system. This is marked at the bottom of the slopes by coloured flags; yellow, yellow and black and black. These flags represent a score of one to five explaining the danger posed by the instability in the snow.
Once on the hill it is important to remember the following:
- Avalanche accidents often occur on steeper slopes. 30-40 degree slopes are prime spots for avalanche accidents. In practical terms this means a slope that is equivalent to a red or black piste. Slopes below this range are usually not steep enough for the snow to be able to slide and above this range the snow tends not to build up in significant amounts as the slope is too steep for the snow to form a slab layer and usually too steep to ski.
- Slab avalanches often occur on the windward side of the hill. Snow is blown from one side of the hill to the other. This "wind loading" causes large amounts of snow to be deposited in one place. This snow bonds well to itself but not to the snow below setting up perfect conditions for a slab avalanche.
- Slab avalanches often fracture along convexities. Where the slope rolls from flat to steep, the snow is under the most stress as the change in gradient causes it to pull itself apart, this is often where avalanches begin.
- The danger may come from the terrain below. Even a small avalanche can knock a skier or snowboarder off their feet and if there are trees, cliffs or lakes below, the avalanche itself may not be fatal but the impact or fall may be.
- Only expose one person to danger at a time. Moving around the off-piste in a small manageable group and only traversing, climbing and skiing potentially dangerous areas one at a time will reduce the risk of triggering an avalanche and make a rescue easier if one does happen.
Although it by no means guarantees off piste safety, following the Hazard Rating System and being aware of the above factors can reduce the chances of being involved in an avalanche accident.
From December to April, Meteo France provides detailed avalanche bulletins for Val Thorens and the surrounding areas. The App FatMap also gives a good indication of which slopes are in the potential avalanche risk range, however it does not guarantee they are safe.