Basic information on avalanches
The risk of avalanche is one of the constant worries of powder-snow skiers, ski-mountaineers and of ski-station management. Many fans of off-piste skiing and ski-mountaineering are concerned by it while others ignore it, either through lack of awareness, incompetence or over-confidence.
Off-piste skiers and ski-mountaineers should not attempt a descent without considering the risks: improvisation should not take the place of basic care. Above all, skiers who often go back to the same areas should be aware how often the danger of avalanches is undervalued in these circumstances. One winter is never the same as the next and there are years when very fragile layers of snow form more easily making slab avalanches more likely.
Reading these pages, you will become aware that it is possible to predict the risk of an avalanche. But there will always be some risk, unless you give up the pleasure of off-piste skiing all together. So it is essential to be able to take responsibility for yourself or, if this is not possible, you should put yourself in the hands of an experienced guide.
How much risk should you take? This is what you should ask yourself before tackling off-piste descents, especially after a long period of enforced inactivity, and when good weather and fresh snow make you really want to go skiing … The answer to this question is not straightforward because where avalanches are concerned the margin between a medium and a large risk is very small. There are many ways a great day's skiing can turn into a drama and chance plays a significant role. Without going into detail on the various studies on snow, we will see how, by following a series of general points on avalanches yu can, from these observations, limit the risks. Finally, we will look at how to behave in the event of an avalanche.
1. - There is always the risk of avalanche or of a snow slide each time you leave the piste; and the risk persists also for a long time after the last snow fall;
2. - If we look at some case studies of avalanches, we can see how in most cases skiers start them themselves;
3. - Sudden atmospheric changes that occur at a medium or high altitude radically alter the structure of the snow cover; the two elements that play the biggest roles are wind and temperature.
Various scales exist to measure avalanche risks and different countries issue "bulletins" based on these. You need to learn how to interpret them remembering that no scale claims to be perfect: it is up to the skier to make choices based on the weather, route, time, the skiiers' experience and also on the mountain: depending on the altitude and exposure, the risks in the "bulletins" can vary considerably from one mountain to the next.
This is the right time to point out the differences with respect to the risk of an avalanche for off-piste skiers and ski-mountaineers:
- for powder, most slopes are often skied by a large number of skiers, who help the different layers of snow to settle; also, after there has been a lot of snow in several resorts, explosives are used to start artificial mini-avalanches in the most dangerous places. But even if artificial avalanches are set off, you need to be careful at the beginning of the season when the snow cover is not yet stable, or after it has been windy or when there has been a change in temperature).
- for ski-mountaineering, it is often the actual ascent with skins that exposes you more than anything else to the risk of avalanche, especially when you need to traverse a long way across the slope. Also, slopes that are not often frequented, an early morning descent, and the reckless technique of some ski-mountaineers, all increase natural and man-made risks.
It should be emphasised that many off-piste skiers follow trail without worrying about the route, the time and essential technical knowledge needed to avoid the risk of avalanche.
On the other hand, some skiiers consider it their prerogative to make the first tracks after a large snowfall on the most beautiful slopes of the region. They should not be imitated and you need to know when to wait and even give up when there is the slightest doubt. Weekend ski-mountaineering trips planned in advance may have to be changed or cancelled without hesitation in cases of danger.
Equip yourself with an ARVA (Search Equipment for Avalanche Victims) and obviously learn how to use it well and practise using it regularly.
Do not ever ski alone and avoid groups that are too large.
Tell someone in the valley exactly where you plan to go skiing.
Skiing down gullies is less risky that down open slopes of average gradient, except when there are significant rises in temperature.
Summer knowledge of locations allows you to choose rocky areas which are safer than mowed pastures and meadows, because snow does not stick as well to the latter.
Choose with care the areas where you need to stop and avoid lingering under large outcrops or crests where the danger can come from above: natural or artificial avalanches started by other skiers who have cut the slope just above, snow sliding.
When there is a blanket of around 60 cm of fresh snow, it is important to choose your tracks with care: preferably choose convex areas: slopes broken up by very steep inclines and by flatter or undulating sections.
As a basic safety rule, it is essential not to tackle off-piste immediately after a heavy snow fall (80 cm of fresh snow or less if the snow has drifted). If you must ski off-piste, it’s better to choose tree-lined areas or zones with lots of undergrowth.
Watch out for the warm Föhn wind, which is responsible for an overall melting of the snow.
After a snow fall accompanied by strong winds, especially at high altitude, the risk of slab avalanches persists until there is an overheating of the snow cover: contrary to general belief, cold does not help snow to settle, except when the pre-existing layer of snow is very wet.
In Spring it is better to ski on the east-facing slopes and to avoid skiing late in the day on the south and west-facing slopes because the sun quickly alters the snow creating the risk of wet-snow avalanches.
Avoid crossing a slope in the middle but, if there is no choice, then you should take the following precautions:
if you have them, loosen your bindings;the wrist bands of your ski poles;your rucksack so it is only on one shoulder;the slope one person at a time, keeping at least 30 or 50 metres apart depending on the slope’s gradient;while doing this, do not lose sight of the slope or your companions who are traversing it.
It is important to keep the risk of small landslides in mind; these can be very significant if they sweep the skier down the valley possibly across crops of rock or seracs. You always need to be able to assess the risks the route might bring depending on the quality of the snow and on the time of descent. In the case of small artificial avalanches, it is better to leave the line of that descent after a few turns to limit the size of the landslide.
You need to be extremely attentive and not get carried away by the thrill of the descent when there is the possibility of danger.
It is important to highlight the importance of skiing with an ARVA. It is good practice to check before starting to ski that the ARVA is switched on and that the batteries are charged. But it’s important to be very clear on one thing: the ARVA does not stop avalanches! For ski-mountaineering it is always good practice to carry a snow spade to dig out a buried person as quickly as possible: statistics demonstrate that survival is not very likely after 30 minutes of being buried.
In summary, avalanches are of two types:
1. - Surface avalanches, when only the top layer slides because it has not stuck to the layer underneath. The thickness of this layer can vary from a few centimetres to well over half a metre.
2. - Base avalanches, when all the snow slides due to extreme instability of the snow cover and the ground is revealed. These avalanches can take different forms:
A) Powder-snow avalanches
After an abundant cold snow fall, this layer is very unstable, because the snow crystals do not stick together.
The risk is much higher if you find yourself on a large slope because powder-snow avalanches happen very quickly, sometimes reaching a speed of 200 kilometres an hour. The snow mixes with the air forming a very dangerous mixture. If anyone becomes enveloped by it, they risk death by asphyxiation for the snow crystals penetrate the lungs through the nose and mouth. When a powder-snow avalanche happens, you must try to escape quickly because the movement of air and light snow can cover impressive distances.
B) Wet-snow avalanches
These are base-level avalanches and are caused by an increase in temperature from the sun and a warm wind. The entire mass of snow stops sticking together, and becomes wet and slippery. The speed of the avalanche is very slow, but the mass of snow is significant and its density can reach 400 kilometres per metre squared. The avalanche descends in the form of lots of balls of snow and creates a passageway, scraping the ground like a bulldozer. When it stops the balls of snow press one against the other suffocating the submerged skier; this snow hardens rapidly like cement making rescue difficult. This is the typical Spring avalanche and it is also the easiest to predict.
C) Slabs formed by the wind
The wind compresses snow crystals one against the other destroying the structure of the crystals and forming fine granules. The granules accumulate forming very instable slabs that do not stick to the lower layer but remain separate as if there were a mattress of air.
These slabs often form in windy areas, under cornices, in crags on the slope and in the bowls overloaded with snow drifts. This snow crashes very easily and the whole slope is affected by the impact of the broken slab. The speed of this type of avalanche depends on the gradient and on the thickness of the slab (from anything around 30 cm to 2 m).
The dangers of slab avalanches are the most unpredictable and deserve the most attention, because they result in three-quarters of all accidents.
Note that certain avalanches are mixed, in that as the quality of the snow varies with altitude, a powder avalanche can occur at high altitude which then sets off a wet-snow avalanche at a lower altitude. In the same way, a slab avalanche on a large slope can become a dangerous powder-snow avalanche.