Why, how and when...
From the moment snow touches the ground a process of transformation begins, called metamorphism. Snow is not deposited all at once and it is certainly not deposited evenly over all the surfaces of a mountain. The wind and the steepness of the slopes create a snowpack layered in all sorts of ways. Uneven surfaces, changes in temperature and differing sunshine on the various sides of a mountain mean that the snow transforms in different ways. The crystals it is made of can melt and the meltwater can go and fill the gaps between one crystal and the next. Along the slopes of a mountain the process of metamorphism does not always manage to make the accumulation mechanically stable. The snow that breaks away and slides down the slope generates the avalanche. When an avalanche breaks away from the top of a slope it gains speed and on the way down it carries with it ever greater quantities of snow, increasing its speed more and more, a speed that can even exceed 300 km/h.
The reasons that can give rise to an avalanche are many and it is not always easy to tell the moment when it will break away: a layer of snow settles on an old layer of snow that has already transformed or has even frozen. The new layer can easily slide over the old one. A skier going down off-piste and cutting across a snowy slope can cause a fracture that will set off an avalanche. A snow cornice that breaks off a ridge and falls onto the slope below, heavily loaded with a thick layer of wind-drifted snow, can trigger an avalanche.
Avalanche risk
The risk of snow falling and breaking away becomes even greater when:
- the snow has not had the chance to transform;
- a thick layer of snow has been deposited;
- low temperatures persist, preventing the process of metamorphism;
- you are on slopes whose steepness is between 28° and 45°.
You could list endless different situations in which an avalanche is generated. However, our aim is to provide the tools to try to predict the release of an avalanche or at least to minimise the risk of getting caught up in one.
To understand whether we are in a risky situation, first of all we will need to observe the snowpack. In the first place an examination of the surface snow will tell us the state of consistency and cohesion of the ice crystals: we will check whether we are dealing with transformed snow, powdery and icy snow, or wet, rotten snow.
In a second stage we can try to explore the layering underneath, to check for icy crusts on which the upper layers might slide. Not having a shovel to hand and not being able to dig too deep with our hands, a first exploration can be done with the shaft of the ice axe. A discontinuity in the way it penetrates can reveal layers that are more or less compact and more or less hard.
After reasoning about the structure and quality of the snowpack, we will need to look at the features of the route we intend to tackle:
- steepness of the slope;
- exposure of the slope itself to the sun;
- presence of cornices;
- signs of warning fractures in the snowpack;
- concave or convex shape of the slope;
- weather conditions;
- presence of gullies or areas naturally free of trees.
Managing to build an overall picture of these factors and of the conditions of the snowpack can tell us how great the risk of an avalanche is.
However, for the less experienced the only advice that still holds is to rely on a good avalanche bulletin service.