Avalanches are a critical event due to a sudden loss of stability in the snow on a slope and subsequent sliding of a portion of the snowpack involved by the fracture, valley-wards.
Detachment may either be spontaneous or induced. In the first case, the avalanche may be caused by factors such as the weight of fresh snow or rise in temperatures.
Induced detachment instead may be of two types: accidental, as happens with people walking or skiing on a slope covered with fresh snow who unintentionally cause an avalanche by their weight; or else programmed, occurring in skiing areas when dangerous slopes are cleared with explosives. Danger of avalanches is closely linked with tourists in the mountains and therefore with a greater exposure of both people, buildings and facilities at risk. Classifying avalanches it not at all easy due to the numerous variables coming into play: type of detachment, type of snow, position of the slip plane, etc..
Types of avalanches
We have a surface layer avalanche when a fracture occurs in the snowpack, whilst we talk about deep-seated avalanches when this occurs at terrain level. Avalanches can also be skimming i.e. in contact with the surface or forming clouds usually consisting of dry snow.
Avalanches can either be spontaneous or induced. There are different causes, but in any case referable to the low cohesion force of the snow mass, causing relevant detachment. A superficial layer of snow lasting for a long time, warmer spring temperatures and the action of relatively heavy rainfalls all contribute towards detachment.
Predicting an avalanche is no easy task, as often there are no forewarning signs before it starts to slide down. Areas prone to avalanches are however known with a certain precision and situations of hazard are reported through the so-called “avalanche bulletins”.
The measures to implement in the event of risk of an avalanche mainly consist in knowing which areas are involved by these phenomena: they are in fact almost always in the same places: high mountain areas, with steep bare rock faces, between 2,000 and 3,000 metres, moreover not covered by vegetation.
It is important to avoid these areas in periods where these detachments are predicted, which are frequent in early spring when an increase in temperatures could be suffice to cause the mass of snow to suddenly start melting.