Winter means cold, mountains, snow and skis . But also the risk of avalanches which, unfortunately, caused 130 victims in Europe last season, of which only 26 in Italy. What causes an avalanche? And can we foresee this risk? The scientific explanation is this: an avalanche occurs when the weight of the snow, deposited on a sloping surface, exceeds the resistance capacity due to the friction between the layers of the snow itself. Snowflakes can in fact be of many types depending on the temperature and humidity in which they formed, and even once deposited on the ground they can create layers with different characteristics (softer or harder, wetter or drier). .


It is precisely when several layers of snow with different characteristics overlap that an alarm bell goes off for avalanche experts. If a layer of snow loses consistency due to rain, rising temperatures or vibrations, such as those caused by skiers or mountaineers, an avalanche can be triggered.

However, the type and location of the snow involved can make the difference between a small avalanche and a catastrophic one. If the weakened snow layer is shallow enough, there is only a sliding of powder snow, similar to sand rolling down a dune, which is not problematic. But if the brittle layer is deep, under a thick, dense blanket of solid snow, it can cause the most dangerous type of avalanche, the slab, in which an entire layer of snow peels off and begins to slide compactly onto the underlying layer.


Johan Gaume of the Federal Polytechnic of Lausanne in Switzerland simulated a slab avalanche, showing that, while sliding, the upper layer of snow undergoes a chaotic series of collisions, frictions and ruptures so that its behavior is less and less similar to that of a solid and increasingly to that of a liquid. Last note, the slope. To have an avalanche, it must be between 25 and 60 degrees. A smaller slope is not enough to make the snow pick up speed, while if the slope is greater the snow rolls constantly without having time to form a sufficient mass.

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