Explain why do avalanches often occur after heavy snowfall?

Physical characteristics of fresh snow

Fresh snow is typically that light and fluffy layer we love for skiing, but be careful: precisely because it is light, it often contains a lot of air. The result: it makes it low-density, fragile, and above all very unstable. When it falls as fine flakes, it piles up without sticking much, forming many small fragile bonds between the crystals. And if the temperatures vary after the fall, these bonds can break or change quickly, completely altering the stability of the snowpack. Furthermore, this brand-new snow can hide beneath it an older, harder, or icy layer, with which it has no affinity: as a result, it easily slides over it, much like a blanket placed on a too-smooth floor. In short, beneath its soft and peaceful appearance, fresh snow often hides sneaky traps: it looks great for photos, but the risk of avalanche is significantly higher when it has just fallen.

Instability of the snowpack after accumulation

When a layer of fresh snow accumulates quickly, the different layers of the snowpack do not bond well with each other. This creates weak spots called fragile layers. These fragile layers act a bit like a poorly stacked pile of plates: everything seems stable until it is disturbed, but a slight jolt or change in weight can trigger a slide. When a lot of snow falls in a short amount of time, the added weight on the surface increases the pressure on these sensitive areas. As a result, the balance becomes precarious, and sometimes a simple vibration is enough to trigger an avalanche.

Weather conditions favorable to avalanches

When it snows a lot in a short time or a storm arrives with strong winds and sudden cold, the snowpack quickly becomes unstable. The violent wind moves the fresh snow and deposits it in uneven layers, creating risky accumulations called wind slabs. A rapid increase in temperature can also worsen the situation, making the snow heavier and weakening its cohesion. In contrast, prolonged very dry cold can prevent a good bond between fresh snow layers, leading to a high risk of sloughing and thus of avalanches.

Impact of terrain and slope on avalanches

Relief plays a crucial role in triggering avalanches. The steeper the slope, the more unstable the accumulated snow becomes. Generally, avalanches occur on slopes ranging from 30 to 45 degrees. Less steep, the snow holds better, and beyond that, it doesn't stay long, falling as soon as it accumulates. The shape of the terrain also matters greatly: areas with uneven ground, such as narrow corridors or funnel-shaped depressions, promote dangerous accumulations. Rocky slopes or grass-covered areas can anchor the snow to the ground less effectively and create fragile layers conducive to avalanches. Even vegetation has its say: forested areas generally reduce the risk of avalanche, but not always completely.

Human activities triggering avalanches after heavy snowfall

After a heavy snowfall, certain human activities clearly promote the triggering of avalanches. Intensive passage by skiers or snowboarders is enough to weigh down and weaken a freshly accumulated snowpack. Even a single isolated skier can trigger an avalanche if they cross an unstable slope. The same goes for snowshoers, snowmobilers, or mountaineers: their movements generate additional tensions that disturb the already precarious balance of the fresh snow layer. Preventive triggering work carried out voluntarily using explosives is also part of common situations following a heavy snowfall. The vibrations caused by these targeted explosions prevent accidental starts. In short, as soon as one steps— or makes noise— on a thick, freshly laid snowpack, one contributes to the avalanche risk.