Avalanches are more likely to occur at the end of the day because temperatures are generally higher at that time, which can cause the surface snow to melt and make the lower layer unstable, thus favoring the triggering of avalanches.
During the day, the sun warms the surface of the snowpack, leading to a gradual melting of the snow on the surface. As a result, the upper layer becomes wet, heavier, and loses its strength. This significantly weakens the overall stability of the snowpack. By late afternoon, after several hours of sun exposure, the snow is both heavier and more fragile. The layers underneath then struggle more to support this increased pressure. It becomes easier for a small disturbance to trigger an avalanche. This is why avalanches often occur at this specific time of day, when the heat has had ample time to affect the snow.
Throughout the day, the snow evolves: the crystals slowly transform due to heat and accumulated weight. The snow then becomes wetter and heavier, which gradually weakens the lower layers, already unstable. This transformation creates areas known as failure zones, where the snow can give way more easily when stressed, thereby increasing instability. As hours pass, these fragile layers lose strength, somewhat like a framework that softens under heat and eventually cracks. This structural weakening significantly raises the risk of an avalanche occurring spontaneously or as soon as any external pressure, even slight, is applied.
In the late afternoon, especially in ski resorts or on popular mountain routes, there are usually many more skiers, snowboarders, and hikers. This increased traffic tends to create additional stresses and vibrations in the snowpack, which can trigger instabilities already weakened by the heat accumulated during the day. The mere fact that there is more movement in a specific area can sometimes cause an avalanche on a slope that is already critical: the snowpack is a bit like a large, fragile pile of cards; it only takes one more small shake for everything to come crashing down. The concentration of outdoor enthusiasts at the end of the day is often caused by final descents from hiking, the end of ski sessions at resorts, or evening walks in the mountains. The more people there are outside, the greater the risk.
Every day with the sun, the surface snow melts slightly and then refreezes when temperatures drop back to the cold. This phenomenon creates a thin ice crust, sometimes fragile, which gradually becomes less stable over the days. Through repeated cycles, small cracks appear and multiply within the snowpack, making it more sensitive to the passage of a skier or even to the natural weight accumulated. With this kind of accumulated fragility, it then takes very little for an avalanche to be triggered, especially at the end of the day when the effects of the melting and refreezing cycle have built up over all the previous hours.
At the end of the day, the sun gradually sets, causing the surface temperature of the snow to slowly cool down. This cooling induces mechanical stresses within the thickness of the snowpack, leading to internal tensions. In short, there is pulling and pushing in all directions beneath the surface, which can further soften or weaken an already unstable structure. This thermal transition period is therefore conducive to sudden collapses: the snow that was barely holding on eventually gives way right at that moment.
The intense noise and vibrations caused by controlled explosions allow experts to proactively reduce the risks of avalanches on regularly used slopes.
Some alpine plants, such as dwarf shrubs or creeping pines, naturally help stabilize the snowpack and prevent the formation of avalanches.
Avalanches do not only occur in high mountains: they can also happen on moderate slopes starting at 30°, when the snow and weather conditions are favorable.
Studies show that the majority of avalanche victims were themselves responsible for triggering the avalanches, highlighting the importance of a good understanding of mountain safety.
Absolutely, south or southwest-facing slopes generally receive more sunlight, leading to increased warming and therefore a heightened risk of avalanches, particularly in the late afternoon when the snowpack reaches its maximum fragility.
The freeze-thaw cycle occurs daily: the snow melts during the day due to temperatures and sunlight, then refreezes at night. In the medium term, this phenomenon significantly weakens the integrity of the snowpack and increases the risk of avalanches at the end of the day.
Yes, several signs can indicate an increased risk of avalanche: visible cracks on the surface, muffled sounds or settling under skis or snowshoes, and also a rapid rise in ambient temperature at the end of the day. Observing these indicators allows for a more cautious approach.
Yes, human activities such as off-piste skiing or hiking done at the end of the day increase the risk of avalanches, as they can cause additional disturbances to snow that is already weakened by daytime warming.
The preparation involves checking avalanche reports and the weather on the morning of the outing, prioritizing routes that are suitable for the time of day, being equipped with appropriate safety gear (Avalanche transceiver, shovel, probe), and ideally avoiding steep slopes exposed to solar warming in the late afternoon.
The riskiest times are generally in the late afternoon, when the snowpack undergoes significant changes due to the high temperatures of the day and the sunlight, promoting structural instability.
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