Avalanche
Overview
An avalanche (雪沙汰, avalanche) is a natural phenomenon in which snow accumulated on a slope rapidly slides downward under the influence of gravity. In a short time it sweeps down tens of tons of snow along with ice, rock, and trees, causing enormous damage to life and property. It is a representative natural disaster of mountainous regions, and it is estimated that more than 150 people worldwide lose their lives to avalanches each year.
Main Content
Definition and Physical Principles
An avalanche occurs when the shear strength of a snow layer cannot withstand the shear stress caused by gravity. In the layered structure formed as snow crystals accumulate, the typical mechanism is for a hard slab resting on top of a weak layer to slide along a fracture. Slope angles of 30 to 45 degrees are the most dangerous; avalanches rarely occur below 25 degrees, and above 50 degrees snow keeps sliding down and does not accumulate on a large scale.
Types of Avalanches
- Slab avalanche: The most dangerous type, accounting for about 90% of fatal accidents. A plate-shaped mass of snow fractures broadly and slides down.
- Loose-snow avalanche: Snow with weak cohesion starts from a point and spreads out in a fan shape.
- Wet avalanche: Occurs in spring when meltwater makes the snow heavy; it is slow but highly destructive.
- Full-depth avalanche: The entire snowpack down to the ground collapses.
- Powder cloud avalanche: Dry powder snow flows like a cloud and can reach speeds of up to 300 km/h.
Conditions for Occurrence
1) Rapid snowfall: When more than 30 cm of new snow accumulates over 24 hours, the risk rises sharply.
2) Slope angle and aspect: North- and northeast-facing slopes retain snow longer, preserving weak layers.
3) Weather changes: Rapid temperature rises, rainfall, and strong winds (forming wind slabs) act as triggers.
4) Weak layers: Surface hoar, faceted crystal layers, and similar features create weak planes.
5) External stimuli: Skiers, snowmobiles, explosive sounds, earthquakes, and similar factors act as triggers.
Danger Ratings
Europe and North America use a five-level danger scale (1 Low to 5 Extreme). The most accidents occur at Level 3, "Considerable," because awareness of danger actually decreases.
Rescue and Survival
If buried in an avalanche, the survival rate is about 90% when rescued within 15 minutes, but after 35 minutes it drops sharply to below 30%. Transceivers (beacons), probes, and shovels are essential equipment for locating buried victims, and recently RECCO reflectors, drones, thermal imaging cameras, and AI-based detection have been used. When buried, it is important to secure an air pocket around the mouth and conserve energy.
Historical Cases
- 1910 Wellington train disaster in the United States: 96 deaths.
- 1970 Peru Yungay earthquake-triggered avalanche: about 20,000 deaths, the worst in history.
- 1999 Galtür, Austria: 31 deaths.
- 2015 Everest Base Camp, Nepal: 22 deaths from an earthquake-triggered avalanche.
- 2023 Marmolada glacier collapse in Italy: 11 deaths.
Forecasting and Management
Each country operates avalanche forecasting centers and issues danger ratings by combining snowpack observations, snowpack tests (snow pits), and weather models. They conduct avalanche control by artificially dropping explosives, or install structures such as snow fences, defense walls, and diversion dams. For mountain roads, closure and detours are the basic response.
Latest Trends
In 2024–2025, variability caused by climate change is a key topic. Winter snowfall varies greatly by region, and as surface hoar layers from dry periods remain and are then covered by late heavy snow, large slab accidents are recurring. In the European Alps, danger levels 4–5 were maintained for a long period in early 2024 due to unusually heavy snowfall, while the Rocky Mountain region of North America observed a pattern of alternating snow drought and heavy snowfall.
On the technological side, AI-based prediction models are drawing attention. Machine learning is used to simulate snow layers and predict accident probability, and satellite SAR imagery is used to detect cracks on slopes early. Drone- and helicopter-based snowpack measurement, digital twins, and real-time risk alerts through smartphone apps have also become common. In addition, as the winter sports population grows and backcountry skiing booms, the importance of education and equipment distribution is increasing, and countries are trending toward recommending or requiring completion of avalanche safety education and carrying beacons.
Related Topics
- [[Natural Disaster]]
- [[Climate Change]]
- [[Mountain Safety]]
- [[Glacier]]
- [[Earthquake]]