Avalanche

An avalanche is a rapid flow of snow down a slope, driven by gravity and the strength of the snowpack. Avalanches can transport snow, air, and debris and range from small loose-snow slides to large slab events. They pose a major natural hazard in mountainous regions and can be triggered by weather, climate conditions, and human activity.

There are two main types: loose-snow avalanches and slab avalanches. A loose-snow avalanche starts at a single point and expands as it descends, often leaving a fan-shaped path. A slab avalanche involves a cohesive layer of snow detaching and sliding as a unit, which can release a large amount of snow and be particularly destructive.

Common triggers include heavy snowfall, rapid warming or rain-on-snow events, wind loading, and earthquakes; human activities such as skiing, snowmobiling, or construction can initiate avalanches. They most commonly occur on slopes between about 30 and 45 degrees, though they can happen on steeper or gentler terrain depending on the snowpack and terrain features.

Forecasting and safety rely on weather forecasts, snowpack stability assessments, and field observations. Forecasters use stability tests, snowpack models, and the avalanche danger scale to communicate risk. In the mountains, travelers are advised to carry avalanche beacons, probes, and shovels, travel with partners, check current avalanche forecasts, avoid high-risk terrain, and use conservative route planning and spacing.

Mitigation measures include controlled releases by trained professionals, terrain management, and public education to reduce exposure in vulnerable areas.