An avalanche is a rapid descent of a large mass of snow, ice, and debris down a mountainside. Understanding where and why avalanches happen involves examining broad geographical areas, specific landscape characteristics, and the dynamic influence of weather and snow conditions.
Global Avalanche Zones
Avalanches are a common occurrence in major mountain ranges characterized by high elevations, snowfall, and steep terrain. The Himalayas in Asia, for instance, experience thousands of avalanches annually due to their towering peaks and heavy snow accumulation. Nepal’s Himalayas are known for their high avalanche occurrence rate, posing risks to climbers and trekkers.
Similarly, the Rocky Mountains in North America, spanning the United States and Canada, are highly prone to avalanches, especially in states like Colorado, Utah, and Wyoming. Europe’s Alps, extending across multiple countries including Switzerland, France, and Italy, also see frequent avalanche activity. In South America, the Andes Mountains, particularly in Peru, are susceptible to avalanches, sometimes triggered by earthquakes. Other regions with notable avalanche activity include Scandinavia and Japan, though Japan, despite its heavy snowfall, generally experiences fewer deadly avalanches compared to Europe or North America.
Terrain Features Prone to Avalanches
Within these mountainous regions, specific terrain features increase the likelihood of avalanche formation. Slope angle is a factor, with most avalanches occurring on slopes ranging from 30 to 45 degrees, where gravity can pull snow downhill. Open slopes and treeless areas are susceptible because they lack anchors that stabilize the snowpack.
The direction a slope faces, known as its aspect, also influences avalanche risk by affecting sun exposure and wind loading. Wind-loaded slopes, especially those on the leeward side of ridges, can accumulate deeper, unstable snow slabs. Terrain traps, such as gullies, cliffs, and depressions, further increase the danger by concentrating avalanche debris and making it more difficult for buried individuals to survive. Convex rolls, where the slope steepens abruptly, can also be common points of avalanche release.
Snowpack and Weather Influence on Occurrence
While terrain features provide the static conditions for avalanches, dynamic environmental factors determine when and where a slide will occur. New snowfall, particularly heavy and rapid accumulation, can destabilize existing snow layers, leading to avalanches. For example, an accumulation exceeding 30 cm per day can result in widespread avalanching.
Wind plays a role by transporting snow and depositing it unevenly, forming dense “wind slabs” on lee slopes that are prone to sliding. Temperature fluctuations can also weaken the snowpack, creating unstable layers. Rising temperatures can lead to wet snow avalanches, as water percolates through the snow, reducing friction between snow grains. Rain falling on snow can also destabilize the snowpack, making it heavier and weakening its internal structure.
Human Presence and Avalanche Locations
Human activity can influence the occurrence of avalanches in certain locations. Areas frequented by people, such as ski resorts, especially their off-piste or unpatrolled sections, are common sites for human-triggered avalanches. Backcountry touring routes and popular hiking trails during winter months also expose individuals to avalanche hazards. In fact, approximately 90% of avalanche fatalities result from individuals triggering the avalanche themselves through their alpine activities.
Transportation corridors, including roads and railways that traverse mountainous terrain, are also areas where avalanches can pose a threat to human life and infrastructure. The presence of people can add the necessary trigger to an unstable snowpack, turning a potentially stable location into an avalanche site.