When a large mass of snow rapidly flows down a mountain slope, the phenomenon is called an avalanche. These events represent a significant natural hazard in mountainous regions, capable of displacing massive volumes of material, including ice, rock, and trees. Avalanches are characterized by their speed and scale, with the largest types capable of reaching velocities up to 80 miles per hour (130 km/h).
The Conditions That Create Instability
The occurrence of an avalanche depends on the precise interaction of terrain, weather, and the structure of the snowpack. Terrain plays a part, as the slope angle must be within a specific range for most slides to initiate. The majority of destructive avalanches release on slopes between 30 and 45 degrees, which is roughly the steepness of a black diamond run at a ski resort. Slopes less steep than 25 degrees rarely experience slides because the pull of gravity is not strong enough to overcome the snow’s friction.
The snowpack structure is a complex, layered system where the weight of the overlying snow (the load) is constantly tested against the strength of the layers beneath it. Failure occurs when the load exceeds the bonding strength within the snowpack. A frequent cause of instability is the presence of a persistent weak layer, which can remain problematic for weeks or even an entire season. These layers are often composed of specific, poorly-bonded snow crystal types, such as facets or depth hoar.
Facets are angular, sugar-like crystals that form when a large temperature difference exists across the snowpack, leading to rapid moisture transfer. Depth hoar is an advanced form of faceted snow, found near the base of a shallow snowpack where the warm ground creates a strong temperature gradient.
Surface hoar is another weak layer, forming as delicate, feather-like crystals on the snow surface on clear, cold nights with little wind. If these fragile crystal layers get buried under new snow, they act as a slippery interface, which can cause the cohesive upper layer to slide catastrophically.
Categorizing Avalanche Types
Avalanches are broadly categorized based on the mechanism of their release, with the two primary types being slab avalanches and loose snow avalanches. Slab avalanches are responsible for the vast majority of human fatalities, accounting for over 90% of deaths in some regions. This type involves a cohesive layer of snow, known as the slab, fracturing and sliding as a single, large block.
The slab sits directly on top of a weak layer, and when the stress from a trigger, such as a skier’s weight or new snowfall, exceeds the weak layer’s strength, the fracture spreads rapidly. A slab avalanche leaves behind a sharp fracture line, called a crown, at the top of the slide path. These slides can reach high speeds quickly and are capable of depositing massive, densely packed chunks of snow.
A loose snow avalanche, also known as a “sluff,” starts at a single point and gathers more surface snow as it descends, fanning out into an inverted “V” shape. These are made up of poorly bonded surface or near-surface snow and are smaller than slab avalanches. Loose snow slides often occur during or immediately after a fresh snowfall or during periods of significant warming.
A loose snow slide can still pose a hazard by sweeping a person into a terrain trap, such as a gully, or over a cliff. Avalanches are also classified by their moisture content, such as dry snow or wet snow avalanches. Wet snow avalanches occur when liquid water infiltrates the snowpack, weakening the bonds between crystals. Though they travel more slowly, their increased mass makes them highly destructive.
Essential Safety and Survival Measures
Before entering any backcountry area, check the local avalanche forecast. Understanding current conditions, including wind loading and temperature fluctuations, is essential to avoiding high-risk areas. Safe travel practices involve exposing only one person at a time to a potential slide path, minimizing the load placed on the snowpack.
Every member of a group must carry safety equipment and know how to use it proficiently. The three essential tools are an avalanche transceiver (beacon), a probe, and a shovel. The transceiver emits a radio signal to locate a buried victim, and the collapsible probe is used to pinpoint their exact location under the snow.
If caught in a slide, try to move toward the edge of the moving snow and deploy an avalanche airbag if one is carried. As the avalanche slows, attempt to create an air pocket in front of the face with an arm before the snow settles and hardens. Since burial can lead to a lack of oxygen and trauma, the victim’s survival depends on the speed and skill of their companions during the rescue.