Avalanche terrain is defined as any area where a mass of snow is likely to slide. Identifying this environment is the most fundamental step in preventing accidents and managing risk for anyone traveling in snowy mountain regions. This terrain is defined by the hazardous combination of a snowpack, a steep slope, and a potential trigger. Recognizing these zones involves assessing the entire landscape’s potential for snow movement.
The Role of Slope Angle
Slope angle is the most important factor determining where dry-snow avalanches are likely to release. Most slab avalanches occur on slopes inclined between 30 and 45 degrees, which balances gravitational pull and friction. The most common angle for an avalanche release is around 38 degrees.
Slopes that are flatter than 25 degrees rarely produce avalanches because the force of gravity acting parallel to the slope is insufficient to overcome the snowpack’s internal friction. Even if a weak layer collapses, the cohesive snow slab lacks the necessary downhill pull to fracture and slide. Conversely, slopes steeper than 45 to 50 degrees tend to shed new snow frequently in smaller, loose snow slides called sluffs. This constant clearing action prevents deep, dense cohesive slabs from accumulating, making large, human-triggered avalanches less common.
Terrain Shape and Structure
Beyond the slope’s average steepness, the geometry of the land influences how snow accumulates and where stress concentrates. Convex rolls are hazardous because the slope angle increases abruptly over a short distance, creating tension in the snowpack. This concentrated stress makes the snow easier to fracture and trigger than on a uniform slope.
Gullies and bowls are significant terrain traps because they funnel and collect snow and avalanche debris, greatly increasing the burial depth for anyone caught in a slide. This funnelling action amplifies the consequences of even a small avalanche. Overhanging masses of snow, known as cornices, present a direct trigger hazard. They can break off onto the slope below, adding a sudden, heavy load that initiates a major avalanche release.
Environmental Influences: Aspect and Elevation
The direction a slope faces, known as its aspect, interacts with sun and wind to modify the snowpack’s stability. Wind is a primary factor in avalanche formation, transporting snow from windward slopes to leeward slopes. This deposition creates dense, wind-loaded snow slabs that are prone to fracture and slide.
Sun exposure affects snowpack stability, particularly in mid-winter and spring. North-facing slopes receive less solar radiation, resulting in colder temperatures that preserve persistent weak layers near the ground. Conversely, south-facing slopes are subjected to more direct sunlight, making them susceptible to wet-snow avalanches during periods of strong warming or in the spring melt cycle. While elevation influences overall temperature and snow depth, these aspect-related effects determine the specific distribution of weak and strong layers within the snowpack.
Identifying the Complete Avalanche Path
The complete avalanche path is a framework for identifying the entire hazardous zone, not just the area where a slide might be triggered. The path consists of three distinct components that must be considered when traveling through mountain terrain. The Starting Zone is the upper area where unstable snow initially fractures and begins to move, typically encompassing the steepest terrain.
Below the starting zone is the Track, the route the moving snow follows as it accelerates down the mountain. The Track is often defined by the absence of mature trees or by “trim lines” on the surrounding vegetation. The path concludes with the Runout Zone, the flatter area at the bottom where the avalanche loses momentum and the debris piles up. Even low-angle terrain must be avoided if it is directly underneath a steep starting zone, as it may be part of the track or runout zone.