Avalanche Terrain Quiz: Slopes That Kill

Slab avalanches most commonly release on slopes between 30 and 45 degrees. Slopes shallower than 30 degrees generally lack sufficient gravitational stress to overcome weak layer strength. Very steep slopes above about 55 degrees tend to slough continuously and cannot accumulate enough snow to form a large slab. The 30 to 45 degree range provides the critical combination of gravitational stress and snow accumulation necessary for slab release.

Terrain traps are topographic features such as gullies, creek beds, cliff bases, and road cuts that increase avalanche burial consequences by concentrating or deepening snow deposits, causing additional falls, or creating obstacles that worsen trauma. Even a small avalanche that would not be fatal in open terrain can kill when it sweeps a person into a terrain trap. Recognizing terrain traps is fundamental to backcountry safety in mountain environments.

North-facing slopes in the Northern Hemisphere receive little direct solar radiation, keeping the snowpack cold throughout winter. This cold environment promotes persistent weak layer formation such as depth hoar and faceted snow that can remain unstable for weeks to months. South-facing slopes receive more solar warming which tends to promote melt-freeze cycles and wet snow conditions in spring rather than persistent cold dry weak layers.

A convex rollover is where a slope transitions from more gentle to steeper terrain. At this location the snowpack bends over the convex feature creating tension zones that make fractures more likely to initiate. Many slab avalanche starting zones are located at or just below convex rollovers, making these terrain features critical to identify and avoid when traveling in avalanche terrain during periods of elevated hazard.

Convex rollovers create snowpack tension promoting fracture, lee slopes accumulate deep wind slab deposits on top of weak layers, and thin snowpack areas over rocks create stress concentrations where fractures can initiate more easily. Dense mature forest on gentle slopes generally stabilizes the snowpack by anchoring snow and breaking weak layer connectivity rather than increasing avalanche initiation probability on those slopes.

A cornice forms as wind transports snow over a ridge and deposits it in an overhanging mass extending beyond the leeward edge. Cornices can collapse under their own weight or from disturbance, sending a large snow mass onto the lee slope and potentially triggering a slab avalanche. They also indicate that wind slab deposits likely exist on the slope below, creating a second independent hazard regardless of whether the cornice itself collapses.

Sparse vegetation offers little resistance to snow movement and no anchoring effect. Dense mature forest can stabilize the snowpack on less steep terrain by anchoring snow and breaking weak layer connectivity and it can slow smaller avalanches. However large destructive avalanches can overcome forest resistance and snap or uproot trees entirely. Forest cover alone should never be relied upon as guaranteed protection from significant avalanche events.

Slope angle measurement is a fundamental and critical skill in avalanche terrain assessment because the 30 to 45 degree slab release zone cannot always be accurately judged by eye alone. People consistently underestimate slope steepness visually. Using inclinometers or smartphone applications to measure slope angles allows travelers to objectively identify when they are within the primary release zone and make informed decisions about exposure and route selection.

Lee-facing bowls act as natural collection zones for wind-transported snow building up thick slab deposits over weak layers. The enclosed geometry means avalanches releasing from multiple aspects within the bowl can funnel into a single path dramatically increasing the volume of snow in the runout zone. This combination of deep snow loading and complex funneling terrain makes bowls some of the most consequential avalanche environments in mountain terrain.

Traveling on slopes below 30 degrees, avoiding terrain traps, and moving one at a time with observers watching from safe zones are all standard safe travel practices in avalanche terrain. Choosing the steepest most direct route maximizes rather than minimizes exposure by increasing the probability of crossing or triggering slab release zones and worsening consequences if an avalanche does occur during travel through the terrain.

Remote triggering occurs when a load applied at one location causes a slab to release on a separate often distant connected slope. A fracture initiates at the trigger point and propagates through a continuous weak layer to a steeper connected slope where the slab releases. Remote triggering is particularly dangerous because the victim may be standing on apparently safe terrain when an avalanche releases unexpectedly on a slope they cannot see or escape from.

The avalanche path encompasses the complete corridor from the starting zone where the avalanche initiates through the track where it accelerates to the runout zone where it deposits. Understanding the full extent of the avalanche path is essential for hazard mapping, infrastructure planning, and route finding because consequences extend well beyond the steep upper release zone to include the entire path down to the furthest runout extent.

A 25-degree slope is generally below the primary 30 to 45 degree slab release zone and is not primary release terrain. However 25-degree slopes can still be in the runout zone of steeper terrain above and must be assessed with that in mind. Avalanche terrain assessment always requires evaluating all connected steeper slopes above the slope being traveled, not only the immediate angle of the terrain underfoot at the time of travel.

Inclinometers, smartphone slope measurement apps, and topographic map analysis are all practical tools for assessing slope angle and identifying avalanche terrain before and during backcountry travel. Shadow length calculation is not a recognized or reliable method for measuring slope steepness and would not provide the accurate angle information needed for safe avalanche terrain assessment and route-finding decisions.

Narrow gullies and couloirs channel and concentrate snow flow dramatically increasing avalanche speed and burial depth compared to open slope avalanches of similar volume. A person inside a gully has no lateral escape route, is surrounded by terrain traps, and can be buried to extreme depths as snow piles in the confined channel. Small avalanches that would be survivable on open slopes can be fatal inside narrow gullies, making them among the most hazardous avalanche terrain features encountered in mountain environments.