An avalanche is the rapid descent of a large mass of snow down a mountain slope. The forces holding the snow in place are suddenly overcome by gravity. For an avalanche to occur, a combination of three factors must align: a sufficiently steep slope, an unstable layering within the snowpack, and a final force to initiate the slide.
The Foundation: Terrain and Slope Angle
Gravity must have enough leverage to pull the snow mass downhill, overcoming the natural friction and cohesion within the snowpack. Most slab avalanches initiate on slopes where the incline measures between 30 and 50 degrees.
Slopes with angles shallower than 30 degrees rarely produce large slides because the gravitational force component is insufficient. Conversely, terrain steeper than 50 degrees tends to shed snow in smaller, frequent bursts, known as sluffing, which prevents the formation of large, cohesive slabs. Terrain features also focus stress; convex rolls—where the slope steepens rapidly—and gullies are common locations for failure initiation due to concentrated tension in the snowpack.
The Necessary Ingredient: Snowpack Structure
The presence of a layered, unstable structure within the snow is the next prerequisite for most dangerous slides. This instability is defined by two main components: the slab and the weak layer. The slab is a cohesive, dense layer of snow that sits on top. This layer has enough internal strength to hold its shape when it breaks free.
Beneath this cohesive slab lies the weak layer, which consists of fragile snow crystals with low bonding strength. Two common forms of weak layers are surface hoar and depth hoar. Surface hoar crystals are feathery flakes that grow on the snow surface during cold, clear nights and become a weak layer if buried by subsequent snowfall.
Depth hoar, or faceted snow, forms deep within the snowpack under a temperature gradient. Water vapor moves from warmer ground layers to colder surface layers, resulting in large, cup-shaped crystals with poor cohesion. A slab avalanche occurs when the weight of the overlying slab applies stress that exceeds the minimal bonding strength of these fragile crystals, causing the weak layer to collapse and fracture. This failure allows the entire cohesive slab to slide rapidly down the incline.
Environmental Stressors: Meteorological Factors
Dynamic weather changes are the forces that destabilize the structure. The most significant factor is wind loading, where strong winds pick up snow from the windward side of a mountain and deposit it rapidly onto the leeward (sheltered) slopes. This process packs the snow into a dense, cohesive wind slab, quickly adding stress to the underlying weak layers.
Rapid accumulation of new snowfall also increases the load on the snowpack. Caution is suggested if 30 centimeters or more falls within a 24-hour period. If this new snow falls quickly or is accompanied by wind, the snowpack does not have enough time to adjust and consolidate, leading to instability.
Temperature change also plays a dynamic role in weakening the snowpack. A rapid warming trend or heavy rainfall can cause meltwater to percolate through the snow, weakening the bonds between layers and potentially triggering a slide. Conversely, severe cold or clear nights can intensify the temperature gradient within the snow, accelerating the formation of dangerous depth hoar crystals in the lower pack.
The Final Act: The Immediate Trigger
An avalanche requires a final disturbance, or trigger. These triggers are categorized as either natural or artificial. Natural triggers include the weight of very heavy snowfall, intense solar radiation or warming, or the force of a falling cornice or rock.
The most common artificial trigger is the weight of a person, such as a skier or snowmobiler. The small, localized pressure from a human is often enough to cause the weak layer crystals to collapse, releasing the stored tension in the slab. This failure can propagate, meaning the fracture line spreads across the slope from the point of initial stress, releasing the avalanche remotely. Professionals sometimes use explosives as an artificial trigger to release the snowpack intentionally under controlled conditions.