An avalanche is a natural phenomenon involving a large mass of snow, ice, and sometimes rock, rapidly moving down a mountainside. These events can attain significant speeds, reaching up to 130 kilometers per hour (80 miles per hour), causing extensive destruction. Understanding avalanche conditions and mechanisms is important for assessing risks in mountainous environments.
Environmental Factors
Avalanches are influenced by environmental conditions, primarily terrain, weather, and snowpack. Terrain influences snow accumulation and release. Avalanches most frequently occur on slopes angled between 30 and 45 degrees, though they can happen on slopes from 25 to 60 degrees. Slopes less steep than 25 degrees generally do not allow snow to slide, while those steeper than 60 degrees tend to sluff snow constantly, preventing large slabs from forming.
Slope shape also plays a role; convex slopes, which steepen, stress the snowpack. Concave slopes may offer compressive support. Aspect, or slope direction, influences sun and wind exposure. North-facing slopes in the Northern Hemisphere receive less direct sunlight, leading to slower snowpack stabilization and persistent weak layers. Trees and large rocks can anchor the snowpack, reducing avalanche risk.
Weather conditions drive avalanche danger, with new snowfall as a primary factor. Heavy snowfall, especially over 2.5 cm (1 inch) per hour, rapidly increases avalanche risk by adding weight to the snowpack. Wind is another major influence, transporting snow from windward slopes to sheltered, leeward areas. This forms dense, cohesive wind slabs, which are highly unstable.
Temperature changes profoundly affect snowpack stability. Rapid warming weakens bonds, increasing instability. Persistent cold prevents new snow from bonding or promotes weak, faceted crystals. These weather elements interact with terrain to dictate snow layer stability.
Snowpack Instability
The snowpack’s internal structure determines its instability and avalanche formation. Snow events like successive snowfalls, wind, and temperature fluctuations create distinct layers within the snowpack. These layers vary in density, crystal type, and strength, dictating overall stability.
Snowpack instability often stems from weak layers, composed of poorly bonded snow crystals. Common types include surface hoar, depth hoar, faceted crystals, graupel, or buried crusts. Surface hoar consists of delicate, feathery crystals forming on the snow surface during cold, clear nights. Once buried, it creates a persistent weak layer. Depth hoar, found near the ground, forms in areas with a strong temperature gradient, resulting in large, cup-shaped crystals with poor bonding.
As new snow accumulates, its weight (stress) exerts pressure on weak layers. If stress becomes too great, the weak layer can collapse or fracture, leading to snowpack failure. Bond strength between snow crystals and layers is important. Poor bonding allows a stronger, cohesive slab to slide over a weaker, underlying layer, initiating an avalanche.
Initiating Forces
Once a snowpack is unstable, an initiating force, or trigger, is required to release an avalanche. These triggers can be natural or human-induced.
Natural triggers include heavy new snowfall, adding weight to an already precarious snowpack. Rapid temperature increases can also trigger avalanches by causing meltwater to penetrate the snowpack and weaken bonds. Other natural triggers include collapsing cornices (overhanging wind-deposited snow on ridges), or rockfalls and icefalls that dislodge snow. Less common, earthquakes can also generate enough vibration to trigger large avalanches.
Most avalanches, particularly fatal ones, are human-triggered. Activities like skiing, snowboarding, or snowmobiling can add the extra weight or stress needed to initiate a slide. Even a person’s weight can fracture a weak layer and trigger an avalanche, especially where the snowpack is highly unstable. Professionals sometimes use explosives to trigger smaller, controlled avalanches in high-risk areas, preventing larger, more dangerous ones.
Avalanche Classifications
Avalanches are categorized by their characteristics and release, providing insight into their behavior and danger. Slab avalanches are the most common and dangerous type, responsible for over 90% of avalanche fatalities. These occur when a cohesive snow layer, or slab, slides as a single unit over a weaker, underlying layer. Slab avalanches have a distinct fracture line at the top and can travel at high speeds, often reaching 130 kilometers per hour.
Loose snow avalanches, also called point release avalanches, begin from a single point and fan out as they descend, gathering more snow. These are typically less dangerous than slab avalanches, involving smaller snow volumes and generally slower speeds. They often occur in new, unconsolidated, or wet snow. While they can knock a person off their feet, deep burial is less likely.
Wet avalanches occur when water penetrates the snowpack, significantly weakening bonds between snow crystals and layers. Wetting can be caused by warming temperatures, rain, or lack of overnight refreezing. Wet avalanches move slower than dry avalanches due to increased water content friction, but their greater mass can be highly destructive. They can manifest as wet slab or wet loose snow avalanches.