An avalanche is the term for a large mass of snow, ice, and associated debris rapidly moving down a mountain slope. This phenomenon represents one of the most significant natural hazards in mountainous regions with seasonal snowpacks. The speed and destructive power of a large avalanche are immense, with some reaching velocities up to 200 miles per hour. The tremendous forces generated by the moving mass are capable of leveling mature forests, destroying buildings, and displacing vehicles.
Classifying Avalanche Types
Avalanches are classified based on their mechanism of release and the consistency of the snow involved. The two primary categories are slab avalanches and loose snow avalanches. Slab avalanches are responsible for the majority of human-caused avalanche fatalities because they involve a cohesive layer of snow detaching from a weaker layer beneath it. The slab moves as a single, large unit, making this type of slide highly destructive and capable of being triggered remotely.
Loose snow avalanches, sometimes called point-release avalanches or sluffs, are less dangerous to people unless they occur in terrain traps or on very long, steep slopes. They start from a single point and fan out as they descend, entraining more unconsolidated snow particles along the way. Loose snow slides can involve either dry or wet snow, which leads to further classification based on temperature and water content.
Dry avalanches occur in colder conditions where the snow is light and powdery, and they can be fast, sometimes generating a turbulent cloud of snow dust. Wet avalanches involve snow saturated with water, usually occurring during periods of warming or rain-on-snow events. Wet slides move slower than their dry counterparts but possess high density and mass, allowing them to exert powerful destructive forces.
Factors Contributing to Slope Instability
An unstable snowpack is a complex structure of layered snow that has accumulated over time. Instability occurs when a strong layer of snow rests on a weak layer that cannot support the load above it. These weak layers are formed through a process called a temperature gradient, where temperature differences between the ground and the snow surface cause snow crystals to change shape.
Depth hoar is a common weak layer that forms near the ground in a shallow snowpack and consists of large, faceted, and poorly bonded crystals. Surface hoar consists of delicate, feathery crystals that grow on the snow surface in clear, cold conditions and become a weak layer if buried by new snowfall. The cohesion, or the strength of the bonds, between snow grains determines whether the snowpack holds together or fractures.
The slope angle is a factor, with most slab avalanches occurring on slopes ranging from 30 to 45 degrees. Slopes less steep than 25 degrees rarely slide because gravity cannot overcome friction, while slopes steeper than 50 degrees tend to sluff off snow frequently, preventing large accumulations. Weather conditions play a large part in instability. Heavy snowfall rates or the rapid deposit of snow by wind, known as wind loading, add weight to the snowpack too quickly for the layers to bond. A sudden increase in temperature or solar radiation can reduce the stiffness of the snow slab, increasing the possibility of a fracture in the underlying weak layer.
Understanding Avalanche Risk Ratings
To communicate the potential danger to the public, experts use standardized systems like the North American Public Avalanche Danger Scale, which has five ascending levels. This scale provides a measure of risk based on the likelihood, size, and distribution of expected avalanches.
- Low: Conditions are generally safe, and travel advice suggests watching for unstable snow only on isolated terrain features.
- Moderate: Indicates heightened avalanche conditions on specific features, advising travelers to evaluate snow and terrain carefully.
- Considerable: Dangerous avalanche conditions exist, where human-triggered slides are likely, and conservative decision-making is essential.
- High: Natural avalanches are likely, and travel in avalanche terrain is not recommended due to very dangerous conditions.
- Extreme: Natural and human-triggered avalanches are certain, and the advice is to avoid all avalanche terrain.