An avalanche is a rapid mass movement of snow, ice, and debris down a mountain slope. The science of predicting these events focuses on forecasting the probability and distribution of instability across a region, rather than pinpointing the exact time and location of a collapse. Avalanche forecasting is a complex risk assessment process that integrates current weather, snowpack history, and terrain analysis to provide actionable safety information. This process culminates in a standardized danger rating that communicates the likelihood of both natural and human-triggered slides.
The Critical Elements of Avalanche Formation
Avalanche formation requires a combination of three factors: a sufficiently steep slope, a slab of cohesive snow resting on a weak layer, and a trigger. The snowpack itself is a stratified record of weather events, with weak layers forming the failure point for most dangerous slab avalanches. These weak layers are created through a process called snow metamorphism, the change in snow crystal structure after deposition.
One type of metamorphism, driven by a high temperature gradient (approximately 10 degrees Celsius per meter or more), forms faceted snow crystals. When this occurs near the base of a shallow snowpack, it can produce large, hollow, sugar-like crystals known as depth hoar, which bond poorly to the layers above. Another common weak layer forms when surface hoar, or frost that grows on the snow surface during clear, cold nights, is subsequently buried by new snowfall.
Weather influences dramatically alter the stability of the snowpack, with wind being the most significant factor besides new snowfall. Wind transports loose snow crystals, breaking them into smaller particles and depositing them onto leeward slopes, or the side sheltered from the wind. This deposited snow quickly sinters and forms a dense, cohesive wind slab that acts as a heavy load on any weak layer beneath it. Wind can load a slope with new snow up to ten times faster than precipitation alone, creating rapid changes in hazard.
Terrain characteristics dictate where avalanches will run. The majority of dangerous slab avalanches initiate on slopes with an angle between 30 and 45 degrees, which is the optimal range for gravity to overcome the snow’s internal strength. Slopes steeper than 50 degrees tend to slough off small, loose snow frequently, preventing the formation of deep, heavy slabs. The slope’s aspect, or direction it faces, also plays a role, as sun-exposed aspects are more prone to melt-freeze cycles, while shaded aspects often preserve persistent weak layers like depth hoar for longer periods.
Methods for Assessing Snowpack Stability
Forecasters use a combination of field observations and advanced technology to quantify the stability of the critical elements. Field work involves digging snow pits to visually inspect the layering and perform stability tests that simulate a trigger event. The Compression Test (CT) is performed on a small, isolated column of snow to quickly identify weak layers and gauge their sensitivity to fracture initiation. The test involves tapping the top of a column with increasing force, with the number of taps recorded to indicate stability.
A more advanced technique is the Extended Column Test (ECT), which isolates a wider column of snow. This test is designed to assess not just the initiation of a fracture, but also its potential to propagate across the width of the column. An ECT result where the fracture spreads across the entire column with minimal force is a strong indicator of high instability and the potential for a large slab avalanche. These field tests provide the ground truth necessary to calibrate broader area forecasts.
Forecasting centers also rely heavily on remote sensing and computer modeling to cover vast mountainous areas. Lidar (Light Detection and Ranging) is an increasingly utilized technology that uses pulsed lasers to create high-resolution, three-dimensional maps of the terrain. By comparing scans taken before and after a storm, forecasters can measure snow depth and distribution with centimeter-scale precision, identifying areas where wind loading has created a particularly heavy slab.
This Lidar data, along with terrain parameters from Geographic Information Systems (GIS) and output from Numerical Weather Models (NWMs), is fed into complex computer models. These models calculate the regional distribution of instability, helping forecasters to refine their hazard assessment across different elevations and aspects. Ultimately, the human forecaster applies expert judgment, integrating historical avalanche data and local knowledge with the model output and field observations to produce the public danger rating.
Interpreting Avalanche Danger Ratings
The output of the forecasting process is communicated to the public using a standardized, five-level danger scale, ranging from Low to Extreme. This scale synthesizes complex variables into a single rating. It is important to recognize that the risk does not increase linearly; the jump from Moderate to Considerable, for instance, represents a disproportionately large increase in hazard.
A Low rating indicates generally stable snow conditions where natural avalanches are unlikely, and human-triggered slides are only possible in isolated terrain features. Moderate conditions mean the snowpack is only temporarily unstable in specific areas, requiring careful evaluation of snow and terrain before travel. The likelihood of a human-triggered slide increases significantly at this level, particularly on steeper, shaded slopes.
The Considerable rating signals dangerous avalanche conditions, where both natural and human-triggered avalanches are possible. At this level, forecasters advise cautious route-finding and conservative decision-making, as small triggers can lead to large slides in the indicated terrain. A High rating means that avalanches are likely, with many unstable slopes, and travel in avalanche terrain is strongly discouraged.
Finally, an Extreme rating is reserved for catastrophic conditions, usually following heavy snowfall or rapid warming, where numerous large, natural avalanches are expected. All travel in avalanche terrain should be avoided entirely under an Extreme rating. Since these ratings are regional and based on average conditions, travelers must remember that microclimates and specific terrain features can create localized instability that is not fully captured by the broad forecast.