An avalanche is a rapid flow of a large mass of snow sliding down a mountain slope. This phenomenon requires three primary elements: a steep enough slope, an unstable layer within the snowpack, and a final force to initiate the slide. Avalanches can be broken down into seven specific factors that contribute to their formation and release. Understanding these causes provides a framework for assessing danger in mountainous terrain, encompassing static physical conditions, dynamic weather elements, and the action that ultimately initiates the failure.
The Necessary Conditions: Terrain and Snowpack
The foundational prerequisites for an avalanche are the static elements of the mountain environment and the snow structure itself. The most important terrain feature is the steepness of the slope, which dictates whether gravity can overcome the snow’s internal friction. Avalanches most frequently occur on slopes between 30 and 45 degrees, often called the “sweet spot” for large-scale slab releases. Slopes steeper than 50 degrees tend to shed snow in smaller, more frequent sloughs, preventing the buildup of a massive slab.
The second cause is the presence of a weak layer, which acts as the failure plane within the snowpack. These fragile layers often consist of faceted snow crystals, such as depth hoar, which are large, angular, and bond poorly to surrounding snow. Facets form when a strong temperature gradient exists, causing rapid water vapor movement and crystal growth. Another common weak layer is surface hoar, delicate feather-like crystals that grow on the snow surface on clear, cold nights and remain fragile once buried by subsequent snowfall.
A third necessary condition is the slab layer, which is the cohesive, overlying mass of snow that slides as a single unit. This slab must be strong enough to transmit the fracture across the slope but rests precariously on top of the weak layer. Slab avalanches are the most dangerous type because they involve a large, dense volume of snow and account for the vast majority of avalanche fatalities. The slab forms as snow settles and bonds, or through the action of wind, creating a dense, brittle structure.
Atmospheric Forces: Weather Factors
Dynamic weather conditions act as external stressors, adding weight and weakening the snowpack structure established by the first three causes. The fourth cause is new snowfall, which immediately increases the load on any underlying weak layers. The rate of loading is a critical factor, as rapid accumulation does not allow the snowpack enough time to adjust and consolidate. Forecasters often flag the accumulation of about 30 centimeters (one foot) of new snow within a 24-hour period as a significant risk indicator.
The fifth factor is wind loading, which can dramatically accelerate the stress applied to a slope. Wind moves snow from the windward side of a ridge and deposits it in dense drifts, known as wind slabs, on the sheltered leeward sides. Wind can deposit snow three to ten times faster than normal snowfall, rapidly creating a heavy slab. This dense, wind-packed snow often rests precariously on a weaker layer, making the slope highly sensitive to fracture.
The sixth cause relates to temperature changes, specifically warming trends that introduce water into the snowpack. Rapid temperature spikes, sun exposure, or rain can cause surface melting and water percolation. This liquid water greatly reduces the strength of the snowpack by destroying the bonds between snow grains, often leading to wet slides. Wet snow instability can follow a diurnal cycle, with danger increasing in the afternoon as temperatures rise and solar radiation peaks.
Initiation of Failure: The Trigger
The seventh and final cause is the external force, or trigger, which provides the final stress needed to initiate the failure of the weak layer. This action tips the delicate balance between the stability of the slab and the strength of the weak layer beneath it. Triggers can be natural, such as a cornice fall, a falling rock, or a spontaneous collapse due to a rapid increase in temperature or heavy snowfall.
Human activity is the most common trigger in fatal accidents, with the victim or a member of their group initiating the slide in approximately 90% of cases. The weight of a skier, snowboarder, or snowmobiler is often enough to cause the weak layer to collapse, propagating a fracture across the slope. This highlights the direct control that backcountry travelers have over the final cause of an avalanche release.
Categorizing the Seven Causes and Risk Assessment
The seven causes of avalanches can be grouped into three categories: static conditions, atmospheric forces, and the final action.
- Static conditions include the necessary Slope Angle, the presence of a Weak Layer, and the formation of a cohesive Slab Layer.
- Atmospheric forces are the dynamic factors of New Snowfall, Wind Loading, and Temperature Changes.
- The final cause is the External Trigger, which sets the whole system in motion.
Since these factors rarely operate in isolation, risk assessment involves synthesizing this knowledge into practical decisions. A simple method is to check the daily avalanche forecast for the region before heading out. This forecast provides information on the current danger rating and the specific nature of the instability.
Travelers should be prepared with essential rescue gear, including a transceiver, probe, and shovel, and practice using them regularly. Recognizing the immediate signs of an unstable snowpack while in the field is crucial, such as recent avalanche activity, audible collapsing sounds, or shooting cracks in the snow. Avoiding all slopes steeper than 30 degrees during periods of elevated danger is the most effective way to eliminate the risk posed by the terrain factor.