An avalanche is an abrupt and rapid flow of snow, often mixed with air and water, down a mountainside. Avalanches are among the biggest dangers in the mountains for both life and property. Avalanches may also comprise of rocks and boulders. See Rock avalanche.
Several types of snow avalanche may occur. Loose snow avalanches occur when the weight of the snowpack exceeds the shear strength within it, and are most common on steeper terrain. In fresh, loose snow the release is usually at a point and the avalanche then gradually widens down the slope as more snow is entrained, usually forming a teardrop appearance. This is in contrast to a slab avalanche. Slab avalanches account for around 90% of avalanche-related fatalities, and occur when there is a strong, stiff layer of snow known as a slab. These are usually formed when snow is deposited by the wind on a lee slope. When the slab fails, the fracture, in a weak layer, very rapidly propagates so that a large area, that can be hundreds of metres in extent and several metres thick, starts moving almost instantaneously. The third starting type is a isothermal avalanche which occurs when the snowpack becomes saturated by water. These tend to also start and spread out from a point.
As avalanches move down the slope they may entrain snow from the snowpack and grow in size. The snow may also mix with the air and form a powder cloud. An avalanche with a powder cloud is known as a powder snow avalanche. The powder cloud is a turbulent suspension of snow particles that flows as a gravity current. Powder snow avalanches are the largest avalanches and can exceed 300 km/h and 10,000,000 tonnes of snow, they can flow for long distance along flat valley bottoms and even up hill for short distances.
Another factor affecting the incidence of avalanches is the nature of the ground surface underneath the snow cover. Full-depth avalanches (avalanches that sweep a slope virtually clean of snow cover) are more common on slopes with smooth ground cover, such as grass or rock slabs. Vegetation plays an important role in anchoring a snowpack; however, in certain instances, boulders or vegetation may actually create weak areas deep within the snowpack.
The structure of the snow pack is a strong predictor of avalanche danger. For an avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below an overlying slab of cohesive snow. Unfortunately, the relationship between easily-observed properties of snow layers (strength, grain size, grain type, temperature, etc.) and avalanche danger are extraordinarily complex; consequently, for each observed snow pack condition, avalanche forecasts and bulletins recommend conservative use of avalanche terrain. Furthermore, the factors influencing snow stability often vary widely within relatively small areas and time scales; which requires that the avalanche risk must always be reassessed.
Various snow composition and deposition characteristics also influence the likelihood of an avalanche. Newly-fallen snow requires time to bond with the snow layers beneath it, especially if the new snow is light and powdery. Shallower snow that lies above or around boulders, plants, and other discontinuities in the slope will weaken from the presence of stronger temperature gradients. Larger and more angular snow crystals are an indicator of weaker bonds within the snow pack, because the sintering process that forms bonds within the snow pack will also cause the snow crystals to become smaller and rounder. Consolidated snow is less likely to slough than either light powdery layers or loose isothermal snow; however, well-consolidated snow is necessary condition for the occurrence of slab avalanches, and can also mask lingering deeper instable layers within a snow pack.
If the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and refreezing of water in the snow strengthens the snowpack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point, may cause a slope to avalanche, especially in spring. Persistent cold temperatures prevent the snow from stabilizing; long cold spells may contribute to the formation of depth hoar, a condition where there is a pronounced temperature gradient, from top to bottom, within the snow. When the temperature gradient becomes sufficiently strong, thin layers of "faceted grains" may form above or below embedded crusts, allowing slippage to occur.
Any wind stronger than a light breeze can contribute to a rapid accumulation of snow on sheltered slopes downwind. Wind pressure at a favorable angle can stabilize other slopes. A "wind slab" is a particularly fragile and brittle structure which is heavily-loaded and poorly-bonded to its underlayment. Even on a clear day, wind can quickly shift the snow load on a slope. This can occur in two ways: by top-loading and by cross-loading. Top-loading occurs when wind deposits snow perpendicular to the fall-line on a slope; cross-loading occurs when wind deposits snow parallel to the fall-line. When a wind blows over the top of a mountain, the leeward, or downwind, side of the mountain experiences top-loading, from the top to the bottom of that lee slope. When the wind blows across a ridge that leads up the mountain, the leeward side of the ridge is subject to cross-loading. Cross-loaded wind-slabs are usually difficult to identify visually.
Snowstorms and rainstorms are important contributors to avalanche danger. Heavy snowfall may cause instability in the existing snowpack, both because of the additional weight and because the new snow has insufficient time to bond to underlying snow layers. Rain has a similar effect. In the short-term, rain causes instability because, like a heavy snowfall, it imposes an additional load on the snowpack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snowpack together. Most avalanches happen during or soon after a storm.
Daytime exposure to sunlight can rapidly destabilize the upper layers of a snowpack. Sunlight reduces the sintering, or necking, between snow grains. During clear nights, the snowpack can strengthen, or tighten, through the process of long-wave radiative cooling. When the night air is significantly cooler than the snowpack, the heat stored in the snow is re-radiated into the atmosphere.
Driving a (non-airborne) avalanche is the component of the avalanche's weight parallel to the slope; as the avalanche progresses any unstable snow in its path will tend to become incorporated, so increasing the overall weight. This force will increase as the steepness of the slope increases, and diminish as the slope flattens. Resisting this are a number of components that are thought to interact with each other: the friction between the avalanche and the surface beneath; friction between the air and snow within the fluid; fluid-dynamic drag at the leading edge of the avalanche; shear resistance between the avalanche and the air through which it is passing, and shear resistance between the fragments within the avalanche itself. An avalanche will continue to accelerate until the resistance exceeds the forward force.
Voellmy used a simple empirical formula based on Bernoulli's principle, treating an avalanche as a sliding block of snow moving with a force that was proportional to the square of the speed of its flow:
He and others subsequently derived other formulae that take other factors into account, with the Voellmy-Salm-Gubler and the Perla-Cheng-McClung models becoming most widely used as simple tools to model flowing (as opposed to airborne) avalanches.
Since the 1990s many more sophisticated models have been developed. In Europe much of the recent work was carried out as part of the SATSIE (Avalanche Studies and Model Validation in Europe) research project supported by the European Commission which produced the leading-edge MN2L model, now in use with the Service Réstitution Terrains en Montagne (Mountain Rescue Service) in France, and D2FRAM (Dynamical Two-Flow-Regime Avalanche Model), which was still undergoing validation as of 2007.
Due to the complexity of the subject, winter travelling in the backcountry (off-piste) is never 100% safe. Good avalanche safety is a continuous process, including route selection and examination of the snowpack, weather conditions, and human factors. Several well-known good habits can also minimize the risk. If local authorities issue avalanche risk reports, they should be considered and all warnings heeded. Never follow in the tracks of others without your own evaluations; snow conditions are almost certain to have changed since they were made. Observe the terrain and note obvious avalanche paths where vegetation is missing or damaged, where there are few surface anchors, and below cornices or ice formations. Avoid traveling below others who might trigger an avalanche.
There are several ways to prevent avalanches and lessen their power and destruction. They are employed in areas where avalanches pose a significant threat to people, such as ski resorts and mountain towns, roads and railways. Explosives are used extensively to prevent avalanches, especially at ski resorts where other methods are often impractical. Explosive charges are used to trigger small avalanches before enough snow can build up to cause a large avalanche. Snow fences and light walls can be used to direct the placement of snow. Snow builds up around the fence, especially the side that faces the prevailing winds. Downwind of the fence, snow buildup is lessened. This is caused by the loss of snow at the fence that would have been deposited and the pickup of the snow that is already there by the wind, which was depleted of snow at the fence. When there is a sufficient density of trees, they can greatly reduce the strength of avalanches. They hold snow in place and when there is an avalanche, the impact of the snow against the trees slows it down. Trees can either be planted or they can be conserved, such as in the building of a ski resort, to reduce the strength of avalanches.
Artificial barriers can be very effective in reducing avalanche damage. There are several types. One kind of barrier (snow net) uses a net strung between poles that are anchored by guy wires in addition to their foundations. These barriers are similar to those used for rockslides. Another type of barrier is a rigid fence like structure (snow fence) and may be constructed of steel, wood or pre-stressed concrete. They usually have gaps between the beams and are built perpendicular to the slope, with reinforcing beams on the downhill side. Rigid barriers are often considered unsightly, especially when many rows must be built. They are also expensive and vulnerable to damage from falling rocks in the warmer months. Finally, there are barriers that stop or deflect avalanches with their weight and strength. These barriers are made out of concrete, rocks or earth. They are usually placed right above the structure, road or railway that they are trying to protect, although they can also be used to channel avalanches into other barriers. Occasionally, earth mounds are placed in the avalanche's path to slow it down.
In some cases avalanche victims are not located until spring thaw melts the snow, or even years later when objects emerge from a glacier.
Chances of a buried victim being found alive and rescued are increased when everyone in a group is carrying and using standard avalanche equipment, and have trained in how to use it. However, like a seat belt in a vehicle, using the right equipment does not justify exposing yourself to unnecessary risks with the hope that the equipment might save your life when it is needed. A beacon, shovel and probe is considered the minimum equipment to carry when exposing yourself to avalanche danger.
Recent digital models also attempt to give visual indications of direction and distance to victims and require less practice to be useful. There are also passive transponder devices that can be inserted into equipment, but they require specialized search equipment that might only be found near an organized sports area.
Probing can be a very time-consuming process if a thorough search is undertaken for a victim without a beacon. In the U.S., 86% of the 140 victims found (since 1950) by probing were already dead. Survival/rescue more than 2 m deep is relatively rare (about 4%). Probes should be used immediately after a visual search for surface clues, in coordination with the beacon search.
Technology to summon outside help is to be used with the knowledge that those responding will likely be performing a body recovery. Only on-site rescuers are in position to render assistance during the brief interval that the victim is most likely to survive.
Other rescue devices are proposed, developed and used, such as avalanche balls, vests and airbags, based on statistics that most deaths are due to suffocation.
Although inefficient, some rescue equipment can be improvised by unprepared parties: ski poles can become short probes, skis or snowboards can be used as shovels. A first aid kit and equipment is useful for assisting survivors who may have cuts, broken bones, or other injuries, in addition to hypothermia.
Survival time is short, if a victim is buried. There is no time to waste before starting a search, and many people have died because the surviving witnesses failed to do even the simplest search.
Witnesses to an avalanche that engulfs people are frequently limited to those in the party involved in the avalanche. Those not caught should try to note the locations where the avalanched person or people were seen. This is such an important priority it should be discussed before initially entering an avalanche area. Once the avalanche has stopped, and there is no danger of secondary slides, these points should be marked with objects for reference. Survivors should then be counted to see who may be lost. If the area is safe to enter, a visual search of the likely burial areas should begin (along a downslope trajectory from the marked points last seen). Some victims are buried partially or shallowly and can be located quickly by making a visual scan of the avalanche debris and pulling out any clothing or equipment found. It may be attached to someone buried.
Alert others if a radio is available, especially if help is nearby, but do NOT waste valuable resources by sending a searcher for help at this point. Switch transceivers to receive mode and check them. Select likely burial areas and search them, listening for beeps (or voices), expanding to other areas of the avalanche, always looking and listening for other clues (movement, equipment, body parts). Probe randomly in probable burial areas. Mark any points where signal was received or equipment found. Only after the first 15 minutes of searching should consideration be given to sending someone for help. Continue scanning and probing near marked clues and other likely burial areas. After 30-60 minutes, consider sending a searcher to get more help, as it is more likely than not that any remaining victims have not survived.
Line probes are arranged in most likely burial areas and marked as searched. Continue searching and probing the area until it is no longer feasible or reasonable to continue. Avoid contaminating the scent of the avalanche area with urine, food, spit, blood, etc, in case search dogs arrive.
The areas where buried victims are most likely to be found are: below the marked point last seen, along the line of flow of the avalanche, around trees and rocks or other obstacles, near the bottom runout of the debris, along edges of the avalanche track, and in low spots where the snow may collect (gullies, crevasses, creeks, ditches along roads, etc). Although less likely, other areas should not be ignored if initial searches are not fruitful.
Once a buried victim is found and his or her head is freed, perform first aid (airway, breathing, circulation/pulse, arterial bleeding, spinal injuries, fractures, shock, hypothermia, internal injuries, etc), according to local law and custom.
Myth: Spitting while covered in snow can determine the direction upwards - Spitting while covered in snow is not possible because when the snow has settled it becomes very solid and most of the time, moving is not possible.
The small Austrian village of Galtür was hit by the Galtür avalanche in 1999. The village was thought to be in a safe zone but the avalanche was exceptionally large and flowed into the village. Thirty-one people died.
In the northern hemisphere winter of 1951-1952 approximately 649 avalanches were recorded in a three month period throughout the Alps in Austria, France, Switzerland, Italy and Germany. This series of avalanches killed around 265 humans and was termed the Winter of Terror.
During World War I, approximately 50,000 soldiers died as a result of avalanches during the mountain campaign in the Alps at the Austrian-Italian front, many of which were caused by artillery fire. However, it is very doubtful avalanches were used deliberately at the strategic level as weapons; more likely they were simply a side effect to shelling enemy troops, occasionally adding to the toll taken by the artillery. Avalanche prediction is nearly impossible; forecasters can only assert the conditions, terrain and relative likelihood of slides with the help of detailed weather reports and from localized snowpack observation. It would be almost impossible to predict avalanche conditions many miles behind enemy lines, making it impossible to intentionally target a slope at risk for avalanches. Also, high priority targets received continual shelling and would be unable to build up enough unstable snow to form devastating avalanches, effectively imitating the avalanche prevention programs at ski resorts.
On Sunday 3rd August 2008 there was an avalanche on K2 Mountain.
In France, most avalanche deaths occur at risk levels 3 and 4. In Switzerland most occur at levels 2 and 3. It is thought that this may be due to national differences of interpretation when assessing the risks.
|Risk Level||Snow Stability||Flag||Avalanche Risk|
|1 - Low||Snow is generally very stable.||Avalanches are unlikely except when heavy loads  are applied on a very few extreme steep slopes. Any spontaneous avalanches will be minor (sluffs). In general, safe conditions.|
|2 - Limited||On some steep slopes the snow is only moderately stable . Elsewhere it is very stable.||Avalanches may be triggered when heavy  loads are applied, especially on a few generally identified steep slopes. Large spontaneous avalanches are not expected.|
|3 - Medium||On many steep slopes  the snow is only moderately or weakly stable.||Avalanches may be triggered on many slopes even if only light loads  are applied. On some slopes, medium or even fairly large spontaneous avalanches may occur.|
|4 - High||On most steep slopes  the snow is not very stable.||Avalanches are likely to be triggered on many slopes even if only light loads  are applied. In some places, many medium or sometimes large spontaneous avalanches are likely.|
|5 - Very High||The snow is generally unstable.||Even on gentle slopes, many large spontaneous avalanches are likely to occur.|
 additional load:
|Size||Runout||Potential Damage||Physical Size|
|1 - Sluff||Small snow slide that cannot bury a person, though there is a danger of falling.||Unlikely, but possible risk of injury or death to people.||length <50 m |
volume <100 m³
|2 - Small||Stops within the slope.||Could bury, injure or kill a person.||length <100 m |
volume <1,000 m³
|3 - Medium||Runs to the bottom of the slope.||Could bury and destroy a car, damage a truck, destroy small buildings or break trees.||length <1,000 m |
volume <10,000 m³
|4 - Large||Runs over flat areas (significantly less than 30°) of at least 50 m in length, may reach the valley bottom.||Could bury and destroy large trucks and trains, large buildings and forested areas.||length >1,000 m |
volume >10,000 m³
|Probability and trigger||Degree and distribution of danger||Recommended action in back country|
|Low (green)||Natural avalanches very unlikely. Human triggered avalanches unlikely. Generally stable snow. Isolated areas of instability.||Travel is generally safe. Normal caution advised.|
|Moderate (yellow)||Natural avalanches unlikely. Human triggered avalanches possible. Unstable slabs possible on steep terrain.||Use caution in steeper terrain|
|Considerable (orange)||Natural avalanches possible. Human triggered avalanches probable. Unstable slabs probable on steep terrain.||Be increasingly cautious in steeper terrain.|
|High (red)||Natural and human triggered avalanches likely. Unstable slabs likely on a variety of aspects and slope angles.||Travel in avalanche terrain is not recommended. Safest travel on windward ridges of lower angle slopes without steeper terrain above.|
|Extreme (red/black border)||Widespread natural or human triggered avalanches certain. Extremely unstable slabs certain on most aspects and slope angles. Large destructive avalanches possible.||Travel in avalanche terrain should be avoided and travel confined to low angle terrain well away from avalanche path run-outs.|
|1||Relatively harmless to people.|
|2||Could bury, injure or kill a person.|
|3||Could bury and destroy a car, damage a truck, destroy a small building or break a few trees.|
|4||Could destroy a railway car, large truck, several buildings or a forest area up to 4 hectares.|
|5||Largest snow avalanche known. Could destroy a village or a forest of 40 hectares.|
|1||Sluff or snow that slides less than 50m (150') of slope distance.|
|2||Small, relative to path.|
|3||Medium, relative to path.|
|4||Large, relative to path.|
|5||Major or maximum, relative to path.|