- This article refers to the natural event. For other uses, see Avalanche (disambiguation)
An avalanche is caused when a build up of snow is released down a slope, and is one of the major dangers faced in the mountains in winter. An avalanche is an example of a gravity current consisting of granular material.
In an avalanche, lots of material or mixtures of different types of material fall or slide rapidly under the force of gravity. Avalanches are often classified by what they are made of, for example snow, ice, rock or soil avalanches. A mixture of these would be called a debris avalanche.
A large avalanche can run for miles, and can create massive destruction of the lower forest and anything else in its path. For example, in Montroc, France, in 1999 300,000 cubic metres of snow slid on a 30 degree slope, achieving a speed of 100 km/h. It killed 12 people in their chalets under 100,000 tons of snow, 5 meters deep. The Mayor of Chamonix was charged with manslaughter. 
During World War I, over 60,000 soldiers died as a result of avalanches during the mountain campaign in the Alps, many of which were caused by artillery fire. However, it is very doubtful avalanches were used as deliberate weapons; more likely they were simply a side benefit to shelling enemy troops, occasionally adding to the toll taken by the artillery. Avalanche prediction is difficult even with detailed weather reports and core samples from the snowpack. 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.
- 1 Causes
- 2 Contributing factors
- 3 Avalanche avoidance
- 4 Human survival and avalanche rescue
- 5 European avalanche risk table
- 6 United States Avalanche Danger Scale
- 7 See also
- 8 External links
- 9 References
Avalanches occur when the load on the upper snow layers exceeds bonding forces (bonding to layer beneath, support from anchors such as rocks and trees, stress support from top or bottom of slope).
Critical load may be exceeded naturally by adding new snow or by rapid loading, by falling ice, cornices and similar means. Forty-three climbers were killed in one avalanche in the Pamirs in 1990 when a serac crumbled on a slope above them and swept them into a crevasse . The bonding forces within a snowpack are affected by temperatures (e.g., a lubricated melt layer, or a fragile crystal layer) before, during and after snowfall.
Avalanches are also triggered by humans - because of the additional weight, kicks during skiing (e.g. during jumps) or intentionally by explosives, slope-cuts and other means. Contrary to common thought, avalanches are rarely caused by loud sounds. More than 90% of avalanche victims worldwide are due to avalanches triggered by the victim or someone in the victim's party.
Determining critical load which would cause a slope avalanche is a complex task involving evaluation of many factors. Some of them are:
- Steepness - slopes under 25 degrees and over 60 degrees have a low avalanche risk because of the angle of repose for snow. Snow does not accumulate significantly on steep slopes and does not easily flow on flat slopes. Distribution of avalanches by slope has a sharp peak between 35 to 45 degrees. That peak hazard lies at around 38 degrees. Unfortunately, slopes with the most dangerous steepness are favourite for skiing.
- Direction - snowpack evolution is influenced by solar heating and wind. In medium latitudes on the northern hemisphere, more accidents happen on shady slopes with northern and north-eastern aspects. Slopes sheltered from the wind tend to gather more snow. Cornices also accumulate on the downwind side of ridges, and can contribute to avalanche danger.
- Profile - convex slopes are statistically more dangerous than concave. Reasons lie partly in human behaviour, and the tensile strength of snow layers versus the compression strength.
- Surface - base avalanches are more common on slopes covered with grass surfaces than slopes with dwarf pines. Boulders or buried vegetation may create weak areas within the snow pack.
Structure of the snowpack determines avalanche danger. Unfortunately relations between easily observable properties of snow layers (strength, grain size, grain type, temperature) and avalanche danger are complex and not yet fully understood. Additionally snow cover varies in space and so does stability of snow.
- New snow - has not had time to bond with the layers below, especially if it is light and powdery.
- Snow depth - snow that is above the layer of boulders and plants on the slope has none of these natural objects to help anchor it to the slope, and is therefore more dangerous. Naturally this is just the type of snow needed for snowsports such as skiing.
- Snow crystal size - Generally speaking, the larger the crystal, the weaker it is.
- Snow compaction - compacted snow is less likely to move than light powdery layers.
Weather determines the evolution of snowpack. The most important factors are heating by solar radiation, radiational cooling, temperature gradients in snow, and snowfall amounts and type. Most avalanches happen during or soon after a storm.
- Temperature - if the temperature is high enough for gentle freeze-thaw cycles to take place, the melting and re-freezing of water in the snow strengthens the snowpack during the freeze cycle and weakens it in the thaw cycle. Temperatures rising significantly over the freezing point may cause the whole slope to avalanche, especially in spring. Persistent cold temperatures cause the snow to not gain stability and may contribute to formation of "depth hoar", where there is a high temperature gradient within the snow. Thin layers of "faceted grains" may form above or below crusts when temperature gradients become strong
- through the crust.
- Wind - anything more than a gentle wind can contribute to rapid build up of snow on sheltered slopes (downwind), while the wind pressure can also stabilize other slopes. "Wind slab" is a particularly fragile brittle structure -- heavily loaded, poorly bonded. Even on a clear day, wind can quickly shift snow-load to the snow pack.
- Heavy snowfall - cause instability, both through the additional weight, and because the snow has insufficient time to bond.
- Rain - in the short-term causes instability through additional load and possible lubrication of lower layers.
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 minimise 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 travel below others who might trigger an avalanche.
- Do not travel alone. There will be no-one to witness your burial and start the rescue.
- Traversing - is to be avoided to minimise cutting across the slope.
- Risk exposure - minimise the number of people on the slope. Maintain separation. Ideally one person should pass over the slope into an avalanche protected area before the next one leaves protective cover. Route selection should also consider what dangers lie above and below the route, and the consequences of an unexpected avalanche (i.e., unlikely to occur, but deadly if it does). Stop or camp only in safe locations. Wear warm gear to delay hypothermia if buried. Plan escape routes.
- Group size - the party should be large enough to perform a rescue, but additional people will increase the disturbance to the slope. Members should be aware of their duties to search.
- Leadership - If you find yourself in a potentially dangerous avalanche situation, you should seriously question the choice of route, why your safety is being put in jeopardy, and what alternatives might be safer than pressing on.
Human survival and avalanche rescue
Even small avalanches are a serious danger to life, even with properly trained and equipped companions who avoid the avalanche. Between 55 and 65 percent of victims buried in the open are killed, and only 80 percent of the victims remaining on the surface survive. (McClung, p.177). Thus, there is at least a one in five chance that you will be killed if caught in a significant avalanche.
- very rapidly from 92% within 15 minutes to only 30% after 35 minutes (victims die of suffocation)
- and near zero after two hours (victims die of injuries or hypothermia)
- (Historically, the chances of survival were estimated at 85% within 15 minutes, 50% within 30 minutes, 20% within one hour).
Consequently it is vital that everyone surviving an avalanche is used in an immediate search and rescue operation, rather than waiting for help to arrive. Additional help can be called once it can be determined if anyone is seriously injured or still remains unaccountable after the immediate search (i.e., after at least 30 minutes of searching). Even in a well equipped country such as France, it typically takes 45 minutes for a helicopter rescue team to arrive, by which time most of the victims are likely to be dead.
In some cases avalanche victims are not located until spring thaw melts the snow, or even years later when objects emerge from a glacier.
Search and rescue equipment
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.
Using an avalanche cord is the oldest form of Equipment - mainly used before beacons became available. The principle is simple - an aprox. 10m long, red cord (similar to parachute cord) is attached to the persons belt. While skiing/snowboarding/walking the cord is dragged along behind the person. If the person gets buried in an avalanche, the light cord stays on top of the snow. Due to the color the cord is easily visible for rescue personnel. Typicly the cord has iron markings every 1m that indicate the direction and length to the victim.
Beacons - known as "beepers", ARVAs (Appareil de Recherche de Victimes en Avalanche, in French), LVS (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche transceivers or under various trade names (including Ortovox, Barryvox, and Pieps), are important for every member of the party. They emit a "beep" via 457kHz radio signal in normal use, but can be switched to receive mode to locate a buried victim up to 80 meters away. Analog receivers provide audible beeps that rescuers interpret to estimate distance to a victim. To be effective, beacons require regular practice. Some older models of beepers operated on a different frequency (2.275 kHz) and a group leader should ensure these are no longer in use.
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.
Mobile phones can seriously disrupt the ability of a beacon to receive a transmitting beacon's signal. Phones should be switched off when searching.
Portable (collapsible) probes can be extended to probe into the snow to locate the exact location of a victim at several yards / metres in depth. When multiple victims are buried, probes should be used to decide the order of rescue, with the shallowest being dug out first since they have the greatest chance of survival.
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 percent of the 140 victims found (since 1950) by probing were already dead. Outside Online, Feb 2000 Survival/rescue more than 2 m deep is relatively rare (about 4 percent). Probes should be used immediately after a visual search for surface clues, in coordination with the beacon search.
When an avalanche stops, the deceleration normally compresses the snow to a hard mass. Shovels are essential for digging through the snow to the victim, as the deposit is too dense to dig with hands or skis. A large scoop and sturdy handle are important. Shovels are also useful for digging snow pits as part of evaluating the snowpack for hidden hazards, such as weak layers supporting large loads.
Other rescue devices are proposed, developed and used, such as Avalanche Ball, Avalung vest and avalanche airbags (most deaths are due to suffocation). There are also passive signalling devices that can be carried or inserted into sports equipment, but they require specialized search equipment which might only be found near an organized sports area. When considering any of theses devices, one should consider that if your group does not recover the avalanche victim within 15 minutes, the chance of survival rapidly decreases. Reliance on technology to summon outside help is used with the knowledge that those responding will likey be performing a body recovery. Your group MUST be capable of self-rescue!! More back-country adventurers are also carrying EPIRBs (Emergency position-indicating rescue beacons) containing GPS. This device can quickly notify search and rescue of an emergency and the general location (within 100 yards), but only if the person with the EPIRB has survived the avalanche and can activate the device manually. With modern mobile phone developments, an emergency GPS transmittter may also become more widely available (again, for use by a rescuer, because a victim may be unconscious or completely immobilised beneath dense snow).
Prior to the arrival of beacons, some brave skiers carried a 50-ft ball of nylon cord, tied to the skier, with direction and distance markings attached at intervals. When traversing a dangerous area, the ball would be dropped or thrown and the unravelled string would trail behind. If caught in an avalanche, the hope was that a segment of the string would be quickly found and indicate the distance and direction to the victim.
Although it will be very inefficient, some rescue equipment can also be hastily improvised: ski poles can become short probes, skis or snowboards can be used as shovels.
Witnesses as rescuers
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 you can live with the fact that your companion may be dead because you didn't try hard enough to find him or her. 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.
Victims caught in an avalanche are advised to try to ski or board towards the side of the avalanche until they fall, then to jettison their equipment and attempt swimming motions. As the snow comes to rest an attempt should be made to preserve an air-space in front of the mouth, and try to thrust an arm, leg or object above the surface, assuming you are still conscious. If it is possible to move once the snow stops enlarge the air space, but minimise movement to maximise the oxygen supply. Warm breath may soon cause a mask of ice to glaze over the snow in your face, sealing it against further air.
European avalanche risk table
In Europe, the avalanche risk is widely rated on the following scale, which was adopted in April 1993 to replace the earlier non-standard national schemes. Descriptions were last updated in May 2003 to enhance uniformity pdf.
|Risk Level||Snow Stability||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.|
- Generally described in more detail in the avalanche bulletin (regarding the altitude, aspect, type of terrain etc.)
 additional load:
- heavy: two or more skiers or boarders without spacing between them, a single hiker or climber, a grooming machine, avalanche blasting.
- light: a single skier or snowboarder smoothly linking turns and without falling, a group of skiers or snowboarders with a minimum 10m gap between each person, a single person on snowshoes.
- gentle slopes: with an incline below about 30°.
- steep slopes: with an incline over 30°.
- very steep slopes: with an incline over 35°.
- extreme steep slopes: extreme in terms of the incline (over 40°), the terrain profile, proximity of the ridge, smoothness of underlying ground.
|Size||Runout||Potential Damage||Physical Size|
|1 - Sluff||Small snow slide that cannot bury a person, though there is a danger of falling.||Relatively harmeless to people||length <50m |
|2 - Small||Stops within the slope.||Could bury, injure or kill a person.||length <100m |
|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 <1,000m |
|4 - Large||Runs over flat areas (significantly less than 30°) of at least 50m in length, may reach the valley bottom.||Could bury and destroy large trucks and trains, large buildings and forested areas.||length >1,000m |
United States Avalanche Danger Scale
In the United States, the following avalanche danger scale is used.
|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.|
- Skiing and Ski Topics
- Pyroclastic flow
- Green Alder
- Wellington, Washington: Site of the worst avalanche in U.S. history, in which 96 people died.
- Avalanche Center
- SLF (Swiss Federal Institute for Snow and Avalanche Research) - leading avalanche research institution
- European avalanche services
- Avalanche Avoidance and Route Planning
- McClung, David and Shaerer, Peter: The Avalanche Handbook, The Mountaineers: 1993. ISBN 0-89886-364-3
- Tremper, Bruce: Staying Alive in Avalanche Terrain, The Mountaineers: 2001. ISBN 0-89886-834-3
- Munter, Werner: 3x3 Lawinen, Bergverlag Rother 2002. ISBN 3-00-010520-4 (partial english translation included in PowderGuide: Managing Avalanche Risk ISBN 0-97248-273-3)