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Avalanche

2007 Schools Wikipedia Selection. Related subjects: Natural Disasters

   An avalanche is a very large slide of snow (or rock) down a
   mountainside, caused when a buildup of snow is released down a slope,
   and is one of the major dangers faced in the mountains. An avalanche
   consists of rapidly moving granular material that has exceeded the
   critical static friction threshold and thereby causes additional
   material to exceed its threshold as well, in a cascading effect.

   In an avalanche, large quantities 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 many 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 (60 mph). It killed
   12 people in their chalets under 100,000 tons of snow, 5 meters (15
   feet) deep. The Mayor of Chamonix was convicted of second-degree murder
   for not evacuating the area, but received a suspended sentence.

   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 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.
   A Himalayan avalanche near Mount Everest.
   A Himalayan avalanche near Mount Everest.

Causes

   Snow avalanches occur when the load on the upper snow layers exceeds
   the bonding forces of a mass of snow (bonding to layer beneath,
   horizontal internal stability, support from anchors such as rocks and
   trees, stress support from top or bottom of slope).

Contributing factors

   All avalanches are caused by an over-burden of material (typically,
   snowpack) that is too massive and unstable for the slope that supports
   it. Determining the critical load, the amount of over-burden which is
   likely to cause an avalanche, is a complex task involving the
   evaluation of a number of factors. These factors include:

Terrain

   Slopes flatter than 25 degrees or steeper than 60 degrees typically
   have a low risk of avalanche. Snow does not accumulate significantly on
   steep slopes; also, snow does not flow easily on flat slopes.
   Avalanches are most likely to occur when the snow's angle of repose is
   between 35 and 45 degrees; the critical angle, the angle at which the
   incidence of avalanches is greatest, is 38 degrees. The rule of thumb
   is: A slope that is flat enough to hold snow but steep enough to ski
   has the potential to generate an avalanche, regardless of the angle.
   However, avalanche risk increases exponentially with use; that is, the
   more a slope is disturbed by skiers, the more likely it is that an
   avalanche will occur.

   The four variables that influence snowpack evolution and composition
   are temperature, precipitation, solar radiation, and wind. In the
   mid-latitudes of the Northern Hemisphere, more avalanches occur on
   shady slopes with northern and north-eastern exposures. (However, when
   the incidence of avalanches are normalized to mid-latitude rates of
   recreational use, no significant difference in hazard for a given
   exposure direction can be found.) The snowpack on slopes with southern
   exposures are strongly influenced by sunshine; daily cycles of surface
   thawing and refreezing create a crust that may tend to stabilize an
   otherwise unstable snowpack, but the crust, once it has been fractured,
   may detach itself from the underlying layers of snow, slide, and
   promote the generation of an avalanche. Slopes in the lee of a ridge or
   other wind obstacle accumulate more snow and are more likely to include
   pockets of abnormally deep snow, windslabs, and cornices, all of which,
   when disturbed, may trigger an avalanche.

   Convex slopes are more dangerous than concave slopes. Part of the
   increase in hazard can be ascribed to human behaviour; skiers enjoy
   propelling themselves into the air by skiing over convex features in
   the snowscape. Another factor contributing to the increased avalanche
   danger on convex slopes is a disparity between the tensile strength of
   snow layers and their compression strength.

   Another factor effecting 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.

Snow structure and characteristics

   The structure of the snowpack is a strong predictor of avalanche
   danger. For an avalanche to occur, it is necessary that a snowpack have
   a weak layer (or instability) below the surface and an overlying slab
   of 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, this is an area that is not yet fully understood.
   Furthermore, snow cover and stability often vary widely within
   relatively small areas, and a risk assessment of a given slope is
   unlikely to remain valid, accurate, or useful for very long.

   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. Snow that lies above boulders or certain types of plants
   has little to help anchor it to the slope. Larger snow crystals,
   generally speaking, are less likely to bond together to form strong
   structures than smaller crystals are. Consolidated snow is less likely
   to sluff than light powdery layers; however, well-consolidated snow is
   more likely to generate unstable slabs.

Weather

   Weather also influences the evolution of snowpack formation. The most
   important factors are heating by the sun, radiational cooling, vertical
   temperature gradients in standing snow, snowfall amounts, and snow
   types.

   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 parallel to the fall-line on a slope; cross-loading occurs when
   wind deposits snow perpendicular 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;
   they also tend to be less stable and more dangerous than top-loaded
   ones.

   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 destablize 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.

Avalanche avoidance

   United States Forest Service avalanche danger advisories.
   United States Forest Service avalanche danger advisories.

   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 travelling below
   others who might trigger an avalanche.

Prevention

   Snow fences in Switzerland
   Snow fences in Switzerland
   Avalanche blasting in French ski resort Tignes (3,600 m)
   Avalanche blasting in French ski resort Tignes (3,600 m)

   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 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 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, mounds of earth are placed in the avalanche's path to
   slow it down.

Safety in Avalanche Terrain

     * Terrain Management - Terrain management involves reducing the
       exposure of an individual to the risks of travelling in avalanche
       terrain by carefully selecting what areas of slopes to travel on.
       Features to be cognizant of include not under cutting slopes
       (removing the physical support of the snow pacl), not traveling
       over convex rolls (areas where the snow pack is under tension),
       staying away from weaknesses like exposed rock, and avoiding areas
       of slopes that expose one to terrain traps (gulleys that can be
       filled in, cliffs over which one can be swept, or heavy timber into
       which one can be carried).

     * Group Management - Group management is the practice of reducing the
       risk of having a member of a group, or a whole group involved in an
       avalanche. 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. Most important of all practice good
       communication with in a group including clearly communicating the
       decisions about safe locations, esape routes, and slope choices,
       and having a clear understanding of every members skills in snow
       travel, avalanche rescue, and route finding.

     * Group size - Group size must balance the hazard of not having
       enough people to effectively carry out a rescue with the risk of
       having to many members of the group to safely manage the risks. It
       is generally recommended not to travel alone. There will be no-one
       to witness your burial and start the rescue.

     * Leadership - Leadership in avalanche terrain requires well defined
       decision making protocols, which are being taught in a growing
       number of courses provided by national avalanche resource centers
       in Europe and North America. Fundamental to leadership in avalanche
       terrain is an honest attempt at assessing ones blind spots (what
       information am I ignoring?) There is a growing body of research
       into the pyschological behaviours and group dynamics that lead to
       avalanche involvement.

Human survival and avalanche rescue

   Avalanche on the backside (east) of Mount Timpanogos, Utah at Aspen
   Grove trail
   Avalanche on the backside (east) of Mount Timpanogos, Utah at Aspen
   Grove trail

   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).

   Research carried out in Italy (Nature vol 368 p21) based on 422 buried
   skiers indicates how the chances of survival drop:
     * very rapidly from 92 percent within 15 minutes to only 30 percent
       after 35 minutes (victims die of suffocation)
     * near zero after two hours (victims die of injuries or hypothermia)

          (Historically, the chances of survival were estimated at 85%
          percent 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 have died.

   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.

Avalanche cords

   Using an avalanche cord is the oldest form of equipment — mainly used
   before beacons became available. The principle is simple. An
   approximately 10 meter long red cord (similar to parachute cord) is
   attached to the person in question's belt. While skiing, snowboarding,
   or 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 colour the cord is easily visible for rescue personnel.
   Typically the cord has iron markings every one meter that indicate the
   direction and length to the victim.

Beacons

   Beacons — known as "beepers", peeps (pieps), ARVAs (Appareil de
   Recherche de Victimes en Avalanche, in French), LVS
   (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche
   transceivers, or various other trade names, are important for every
   member of the party. They emit a "beep" via 457kHz radio signal in
   normal use, but may 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.

Probes

   Portable probe, collapsed
   Portable probe, collapsed

   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 % 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.

Shovels

   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. Not to mention a large
   number of diggers. Shovels are also useful for digging snow pits as
   part of evaluating the snow pack for hidden hazards, such as weak
   layers supporting large loads.

Other devices

   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. 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 the 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 likely be performing a body recovery. Any group
   that wants to survive must be capable of self-rescue. More back-country
   adventurers are also carrying EPIRBs ( Emergency Position-Indicating
   Radio 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 transmitter may also become more widely
   available (again, for use by a rescuer, because a victim may be
   unconscious or completely immobilised beneath dense snow).

   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.

   A first aid kit and equipment will also be useful for assisting
   survivors who may have cuts, broken bones, or other injuries, in
   addition to hypothermia.

Witnesses as rescuers

   Periodic winter avalanches on this 800 m high slope transport woody
   debris to the flat in the foreground.
   Periodic winter avalanches on this 800 m high slope transport woody
   debris to the flat in the foreground.

   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.

Victims

   Victims caught in an avalanche are advised to try to ski or board
   toward 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. Then you
   must be found or there is not much hope of living.

Case Example

   An experienced skier participating in a guided trip experienced the
   effects of an avalanche first-hand. As they set out in the morning, the
   party experienced "the most stable conditions they could remember."
   However, during the next 48 hours, the temperature increased, and the
   wind rose, creating unstable conditions on the mountain. On the tour,
   the group found themselves a short distance off-course and traversed
   below a sub-peak. The unstable snowpack underfoot fractured, triggering
   an avalanche. The mass of snow impacted the man from behind, thrusting
   him down the hill head-first with his skis trailing behind. Traveling
   at the speed of the slide, his knees were wrenched continuously.
   Eventually, he was dragged under the flowing snow and cemented into
   place. With his nose and mouth filled with snow, his screams could only
   be heard within a few feet of his position. After a short time, the
   skier was breathing his own exhaled carbon dioxide, and his body
   sensations began to dwindle. After roughly ten minutes in that state,
   he was located using a probe line. Once he was uncovered, CPR and
   rescue-breathing was administered. The skier was saved and lives to
   tell about it.

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. ^

   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.

   Stability:
     * 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
       10 m gap between each person, a single person on snowshoes.

   Gradient:
     * gentle slopes: with an incline below about 30°.
     * steep slopes: with an incline over 30°.
     * very steep slopes: with an incline over 35°.
     * extremely steep slopes: extreme in terms of the incline (over 40°),
       the terrain profile, proximity of the ridge, smoothness of
       underlying ground.

European avalanche size table

   Avalanche size:
   Size Runout Potential Damage Physical Size
   1 - Sluff Small snow slide that cannot bury a person, though there is a
   danger of falling. Relatively harmless 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

North American Avalanche Danger Scale

   In the United States and Canada, 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.

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