Avalanche

Avalanche

Overview
An avalanche is a sudden rapid flow of snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

 down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope.
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Encyclopedia
An avalanche is a sudden rapid flow of snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

 down a slope, occurring when either natural triggers or human activity causes a critical escalating transition from the slow equilibrium evolution of the snow pack. Typically occurring in mountainous terrain, an avalanche can mix air and water with the descending snow. Powerful avalanches have the capability to entrain ice, rocks, trees, and other material on the slope. Avalanches are primarily composed of flowing snow, and are distinct from mudslides
Mudflow
A mudslide is the most rapid and fluid type of downhill mass wasting. It is a rapid movement of a large mass of mud formed from loose soil and water. Similar terms are mudflow, mud stream, debris flow A mudslide is the most rapid (up to 80 km/h, or 50 mph) and fluid type of downhill mass...

, rock slides
Landslide
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

, and serac
Serac
A serac is a block or column of ice formed by intersecting crevasses on a glacier. Often house-sized or larger, they are dangerous to mountaineers since they may topple with little warning...

 collapses on an icefall
Icefall
An icefall is a portion of some glaciers characterized by rapid flow and a chaotic crevassed surface. Perhaps the most conspicuous consequence of glacier flow, icefalls occur where the glacier bed steepens and/or narrows...

. In contrast to other natural events which can cause disasters, avalanches are not rare or random events and are endemic to any mountain range that accumulates a standing snow pack. In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.

Avalanches are classified by their morphological characteristics and are rated by either their destructive potential, or the mass of the downward flowing snow. Some of the morphological characteristics used to classify avalanches include the type of snow involved, the nature of the failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction and elevation. The size of an avalanche, its mass and its destructive potential are rated on a logarithmic scale
Logarithmic scale
A logarithmic scale is a scale of measurement using the logarithm of a physical quantity instead of the quantity itself.A simple example is a chart whose vertical axis increments are labeled 1, 10, 100, 1000, instead of 1, 2, 3, 4...

, typically of 5 categories, with the precise definition of the categories depending on the observation system or forecast region.

Formation and occurrences


Avalanches occur only when the stress on the snow exceeds the shear, ductile and tensile strength either within the snow pack or at the contact of the base of the snow pack with the ground or rock surface. A number of the forces acting on a snow pack can be readily determined. For example, there is little problem in calculating the weight of the snow. However, other factors are much more difficult to determine. It is very difficult to estimate the shear, ductile and tensile strengths within the snow pack or relative to the ground below. These strengths vary with the type of snow crystal and the bonding between them. The thermo-mechanical properties of the snow crystals in turn depend on the local conditions they have experienced, such as temperature and humidity. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of snow packs over time and predict the shear yield stress. A complicating factor is the large spatial variability in the data that is typical.

Classification and terminology


All avalanches share common elements: a trigger which causes the avalanche, a start zone from which the avalanche originates, a slide path along which the avalanche flows, a run out where the avalanche comes to rest, and a debris deposit which is the accumulated mass of the avalanched snow once it has come to rest. As well avalanches have a failure layer that propagates the failure and the bed surface along which the snow initially slides, in most avalanches the failure layer and the bed surface are the same. Additionally slab avalanches have a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a shallow staunch fracture at the bottom of the start zone. The crown and flank fractures are vertical walls in the snow delineating the snow that was entrained in the avalanche from the snow that remained on the slope.

The nature of the failure of the snow pack is used to morphologically classify the avalanche. Slab avalanches are generated when an additional load causes a brittle failure of a slab that is bridging a weak snow layer; this failure is propagated through fracture formation in the bridging slab. Loose snow, point release, and isothermal avalanches are generated when a stress causes a shear failure in a weak interface, either within the snow pack, or at the base. When the failure occurs at the base they are known as full depth avalanches. Spin drift avalanches occur when wind lifted snow is funneled into a steep drainage from above the drainage.

Loose snow avalanche
Loose snow avalanche
A loose snow avalanche is an avalanche formed in snow with little internal cohesion among individual snow crystals. Usually very few fatalities occur from loose snow avalanches, as the avalanches have a tendency to break beneath the person and are usually small even having a path as small as a few...

s occur in freshly fallen snow that has a lower density 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, cohesive layer of snow known as a slab. These are usually formed when falling snow is deposited by the wind on a lee slope, or when loose ground snow is transported elsewhere. When there is a failure in a weak layer, a fracture very rapidly propagates so that a large area, that can be hundreds of meters in extent and several meters thick, starts moving almost instantaneously.

A third starting type is a wet snow avalanche or isothermal avalanche, which occurs when the snow pack becomes saturated by water. These tend to also start and spread out from a point. When the percentage of water is very high they are known as slush flows and they can move on very shallow slopes.

Among the largest and most powerful of avalanches, powder snow avalanches can exceed speeds of 300 km/h, and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even up hill for short distances. A powder snow avalanche is a powder cloud that forms when an avalanche accelerates over an abrupt change in slope, such as a cliff band, causing the snow to mix with air. This turbulent suspension of snow particles then flows as a gravity current
Gravity current
In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents"...

.

Terrain



Terrain affects avalanche occurrence and development through three factors: First, terrain affects the evolution of the snow pack by determining the meteorological exposure of the snow pack. Second, terrain affects the stability of the snow pack, through the geometry and ground composition of the slope. Third, the down slope features of the terrain affects the path and consequences of a flowing avalanche.

For a slope to generate an avalanche it must be simultaneously capable of retaining snow, and allowing snow to accelerate once set in motion. The angle of the slope that can hold snow depends on the ductile and shear strength of the snow, which is determined by the temperature and moisture content of the snow. Drier and colder snow, with lower ductile and shear strength, will only bond to lower angle slopes; while wet and warm snow, with higher ductile and shear strength, can bound to very steep surfaces. In particular, in coastal mountains, such as the Cordillera del Paine
Cordillera del Paine
The Cordillera del Paine is a small but spectacular mountain group in Torres del Paine National Park in Chilean Patagonia. It is located north of Punta Arenas, and about 1,960 km south of the Chilean capital Santiago. It belongs to the Commune of Torres del Paine in Última Esperanza Province...

 region of Patagonia
Patagonia
Patagonia is a region located in Argentina and Chile, integrating the southernmost section of the Andes mountains to the southwest towards the Pacific ocean and from the east of the cordillera to the valleys it follows south through Colorado River towards Carmen de Patagones in the Atlantic Ocean...

, deep snow packs collect on vertical, and overhanging, rock faces. The angle of slope that can allow moving snow to accelerate depends on the shear strength of the snow. Snow that has been water saturated to the point of slush can accelerate on shallow angled terrain; while a cohesive snow pack will not accelerate on very steep slopes, such as the typical snow pack in the Chugach Mountains
Chugach Mountains
The Chugach Mountains of southern Alaska are the northernmost of the several mountain ranges that make up the Pacific Coast Ranges of the western edge of North America. The range is about 500 km long, running generally east-west. Its highest point is Mount Marcus Baker, at , but most of its...

 of Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...

.

The snow pack on slopes with sunny exposures are strongly influenced by sunshine
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

. Daily cycles of mild thawing and refreezing can stabilize the snow pack by promoting settlement, strong freeze thaw cycles will result in the formation of surface crusts during the night, and the formation of unstable isothermal snow during the day. 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, wind slabs, and cornices
Cornice (climbing)
A snow cornice or simply cornice is an overhanging edge of snow on a ridge or the crest of a mountain. They form by wind blowing snow over the crest of the mountain, so they often form on the leeward sides of mountains...

, all of which, when disturbed, may trigger an avalanche. Conversely a windward slope will be bare of snow.

The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally convex
Convex function
In mathematics, a real-valued function f defined on an interval is called convex if the graph of the function lies below the line segment joining any two points of the graph. Equivalently, a function is convex if its epigraph is a convex set...

 slopes are less stable than concave
Concave function
In mathematics, a concave function is the negative of a convex function. A concave function is also synonymously called concave downwards, concave down, convex upwards, convex cap or upper convex.-Definition:...

 slopes, because of the disparity between the tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...

 of snow layers and their compressive strength
Compressive strength
Compressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...

. The composition and structure of the ground surface beneath the snow pack influences the stability of the snow pack, either being a source of strength or weakness. Thickly forested areas are unlikely to be avalanche paths, however boulders and sparsely distributed vegetation can create weak areas deep within the snow pack, through the formation of strong temperature gradients. 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.

Avalanches follow drainages down slope, frequently sharing drainage features with summertime watersheds. At and below tree line these drainages are well defined by vegetation boundaries where the avalanches have prevented the growth of large vegetation. Engineered drainages, such as the avalanche dam
Avalanche dam
Avalanche dams are a type of avalanche control structures used for protection of inhabited areas, roads, electric lens, etc., from avalanches. The two major types are deflection and catchment dams...

 on Mount Stephen
Mount Stephen
Mount Stephen is a mountain located in the Kicking Horse River Valley of Yoho National Park, ½ km east of Field. The mountain was named in 1886 for George Stephen, the first president of the Canadian Pacific Railway....

 in Kicking Horse Pass
Kicking Horse Pass
Kicking Horse Pass is a high mountain pass across the Continental Divide of the Americas of the Canadian Rockies on the Alberta/British Columbia border, and lying within Yoho and Banff National Parks...

, have been constructed to protect people and property, by redirecting the flow of avalanches. Deep debris deposits from avalanches will collect in catchments at the terminus of a run out, such as gullies, and river beds.

Slopes flatter than 25 degrees or steeper than 60 degrees typically have a lower incidence of avalanche involvement, likewise slopes with windward and sunny exposure have a lower incidence of avalanche involvement . Human triggered avalanches have the greatest incidence when the snow's angle of repose
Angle of repose
The angle of repose or, more precisely, the critical angle of repose, of a granular material is the steepest angle of descent or dip of the slope relative to the horizontal plane when material on the slope face is on the verge of sliding. This angle is in the range 0°–90°.When bulk granular...

 is between 35 and 45 degrees; the critical angle, the angle at which the human incidence of avalanches is greatest, is 38 degrees. But when the incidence of human triggered avalanches are normalized by the rates of recreational use hazard increases uniformly with slope angle, and no significant difference in hazard for a given exposure direction can be found. 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.

Snow structure and characteristics


The snow pack is composed of deposition layers of snow that are accumulated over time. The deposition layers are stratified parallel to the ground surface on which the snow falls. Each deposition layer indicates a distinct meteorological condition during which the snow was accumulated. Once deposited a snow layer will continue to evolve and develop under the influence of the meteorological conditions that prevail after deposition.

For an avalanche to occur, it is necessary that a snow pack have a weak layer (or instability) below a slab of cohesive snow. In practice the mechanical and structural determinants of snow pack stability are not directly observable outside of laboratories, thus the more easily observed properties of the snow layers (e.g. penetration resistance, grain size, grain type, temperature) are used as proxy measurements of the mechanical properties of the snow (e.g. tensile strength, friction coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snow pack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snow pack vary widely within small areas and time scales, resulting in an inability to extrapolate point observations of snow layers. Second, the understanding of the relationship between the readily observable snow pack characteristics and the snow pack's critical mechanical properties has not been completely developed.

While the deterministic relationship between snow pack characteristics and snow pack stability is still a matter of ongoing scientific study, there is a growing empirical understanding of the snow composition and deposition characteristics that influence the likelihood of an avalanche. Observation and experience has shown that newly fallen snow requires time to bond with the snow layers beneath it, especially if the new snow falls during very cold and dry conditions. Shallower snow, that can lie above or around boulders, plants, and other discontinuities in the slope, will weaken from the presence of a stronger temperature gradient. Larger and more angular snow crystals are an indicator of weaker bonds within the snow pack, because the sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

 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 loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of slab avalanches, and will mask persistent instabilities within a snow pack. The empirical understanding of the factors influencing snow stability only places broad predictive bounds on the stability of the snow, consequently a conservative use of avalanche terrain, well within the recommended guidelines of the local avalanche forecasts and bulletins, is always recommended.

Weather



Avalanches can only occur in a standing snow pack. Typically winter seasons and high altitudes have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a snow pack. The evolution of the snow pack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snow pack. Among the critical factors controlling snow pack evolution are: heating by the sun, radiational cooling, vertical temperature gradient
Temperature gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees per unit length...

s in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snow pack; and conversely very cold, windy, or hot weather will weaken the snow pack.

At temperatures close to the freezing point of water, or during times of moderate solar radiation, a gentle freeze-thaw cycle will take place. The melting and refreezing of water in the snow strengthens the snow pack during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature, to a point significantly above the freezing point of water, may cause a slope to avalanche, especially in the spring.

Persistent cold temperatures can either prevent the snow from stabilizing or destabilize a snow pack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snow pack is close to freezing; unless the snow pack is standing on glaciated terrain, in which case the temperature at the base of the snow pack can be significantly below freezing. When a temperature gradient greater than 10oC change per vertical meter of snow is sustained for more than a day depth hoar
Depth hoar
Depth hoares are large crystals occurring at the base of a snowpack that form from when uprising water vapor freezes onto existing snow crystal. Depth hoares are sparkly large grains with facets that can be cup-shaped and that are up to 10 mm in diameter...

 will form in the snow pack, through the thermal transport of moisture away from the depth hoar
Depth hoar
Depth hoares are large crystals occurring at the base of a snowpack that form from when uprising water vapor freezes onto existing snow crystal. Depth hoares are sparkly large grains with facets that can be cup-shaped and that are up to 10 mm in diameter...

 along the temperature gradient, from bottom to top. This layer of depth hoar
Depth hoar
Depth hoares are large crystals occurring at the base of a snowpack that form from when uprising water vapor freezes onto existing snow crystal. Depth hoares are sparkly large grains with facets that can be cup-shaped and that are up to 10 mm in diameter...

 becomes a persistent weakness in the snow pack, characterized by faceted grains forming either above or below crusts and slabs. When a slab lying on top of this persistent weakness is loaded by a force above the tensile and ductile strength of the slab and the shear strength of the persistent weak layer, the persistent weak layer will fail and generate an avalanche.

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
Fall line (skiing)
In alpine skiing, a fall line refers to the line down a mountain or hill which is most directly downhill. That is, the direction a ball would roll if it were free to move on the slope under gravity...

 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 will cause instability in the existing snow pack, 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 snow pack; and, once rainwater seeps down through the snow, it acts as a lubricant, reducing the natural friction between snow layers that holds the snow pack together. Most avalanches happen during or soon after a storm.

Daytime exposure to sunlight will rapidly destabilize the upper layers of a snow pack. Sunlight reduces the sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...

, or necking
Necking (engineering)
Necking, in engineering or materials science, is a mode of tensile deformation where relatively large amounts of strain localize disproportionately in a small region of the material. The resulting prominent decrease in local cross-sectional area provides the basis for the name "neck"...

, between snow grains. During clear nights, the snow pack can strengthen, or tighten, through the process of long-wave radiative cooling. When the night air is significantly cooler than the snow pack, the heat stored in the snow is re-radiated into the atmosphere.

Triggers


Avalanches are always caused by an external stress on the snow pack; they are not random or spontaneous events. Natural triggers of avalanches include additional precipitation, radiative and convective heating, rock fall, ice fall, and other sudden impacts; however, even a snow pack held at a constant temperature, pressure, and humidity will evolve over time and develop stresses, often from the downslope creep of the snow pack. Human triggers of avalanches include skiers, snowmobiles, and controlled explosive work. The triggering stress load can be either localized to the failure point, or remote. Localized triggers of avalanches are typified by point releases from solar heated rocks. Remotely triggered avalanches occur when a tensile stress wave is transmitted through the slab to the start zone, once the stress wave reaches the start zone a fracture initiates and propagates the failure. Of exceptional note is that avalanches can not only entrain additional snow within the failing slab, but can also, given the sufficient accumulation of overburden due to a smaller avalanche, step down and trigger deeper slab instabilities that would be more resilient against smaller stresses. The triggering of avalanches is an example of critical phenomenon.

Dynamics


When an avalanche occurs, as the snow slides down the slope any slab present begins to fragment into increasingly smaller tumbling fragments. If the fragments become small enough the avalanche takes on the characteristics of a fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....

. When sufficiently fine particles are present they can become airborne and, given a sufficient quantity of airborne snow, this portion of the avalanche can become separated from the bulk of the avalanche and travel a greater distance as a powder snow avalanche. Scientific studies using radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

, following the 1999 Galtür avalanche disaster
Galtür Avalanche
The Galtür Avalanche was an avalanche that occurred on February 23, 1999 in the Alpine village of Galtür, Austria. The avalanche killed 31 people and is considered the worst Alpine avalanche in 40 years. Three major weather systems originating from the Atlantic accounted for large snow falls...

, confirmed suspicions that a saltation layer
Saltation (geology)
In geology, saltation is a specific type of particle transport by fluids such as wind or water. It occurs when loose material is removed from a bed and carried by the fluid, before being transported back to the surface...

 forms between the surface and the airborne components of an avalanche, which can also separate from the bulk of the avalanche.

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.

Modelling


Attempts to model avalanche behaviour date from the early 20th century, notably the work of Professor Lagotala in preparation for the 1924 Winter Olympics
1924 Winter Olympics
The 1924 Winter Olympics, officially known as the I Olympic Winter Games, were a winter multi-sport event which was held in 1924 in Chamonix, France...

 in Chamonix
Chamonix
Chamonix-Mont-Blanc or, more commonly, Chamonix is a commune in the Haute-Savoie département in the Rhône-Alpes region in south-eastern France. It was the site of the 1924 Winter Olympics, the first Winter Olympics...

. His method was developed by A. Voellmy and popularised following the publication in 1955 of his Ueber die Zerstoerungskraft von Lawinen (On the Destructive Force of Avalanches).

Voellmy used a simple empirical formula, treating an avalanche as a sliding block of snow moving with a drag 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 powder snow) 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
European Commission
The European Commission is the executive body of the European Union. The body is responsible for proposing legislation, implementing decisions, upholding the Union's treaties and the general day-to-day running of the Union....

 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.

Avalanche avoidance





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. 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.

Prevention



Preventative measures are employed in areas where avalanches pose a significant threat to people, such as ski resort
Ski resort
A ski resort is a resort developed for skiing and other winter sports. In Europe a ski resort is a town or village in a ski area - a mountainous area, where there are ski trails and supporting services such as hotels and other accommodation, restaurants, equipment rental and a ski lift system...

s and mountain towns, roads and railways. There are several ways to prevent avalanches and lessen their power and destruction; active preventative measures reduce the likelihood and size of avalanches by disrupting the structure of the snow pack; passive measures reinforce and stabilize the snow pack in situ. The simplest active measure is by repeatedly traveling on a snow pack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming. Explosives are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snow pack, and removing over burden that can result in larger avalanches. Explosive charges are delivered by a number of methods including hand tossed charges, helicopter dropped bombs, Gazex concussion lines, and ballistic projectiles launched by air cannons and artillery. Passive preventive systems such as Snow fence
Snow fence
A snow fence is a structure, similar to a sand fence, that forces drifting snow to accumulate in a desired place. They are primarily employed to minimize the amount of snowdrift on roadways and railways. Farmers and ranchers may use temporary snow fences to create large drifts in basins for a...

s 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 wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

s. 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 tree
Tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...

s, 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.

To mitigate the effect 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 rockslide
Rockslide
A rockslide is a type of landslide caused by rock failure in which part of the plane of failure passes through intact rock and where material collapses en masse and not in individual blocks.The mode of failure is different from that of a rock-fall....

s. Another type of barrier is a rigid fence like structure (snow fence
Snow fence
A snow fence is a structure, similar to a sand fence, that forces drifting snow to accumulate in a desired place. They are primarily employed to minimize the amount of snowdrift on roadways and railways. Farmers and ranchers may use temporary snow fences to create large drifts in basins for a...

) and may be constructed of steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

, wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 or pre-stressed concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

. 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. In addition to industrially manufactured barriers, landscaped barriers, called avalanche dam
Avalanche dam
Avalanche dams are a type of avalanche control structures used for protection of inhabited areas, roads, electric lens, etc., from avalanches. The two major types are deflection and catchment dams...

s 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. Finally, along transportation corridors, large shelters, called snow shed
Snow shed
An avalanche snow bridge or simply snow bridge is a type of rigid snow-supporting structure for avalanche control . Avalanche bridges can be made of steel, prestressed concrete frames, or timber....

s, can be built directly in the slide path of an avalanche to protect traffic from avalanches.

Safety in avalanche terrain

  • Terrain management - Terrain management involves reducing the exposure of an individual to the risks of traveling 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 pack), 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. Minimize the number of people on the slope, and maintain separation. Ideally one person should pass over the slope into an area protected from the avalanche hazard 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. In determining the size of the group balance the hazard of not having enough people to effectively carry out a rescue with the risk of having too many members of the group to safely manage the risks. It is generally recommended not to travel alone, because there will be no-one to witness your burial and start the rescue. Additionally, avalanche risk increases with use; that is, the more a slope is disturbed by skiers
    Skiing
    Skiing is a recreational activity using skis as equipment for traveling over snow. Skis are used in conjunction with boots that connect to the ski with use of a binding....

    , the more likely it is that an avalanche will occur. Most important of all practice good communication with in a group including clearly communicating the decisions about safe locations, escape routes, and slope choices, and having a clear understanding of every members skills in snow travel, avalanche rescue, and route finding.

  • Risk Factor Awareness - Risk factor awareness in avalanche safety requires gathering and accounting for a wide range of information such as the meteorological history of the area, the current weather and snow conditions, and equally important the social and physical indicators of the group.

  • Leadership - Leadership in avalanche terrain requires well defined decision making protocols that use the observed risk factors. These decision making frameworks are taught in a variety of courses provided by national avalanche resource centers in Europe and North America. Fundamental to leadership in avalanche terrain is honestly assessing and estimating the information that was ignored or overlooked. Recent research has shown that there are strong psychological and group dynamic determinants that lead to avalanche involvement.

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).

Research carried out in Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

http://www.avalanche.org/moonstone/rescue/avalanche%20survival%20chances.htm based on 422 buried skiers
Skiing
Skiing is a recreational activity using skis as equipment for traveling over snow. Skis are used in conjunction with boots that connect to the ski with use of a binding....

 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
    Suffocation
    Suffocation is the process of Asphyxia.Suffocation may also refer to:* Suffocation , an American death metal band* "Suffocation", a song on Morbid Angel's debut album, Altars of Madness...

    )
  • near zero after two hours (victims die of injuries
    Physical trauma
    Trauma refers to "a body wound or shock produced by sudden physical injury, as from violence or accident." It can also be described as "a physical wound or injury, such as a fracture or blow." Major trauma can result in secondary complications such as circulatory shock, respiratory failure and death...

     or hypothermia
    Hypothermia
    Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

    )
(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
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan 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. A beacon, shovel and probe is considered the minimum equipment to carry for companion rescue. Organized rescue involves ski patrols and mountain rescue teams who are often equipped with other technologies to search for buried victims. Rescue equipment can make a difference, and in 2010 the French National Association for the Study of Snow and Avalanches (ANENA) recommended that all off-piste skiers should carry beacons, probes, shovels, and Recco reflectors.

Avalanche cords


The use of avalanche cords goes back just over 100 years to a Bavarian mountaineer named Eugen Oertel. In the United States the concept was recommended as early as 1908 in the Colorado newspaper - the Ouray Herald (November 13) - when the editor repeated the suggestion that miners in the San Juans Mounains adopt "snowslide ribbons" to safeguard their travels to and from the mines. The principle is simple. An approximately 15 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. The assumption is that if the person gets buried in an avalanche, the light cord stays on top of the snow. Due to the color the cord would be easily visible for companions. Commercial avalanche cords have metal markings every one to three meters indicating the direction and length to the victim.

Avalanche cords were popular before beacons became available, and while cords were thought to be effective markers there was never any proof of their effectiveness. In the 1970s Melchoir Schild of the Swiss Federal Institute for Snow and Avalanche Research (SLF) reviewed 30 years of Swiss avalanche accidents and rescues from 1944/45 to 1973/74. Of the 2042 avalanche victims he found only seven cases where avalanche cords were used (not including the 2 mentioned above). In five cases part of the cord was visible on the surface but so too were part of the victims. In the sixth case the victim was completely buried, but part of the cord was visible. Sadly, this victim died of trauma. In the seventh case the completely buried avalanche cord was located by an avalanche rescue dog, however, the cord had become detached from the victim. Her body was found much later. In Knox Williams' and Betsy Armstrong's 1986 book The Avalanche Book they cite an early 1970s study where avalanche cords were tested on sandbag dummies. The dummies were placed onto steep slopes where explosives were used to trigger avalanches. Trials showed a portion of the cord remained on the surface only 40% of the time. The other 60% of the time the cord was completely buried along with the dummy. Typically the cord had spooled around the dummy. 1975, at a symposium of avalanche rescue experts hosted by the International Foundation Vanni Eigenmann, Schild concluded, “On the basis of these results the avalanche cord can no longer be considered reliable.”

In the United States there have been two accidents with five buried victims, all wearing avalanche cords. In one accident, an avalanche cord remained on the surface. In the second accident five ski mountaineers with cords deployed triggered an avalanche. One skier was partly buried, but his four friends and cords were completely buried. Moderate snow and the loss of the survivor’s eyeglasses only worsened the situation. The search was called off a few days later. The four were eventually found many months later after their bodies with attached cords melted out of the snow. On one victim the cord was wrapped tightly around the body.

The avalanche cord should not be used or accepted as being an adequate sole safety measure.

Beacons



Beacons — known as "beepers", peeps (pieps), ARVAs (Appareil de Recherche de Victimes en Avalanche, in French), LVS (Lawinen-Verschütteten-Suchgerät, Swiss German), avalanche transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...

s, or various other trade names, are important for every member of the party. They emit a "beep" via 457 kHz 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 use the receiver effectively requires 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.

Since about 2000 nearly all avalanche rescue transceivers use digital displays to give visual indications of direction and distance to victims. Most users find these beacons easier to use, but to be effective still requires considerable practice by the user. Beacons are the primary rescue tool for companion rescue and are considered active devices because the user must learn to use and care for their device.

Probes



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. http://outsideonline.com/outside/magazine/200002/200002ava_whitedeath7.html Survival/rescue more than 2 m deep is rare (about 4%). Probes should be used immediately after a visual search for surface clues, in coordination with the beacon search.

Shovels


Even when the snowpack consists of loose powder, avalanche debris is hard and dense. The energy of the avalanche causes the snow to melt, and the debris refreezes immediately after it stops.

Shovels are essential for digging through the snow to the victim, as the snow is often too dense to dig with hands or skis. A large strong scoop and sturdy handle are important. Plastic shovels often break, whereas metal ones are less prone to failure.

As excavation of the avalanche victim is extremely time-consuming and many buried victims suffocate before they can be reached, shovelling technique is an essential element of rescue.

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.

RECCO Rescue System


The Recco system is used by organized rescue services around the world. The Recco system is a two-part system where the rescue team uses a small hand-held detector. The detector receives a directional signal that is reflected back from a small, passive, transponder called a reflector that is included into outerwear, boots, helmets, and body protection. Recco reflectors are not a substitute for avalanche beacons. The Recco signal does not interfere with beacons. In fact, the current Recco detector also has an avalanche beacon receiver (457 kHz) so one rescuer can search for a Recco signal and a beacon signal at the same time.

Avalung


Recently, a device called an Avalung has been introduced for use in avalanche terrain. The device consists of a mouth piece, a flap valve, an exhaust pipe, and an air collector. Several models of Avalung either mount on one's chest or integrate in a proprietary backpack.

During an avalanche burial, victims not killed by trauma usually suffer from asphyxiation as the snow around them melts from the heat of the victim's breath and then refreezes, disallowing oxygen flow to the victim and allowing toxic levels of CO2 to accumulate. The Avalung ameliorates this situation by drawing breath over a large surface area in front and pushing the warm exhaled carbon dioxide behind. This buys additional time for rescuers to dig the victim out.

Avalanche airbags


Avalanche airbags help a person avoid burial by making the user an even larger object relative to the moving snow, which forces the person toward the surface. Avalanche airbags work on the principle of inverse segregation. Avalanches, like mixed nuts and breakfast cereal are considered granular materials and behave fluid-like (but are not liquids) where smaller particles settle to the bottom of the flow and larger particles rise to the top. Provided the airbag is properly deployed, the chances of a complete burial are significantly reduced.

Other devices


More backcountry adventurers are also carrying Satellite Electronic Notification Devices (SEND) to quickly alert rescuers to a problem. These devices include the SPOT Messenger, Emergency Position-Indicating Radio Beacon
Emergency Position-Indicating Radio Beacon
Distress radio beacons, also known as emergency beacons, ELT or EPIRB, are tracking transmitters which aid in the detection and location of boats, aircraft, and people in distress. Strictly, they are radiobeacons that interface with worldwide offered service of Cospas-Sarsat, the international...

 (EPIRB) or Personal Locating Beacons (PLBs) containing the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

 (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. Survivors should also try to use a mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 to notify emergency personnel. Unlike the other devices mentioned above, the mobile phone (or satellite phone) provides two-way communications with rescuers.

On-site rescuers (usually companions) are in the best position to save a buried victim. However, organized rescue teams can sometimes respond very quickly to assist in the search for a buried victim. The sooner organized rescue can be notified the sooner they can respond, and this difference can mean the difference in living or dying for a critically injured patient. The International Commission for Alpine Rescue recommends, “early notification is essential, e.g., by mobile phone, satellite phone, or radio, wherever possible”

Other rescue devices are proposed, developed and used, such as avalanche balls, vests and airbags, based on statistics indicating that decreasing the depth of burial increases the chances of survival.

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
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...

.

Self-rescue


Victims caught in an avalanche are advised to try to escape to side of the avalanche. If not possible, grab onto a tree, brush or rock (each second one hangs on lets snow pass by that cannot bury one). If knocked off one's feet, then jettison their equipment (if possible) and FIGHT for your life. Rolling like a log may help one escape to the side. Conventional wisdom says to make swimming motions to stay on the surface. Anecdotal stories tell successes; however, analysis of avalanche motion and physics dispute swimming. Avalanches stop quickly and if under the snow it is critical to get a hand in front of the face to create an airspace before the snow stops. If one is near the surface they may try to thrust an arm, leg or object above the surface. If it is possible, try to break free once the snow stops. If unable to move do not struggle except to enlarge the air space.

Companion rescue



Survival time is short, if a victim is buried. The search for victims must start immediately; many people have died because the surviving companions or 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 persons were last seen. In fact, anyone planning to enter an avalanche area should discuss this step as part of their preparation. Once the avalanche has stopped and the danger of secondary slides has passed, witnesses should mark these points with objects for reference. Then, survivors should take a headcount to determine who may be lost. If the area is safe to enter, the searchers should visually scan along a downslope trajectory from the marked points last seen. Victims who are partially or shallowly buried can often be located quickly by visually scanning the avalanche debris and pulling out clothing or equipment that may be attached to someone buried.

Because survival rates plummet as time passes, do not send a searcher for help until you feel you can do no more. However, do use your mobile phone or radio to call for help as soon as you suspect a burial. Generally, the telephone connection will be better from the top of a slope than from the bottom. Go to and mark the Last Seen Area, 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. Continue scanning and probing near marked clues and other likely burial areas. After 30 to 60 minutes, consider sending a searcher to get more help, because at this point, the remaining victims have probably 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.

Buried victims are most likely to be found--
  • 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
  • In low spots where the snow may collect (gullies, crevasses, creeks, ditches along roads, etc.)


Although less likely, check other areas if initial searches are not fruitful.

Once buried victims are found and their heads and chests are freed, perform first aid (airway, breathing, circulation/pulse, arterial bleeding, spinal injuries, fractures, shock, hypothermia, internal injuries, etc.), according to local law and custom.

Organized rescue


Professional and volunteer rescue teams respond when a victim needs more help than their companions can provide. Traditionally, organized rescue responded after companion rescue efforts failed. However, today, thanks to mobile telephones, helicopters and snow-machines, the distinction between organized and companion rescue sometimes blur together as organized rescue can respond quickly to assist companions. In a some cases in recent years, organized rescue has even replaced companion rescue and saved lives when organized rescue teams reached the debris before the victim's companions.

There are four primary goals of any rescue operation and in organized rescue the goals can be initiated simultaneously.
  • Immediate search: get rescuers to the site; find and uncover buried victims.
  • Medical: care for victims and companions
  • Transport/evacuation: transport rescuers in quickly and safely; get victims out and to advanced medical care; return rescuers safely
  • Support/Logistics: care for rescuers in the field (food, shelter, rest and replacement)

Immediate search


The first teams travel fast and light to locate and uncover buried victims. These teams carry basic rescue equipment, including rescue dogs and RECCO detectors, and emergency-care gear. These rescuers are generally not equipped for prolonged operations.

Medical


While the immediate search teams carry some basic medical equipment, a special team that can provide advanced life-support follows quickly. This team usually includes paramedic, trauma nurse, or physician, and may also transport in a rescue toboggan and other equipment needed to revive, stabilize, protect and transport their patient.

Transport/evacuation


Upon the first alert of an avalanche incident the rescue leader will appoint a team to arrange transportation for both rescuers and patients.

Support/logistics


Rescue leaders will assess the complexity of the search and rescue operation to determine and anticipate the needs for support. Every incident is different depending upon the number of victims, avalanche danger, weather conditions, terrain, access, availability of rescuers, etc. Support includes getting appropriate resources of people and equipment, transporting the resources, caring for and replacing rescuers.

In the United States all agencies are mandated to manage search and rescue operations, including avalanche, under the Incident Command System (ICS)
Incident Command System
The Incident Command System is "a systematic tool used for the command, control, and coordination of emergency response" according to the United States Federal Highway Administration...

.

Myths about avalanches


Myth: Avalanches can be triggered by shouting - Avalanches cannot be triggered by sound as the forces exerted by the pressures in sound waves are far too low. The very large shockwaves produced by explosions can trigger avalanches, however, if they are close enough to the surface.

Myth: Spitting while covered in snow can determine the direction upwards - Spitting while covered in snow is not helpful because when the snow has settled it becomes very solid and most of the time, moving is not possible.

Notable avalanches



Two avalanches occurred in March 1910 in the Cascade and Selkirk Mountain ranges; On March 1 the Wellington avalanche
Wellington avalanche
The Wellington avalanche was the worst avalanche, measured in terms of lives lost, in the history of the United States.For nine days at the end of February 1910, the little town of Wellington, Washington was assailed by a terrible blizzard. Wellington was a Great Northern Railway stop high in the...

 killed 96 in Washington State, United States. Three days later 62 railroad workers were killed in the Rogers Pass avalanche in British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

, Canada.

During World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....

 at the Austrian-Italian
Italian Campaign (World War I)
The Italian campaign refers to a series of battles fought between the armies of Austria-Hungary and Italy, along with their allies, in northern Italy between 1915 and 1918. Italy hoped that by joining the countries of the Triple Entente against the Central Powers it would gain Cisalpine Tyrol , the...

 front, many of which were caused by artillery
Artillery
Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...

 fire. Some 10,000 men, from both sides, lost their lives in avalanches in December 1916. However, it is very doubtful avalanches were used deliberately at the tactical 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
Snowpack
Snowpack forms from layers of snow that accumulate in geographic regions and high altitudes where the climate includes cold weather for extended periods during the year. Snowpacks are an important water resource that feed streams and rivers as they melt. Snowpacks are the drinking water source for...

 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 resort
Ski resort
A ski resort is a resort developed for skiing and other winter sports. In Europe a ski resort is a town or village in a ski area - a mountainous area, where there are ski trails and supporting services such as hotels and other accommodation, restaurants, equipment rental and a ski lift system...

s.

In the northern hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

 winter of 1950-1951 approximately 649 avalanches were recorded in a three month period throughout the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....

 in Austria
Austria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...

, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

, Switzerland
Switzerland
Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 and Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

. This series of avalanches killed around 265 humans and was termed the Winter of Terror
Winter of Terror (1951)
Winter of Terror is a term used to describe the three month period during the winter of 1950-1951 when a previously unrecorded number of avalanches took place in the Alps...

.

In 1993, the Bayburt Üzengili avalanche
1993 Bayburt Üzengili avalanche
The 1993 Bayburt Üzengili avalanche was an avalanche that occurred on January 18, 1993 at around 07:45 local time in Üzengili village of Bayburt Province in north-eastern Turkey...

 killed 60 individuals in Üzengili in the province of Bayburt
Bayburt
Bayburt is a city in northeast Turkey lying on the Çoruh River, and is the provincial capital of Turkey's Bayburt Province.Bayburt was once an important center on the ancient Silk Road and it was visited by Marco Polo and Turkish excursionist Evliya Celebi. Remains of its Byzantine castle still...

, Turkey
Turkey
Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

.

A large avalanche in Montroc, France, in 1999, 300,000 cubic metres of snow slid on a 30° 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 ft) deep. The mayor of Chamonix
Chamonix
Chamonix-Mont-Blanc or, more commonly, Chamonix is a commune in the Haute-Savoie département in the Rhône-Alpes region in south-eastern France. It was the site of the 1924 Winter Olympics, the first Winter Olympics...

 was convicted of second-degree murder for not evacuating the area, but received a suspended sentence.

The small Austrian village of Galtür
Galtür
Galtür is a village and ski resort in the upper Paznaun valley in Tyrol . Located in the Central Eastern Alps 35 km southwest of Landeck near the border of Vorarlberg and Switzerland, its population is about 878.-History:...

 was hit by the Galtür avalanche
Galtür Avalanche
The Galtür Avalanche was an avalanche that occurred on February 23, 1999 in the Alpine village of Galtür, Austria. The avalanche killed 31 people and is considered the worst Alpine avalanche in 40 years. Three major weather systems originating from the Atlantic accounted for large snow falls...

 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.

European avalanche risk table


In Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, 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. http://www.slf.ch/laworg/muenchen2003-fr.pdf

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 [2] 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 [1]. Elsewhere it is very stable. Avalanches may be triggered when heavy [2] loads are applied, especially on a few generally identified steep slopes. Large spontaneous avalanches are not expected.
3 - Medium On many steep slopes [1] the snow is only moderately or weakly stable. Avalanches may be triggered on many slopes even if only light loads [2] are applied. On some slopes, medium or even fairly large spontaneous avalanches may occur.
4 - High On most steep slopes [1] the snow is not very stable. Avalanches are likely to be triggered on many slopes even if only light loads [2] 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.


[1] Stability:
  • Generally described in more detail in the avalanche bulletin (regarding the altitude, aspect, type of terrain etc.)


[2] additional load:
  • heavy: two or more skiers or boarders without spacing between them, a single hiker
    Hiking
    Hiking is an outdoor activity which consists of walking in natural environments, often in mountainous or other scenic terrain. People often hike on hiking trails. It is such a popular activity that there are numerous hiking organizations worldwide. The health benefits of different types of hiking...

     or climber
    Climbing
    Climbing is the activity of using one's hands and feet to ascend a steep object. It is done both for recreation and professionally, as part of activities such as maintenance of a structure, or military operations.Climbing activities include:* Bouldering: Ascending boulders or small...

    , 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 snowshoe
    Snowshoe
    A snowshoe is footwear for walking over the snow. Snowshoes work by distributing the weight of the person over a larger area so that the person's foot does not sink completely into the snow, a quality called "flotation"....

    s.


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. 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

North American Avalanche Danger Scale


In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, the following avalanche danger scale is used. Descriptors vary depending on country.

Canadian classification for avalanche size


The Canadian classification for avalanche size is based upon the consequences of the avalanche. Half sizes are commonly used.
Size Destructive Potential
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.

United States classification for avalanche size

Size Destructive Potential
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.

Related Flows

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    Landslide
    A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments...

  • Mudflow
    Mudflow
    A mudslide is the most rapid and fluid type of downhill mass wasting. It is a rapid movement of a large mass of mud formed from loose soil and water. Similar terms are mudflow, mud stream, debris flow A mudslide is the most rapid (up to 80 km/h, or 50 mph) and fluid type of downhill mass...

  • Pyroclastic flow
    Pyroclastic flow
    A pyroclastic flow is a fast-moving current of superheated gas and rock , which reaches speeds moving away from a volcano of up to 700 km/h . The flows normally hug the ground and travel downhill, or spread laterally under gravity...

  • Debris flow
    Debris flow
    A debris flow is a fast moving, liquefied landslide of unconsolidated, saturated debris that looks like flowing concrete. It is differentiated from a mudflow in terms of the viscosity and textural properties of the flow. Flows can carry material ranging in size from clay to boulders, and may...

  • Slush flow
    Slush flow
    Slush flows are a type of snow avalanche that occurs when the snow pack becomes saturated with water. They most commonly occur at higher latitudes in the spring. Because of their high water content they can flow down gentle slopes of only a few degrees...

  • Lahar
    Lahar
    A lahar is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of...

  • Gravity current
    Gravity current
    In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents"...


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