Ligne Saint-Gervais - Vallorcine - avalanche 2006 - 08

An avalanche is a rapid flow of

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

down a

slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...

, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or

snowpack Snowpack forms from layers of snow that accumulate in geographic regions and high elevations where the climate includes cold weather for extended periods during the year. Snowpacks are an important water resource that feed streams and rivers as th ...

weakening, or by external means such as humans, animals, and

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

s. Primarily composed of flowing snow and air, large avalanches have the capability to capture and move ice, rocks, and trees. Avalanches occur in two general forms, or combinations thereof: slab avalanches made of tightly packed snow, triggered by a collapse of an underlying weak snow layer, and loose snow avalanches made of looser snow. After being set off, avalanches usually accelerate rapidly and grow in mass and volume as they capture more snow. If an avalanche moves fast enough, some of the snow may mix with the air, forming a powder snow avalanche. Though they appear to share similarities, avalanches are distinct from slush flows,

mudslides A mudflow or mud flow is a form of mass wasting involving fast-moving flow of debris that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significa ...

, rock slides, and

serac A serac (from Swiss French ''sérac'') is a block or column of glacial ice, often formed by intersecting crevasses on a glacier. Commonly house-sized or larger, they are dangerous to mountaineers, since they may topple with little warning. Even ...

collapses. They are also different from large scale movements of ice. Avalanches can happen in any mountain range that has an enduring snowpack. They are most frequent in winter or spring, but may occur at any time of year. In mountainous areas, avalanches are among the most serious

natural hazard A natural hazard is a natural phenomenon that might have a negative effect on humans and other animals, or the environment. Natural hazard events can be classified into two broad categories: geophysical and biological. An example of the distinct ...

s to life and property, so great efforts are made in

avalanche control Avalanche control or avalanche defense activities reduce the hazard avalanches pose to human life, activity, and property.dynamics.

Formation

Loose snow and slab avalanches near mt shuksan

Snowboarder triggered soft slab near mount baker

Most avalanches occur spontaneously during storms under increased load due to snowfall and/or

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

. The second largest cause of natural avalanches is metamorphic changes in the snowpack such as melting due to solar radiation. Other natural causes include rain, earthquakes, rockfall and icefall. Artificial triggers of avalanches include skiers, snowmobiles, and controlled explosive work. Contrary to popular belief, avalanches are not triggered by loud sound; the pressure from sound is orders of magnitude too small to trigger an avalanche. Avalanche initiation can start at a point with only a small amount of snow moving initially; this is typical of wet snow avalanches or avalanches in dry unconsolidated snow. However, if the snow has sintered into a stiff slab overlying a weak layer then fractures can propagate very rapidly, so that a large volume of snow, that may be thousands of cubic metres, can start moving almost simultaneously. A snowpack will fail when the load exceeds the strength. The load is straightforward; it is the weight of the snow. However, the strength of the snowpack is much more difficult to determine and is extremely heterogeneous. It varies in detail with properties of the snow grains, size, density, morphology, temperature, water content; and the properties of the bonds between the grains.McClung, David and Shaerer, Peter: The Avalanche Handbook, The Mountaineers: 2006. These properties may all metamorphose in time according to the local humidity, water vapour flux, temperature and heat flux. The top of the snowpack is also extensively influenced by incoming radiation and the local air flow. One of the aims of avalanche research is to develop and validate computer models that can describe the evolution of the seasonal snowpack over time. A complicating factor is the complex interaction of terrain and weather, which causes significant spatial and temporal variability of the depths, crystal forms, and layering of the seasonal snowpack.

Slab avalanches

Slab avalanches form frequently in snow that has been deposited, or redeposited by wind. They have the characteristic appearance of a block (slab) of snow cut out from its surroundings by fractures. Elements of slab avalanches include the following: a crown fracture at the top of the start zone, flank fractures on the sides of the start zones, and a fracture at the bottom called the stauchwall. 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. Slabs can vary in thickness from a few centimetres to three metres. Slab avalanches account for around 90% of avalanche-related fatalities in backcountry users.

Powder snow avalanches

The largest avalanches form turbulent suspension currents known as powder snow avalanches or mixed avalanches, a kind of gravity current. These consist of a powder cloud, which overlies a dense avalanche. They can form from any type of snow or initiation mechanism, but usually occur with fresh dry powder. They can exceed speeds of , and masses of 10,000,000 tonnes; their flows can travel long distances along flat valley bottoms and even uphill for short distances.

Wet snow avalanches

In contrast to powder snow avalanches, wet snow avalanches are a low velocity suspension of snow and water, with the flow confined to the track surface (McClung, first edition 1999, page 108). The low speed of travel is due to the friction between the sliding surface of the track and the water saturated flow. Despite the low speed of travel (≈10–40 km/h), wet snow avalanches are capable of generating powerful destructive forces, due to the large mass and density. The body of the flow of a wet snow avalanche can plough through soft snow, and can scour boulders, earth, trees, and other vegetation; leaving exposed and often scored ground in the avalanche track. Wet snow avalanches can be initiated from either loose snow releases, or slab releases, and only occur in snowpacks that are water saturated and isothermally equilibrated to the melting point of water. The isothermal characteristic of wet snow avalanches has led to the secondary term of isothermal slides found in the literature (for example in Daffern, 1999, page 93).Daffern, Tony: Avalanche Safety for Skiers, Climbers and Snowboarders, Rocky Mountain Books: 1999. At temperate latitudes wet snow avalanches are frequently associated with climatic avalanche cycles at the end of the winter season, when there is significant daytime warming.

Ice avalanche

An ice avalanche occurs when a large piece of ice, such as from a serac or calving glacier, falls onto ice (such as the Khumbu Icefall), triggering a movement of broken ice chunks. The resulting movement is more analogous to a rockfall or a landslide than a snow avalanche. They are typically very difficult to predict and almost impossible to mitigate.

Avalanche pathway

As an avalanche moves down a slope it follows a certain pathway that is dependent on the slope's degree of steepness and the volume of snow/ice involved in the

mass movement Mass movement may refer to: * Mass movement (geology), the movement of rock and soil down slopes due to gravity * Mass movement (politics) A mass movement denotes a political party or movement which is supported by large segments of a population. P ...

. The origin of an avalanche is called the Starting Point and typically occurs on a 30–45 degree slope. The body of the pathway is called the Track of the avalanche and usually occurs on a 20–30 degree slope. When the avalanche loses its momentum and eventually stops it reaches the Runout Zone. This usually occurs when the slope has reached a steepness that is less than 20 degrees. These degrees are not consistently true due to the fact that each avalanche is unique depending on the stability of the

that it was derived from as well as the environmental or human influences that triggered the mass movement.

Injuries and deaths

People caught in avalanches can die from

suffocation Asphyxia or asphyxiation is a condition of deficient supply of oxygen to the body which arises from abnormal breathing. Asphyxia causes generalized hypoxia, which affects primarily the tissues and organs. There are many circumstances that can i ...

, trauma, or hypothermia. From "1950 - 1951 to 2020 - 2021" there were 1,169 people who died in avalanches in the United states. For the 11-year period ending April 2006 445 people died in avalanches throughout North America. On average, 28 people die in avalanches every winter in the United States. In 2001 it was reported that globally an average of 150 people die each year from avalanches. Three of the deadliest recorded avalanches have killed over a thousand people each.

Terrain, snowpack, weather

Doug Fesler and Jill Fredston developed a conceptual model of the three primary elements of avalanches: terrain, weather, and snowpack. Terrain describes the places where avalanches occur, weather describes the meteorological conditions that create the snowpack, and snowpack describes the structural characteristics of snow that make avalanche formation possible.

Terrain

Avalanche formation requires a slope shallow enough for snow to accumulate but steep enough for the snow to accelerate once set in motion by the combination of mechanical failure (of the snowpack) and gravity. The angle of the slope that can hold snow, called the

angle of repose The angle of repose, or critical angle of repose, of a granular material is the steepest angle of descent or dip relative to the horizontal plane to which a material can be piled without slumping. At this angle, the material on the slope fac ...

, depends on a variety of factors such as crystal form and moisture content. Some forms of drier and colder snow will only stick to shallower slopes, while wet and warm snow can bond to very steep surfaces. In particular, in coastal mountains, such as the Cordillera del Paine region of Patagonia, deep snowpacks collect on vertical and even overhanging rock faces. The slope angle that can allow moving snow to accelerate depends on a variety of factors such as the snow's shear strength (which is itself dependent upon crystal form) and the configuration of layers and inter-layer interfaces. The snowpack on slopes with sunny exposures is strongly influenced by

sunshine Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when th ...

. Diurnal cycles of thawing and refreezing can stabilize the snowpack by promoting settlement. Strong freeze-thaw cycles result in the formation of surface crusts during the night and of unstable surface snow during the day. Slopes in the lee of a ridge or of another wind obstacle accumulate more snow and are more likely to include pockets of deep snow,

wind slabs Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...

, and cornices, all of which, when disturbed, may result in avalanche formation. Conversely, the snowpack on a windward slope is often much shallower than on a lee slope. Avalanche path 7271

Avalanches and avalanche paths share common elements: a start zone where the avalanche originates, a track along which the avalanche flows, and a runout zone where the avalanche comes to rest. The debris deposit is the accumulated mass of the avalanched snow once it has come to rest in the run-out zone. For the image at left, many small avalanches form in this avalanche path every year, but most of these avalanches do not run the full vertical or horizontal length of the path. The frequency with which avalanches form in a given area is known as the return period. The start zone of an avalanche must be steep enough to allow snow to accelerate once set in motion, additionally convex slopes are less stable than

concave Concave or concavity may refer to: Science and technology * Concave lens * Concave mirror Mathematics * Concave function, the negative of a convex function * Concave polygon, a polygon which is not convex * Concave set * The concavity In ca ...

slopes, because of the disparity between the

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

of snow layers and their

compressive strength In mechanics, compressive strength or compression strength is the capacity of a material or structure to withstand loads tending to reduce size (as opposed to tensile strength which withstands loads tending to elongate). In other words, compre ...

. The composition and structure of the ground surface beneath the snowpack influences the stability of the snowpack, either being a source of strength or weakness. Avalanches are unlikely to form in very thick forests, but boulders and sparsely distributed vegetation can create weak areas deep within the snowpack 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, such as grass or rock slabs. Generally speaking, avalanches follow drainages down-slope, frequently sharing drainage features with summertime watersheds. At and below

tree line The tree line is the edge of the habitat at which trees are capable of growing. It is found at high elevations and high latitudes. Beyond the tree line, trees cannot tolerate the environmental conditions (usually cold temperatures, extreme snowp ...

, avalanche paths through drainages are well defined by vegetation boundaries called

trim line A trim line, also written as trimline, is a clear line on the side of a valley formed by a glacier. The line marks the most recent highest extent of the glacier. The line may be visible due to changes in color to the rock or to changes in vege ...

s, which occur where avalanches have removed trees and prevented regrowth of large vegetation. Engineered drainages, such as the

avalanche dam on Mount Stephen in Kicking Horse Pass An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and earth ...

, 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 avalanches. Human-triggered avalanches have the greatest incidence when the snow's

is between 35 and 45 degrees; the critical angle, the angle at which human-triggered avalanches are most frequent, is 38 degrees. When the incidence of human triggered avalanches is normalized by the rates of recreational use, however, 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.''

Snowpack structure and characteristics

The snowpack is composed of ground-parallel layers that accumulate over the winter. Each layer contains ice grains that are representative of the distinct meteorological conditions during which the snow formed and was deposited. Once deposited, a snow layer continues to evolve under the influence of the meteorological conditions that prevail after deposition. For an avalanche to occur, it is necessary that a snowpack have a weak layer (or instability) below a slab of cohesive snow. In practice the formal mechanical and structural factors related to snowpack instability 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 index measurements of the mechanical properties of the snow (e.g.

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...

coefficients, shear strength, and ductile strength). This results in two principal sources of uncertainty in determining snowpack stability based on snow structure: First, both the factors influencing snow stability and the specific characteristics of the snowpack vary widely within small areas and time scales, resulting in significant difficulty extrapolating point observations of snow layers across different scales of space and time. Second, the relationship between readily observable snowpack characteristics and the snowpack's critical mechanical properties has not been completely developed. While the deterministic relationship between snowpack characteristics and snowpack 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. If ambient air temperatures are cold enough, shallow snow above or around boulders, plants, and other discontinuities in the slope, weakens from rapid crystal growth that occurs in the presence of a critical temperature gradient. Large, angular snow crystals are indicators of weak snow, because such crystals have fewer bonds per unit volume than small, rounded crystals that pack tightly together. Consolidated snow is less likely to slough than loose powdery layers or wet isothermal snow; however, consolidated snow is a necessary condition for the occurrence of

slab avalanche An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and earth ...

s, and persistent instabilities within the snowpack can hide below well-consolidated surface layers. Uncertainty associated with the empirical understanding of the factors influencing snow stability leads most professional avalanche workers to recommend conservative use of avalanche terrain relative to current snowpack instability.

Weather

Avalanches only occur in a standing diksnowpack. Typically winter seasons at high latitudes, high altitudes, or both have weather that is sufficiently unsettled and cold enough for precipitated snow to accumulate into a seasonal snowpack. Continentality, through its potentiating influence on the meteorological extremes experienced by snowpacks, is an important factor in the evolution of instabilities, and consequential occurrence of avalanches faster stabilization of the snowpack after storm cycles. The evolution of the snowpack is critically sensitive to small variations within the narrow range of meteorological conditions that allow for the accumulation of snow into a snowpack. Among the critical factors controlling snowpack evolution are: heating by the sun,

radiational cooling In the study of heat transfer, radiative cooling is the process by which a body loses heat by thermal radiation. As Planck's law describes, every physical body spontaneously and continuously emits electromagnetic radiation. Radiative cooling ha ...

, vertical

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 degree ...

s in standing snow, snowfall amounts, and snow types. Generally, mild winter weather will promote the settlement and stabilization of the snowpack; conversely, very cold, windy, or hot weather will weaken the snowpack. 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 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 of water, may cause avalanche formation at any time of year. Persistent cold temperatures can either prevent new snow from stabilizing or destabilize the existing snowpack. Cold air temperatures on the snow surface produce a temperature gradient in the snow, because the ground temperature at the base of the snowpack is usually around 0 °C, and the ambient air temperature can be much colder. When a temperature gradient greater than 10 °C change per vertical meter of snow is sustained for more than a day, angular crystals called

depth hoar Depth hoar, also called sugar snow or temperature gradient snow (or TG snow), are large snow-crystals occurring at the base of a snowpack that form when uprising water vapor deposits, or desublimates, onto existing snow crystals. Depth hoar cr ...

or facets begin forming in the snowpack because of rapid moisture transport along the temperature gradient. These angular crystals, which bond poorly to one another and the surrounding snow, often become a persistent weakness in the snowpack. When a slab lying on top of a persistent weakness is loaded by a force greater than the strength of the slab and persistent weak layer, the persistent weak layer can 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 slabs form quickly and, if present, weaker snow below the slab may not have time to adjust to the new load. Even on a clear day, wind can quickly load a slope with snow by blowing snow from one place to another. Top-loading occurs when wind deposits snow from the top of a slope; cross-loading occurs when wind deposits snow parallel to the slope. 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 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 will rapidly destabilize the upper layers of the snowpack if the sunlight is strong enough to melt the snow, thereby reducing its hardness. During clear nights, the snowpack can re-freeze when ambient air temperatures fall below freezing, through the process of long-wave radiative cooling, or both. Radiative heat loss occurs when the night air is significantly cooler than the snowpack, and the heat stored in the snow is re-radiated into the atmosphere.

Dynamics

When a slab avalanche forms, the slab disintegrates into increasingly smaller fragments as the snow travels downhill. If the fragments become small enough the outer layer of the avalanche, called a saltation layer, takes on the characteristics of a

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

. 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 is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...

, following the 1999 Galtür avalanche disaster, confirmed the hypothesis that a saltation layer forms between the surface and the airborne components of an avalanche, which can also separate from the bulk of the avalanche. Driving an 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 The 1924 Winter Olympics, officially known as the I Olympic Winter Games (french: Iers Jeux olympiques d'hiver) and commonly known as Chamonix 1924 ( frp, Chamôni 1924), were a winter multi-sport event which was held in 1924 in Chamonix, France ...

in Chamonix.Snow Avalanches
Christophe Ancey 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: ::

\textrm = \frac   \,  \,  \,\!

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 The European Commission (EC) is the executive of the European Union (EU). It operates as a cabinet government, with 27 members of the Commission (informally known as "Commissioners") headed by a President. It includes an administrative body o ...

which produced the leading-edge MN2L model, now in use with the ''Service Restauration des Terrains en Montagne'' (Mountain Rescue Service) in France, and D2FRAM (Dynamical Two-Flow-Regime Avalanche Model), which was still undergoing validation as of 2007. Other known models are the SAMOS-AT avalanche simulation software and the RAMMS software.

Human involvement

Prevention

Preventative measures are employed in areas where avalanches pose a significant threat to people, such as

ski resort A ski resort is a resort developed for skiing, snowboarding, and other winter sports. In Europe, most ski resorts are towns or villages in or adjacent to a ski area – a mountainous area with pistes (ski trails) and a ski lift system. In North ...

s, mountain towns, roads, and railways. There are several ways to prevent avalanches and lessen their power and develop preventative measures to reduce the likelihood and size of avalanches by disrupting the structure of the snowpack, while passive measures reinforce and stabilize the snowpack ''in situ''. The simplest active measure is repeatedly traveling on a snowpack as snow accumulates; this can be by means of boot-packing, ski-cutting, or machine grooming.

Explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An expl ...

s are used extensively to prevent avalanches, by triggering smaller avalanches that break down instabilities in the snowpack, and removing overburden 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 A snow fence, similar to a sand fence, is a barrier that forces windblown, 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 use snow fe ...

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 is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hou ...

s. Downwind of the fence, snow build-up 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 In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...

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. In turn, socio-environmental changes can influence the occurrence of damaging avalanches: some studies linking changes in land-use/land-cover patterns and the evolution of snow avalanche damage in mid latitude mountains show the importance of the role played by vegetation cover, that is at the root of the increase of damage when the protective forest is deforested (because of demographic growth, intensive grazing and industrial or legal causes), and at the root of the decrease of damage because of the transformation of a traditional land-management system based on

overexploitation Overexploitation, also called overharvesting, refers to harvesting a renewable resource to the point of diminishing returns. Continued overexploitation can lead to the destruction of the resource, as it will be unable to replenish. The term app ...

into a system based on land marginalization and reforestation, something that has happened mainly since the mid-20th century in mountain environments of developed countries

Mitigation

In many areas, regular avalanche tracks can be identified and precautions can be taken to minimize damage, such as the prevention of development in these areas. To mitigate the effect of avalanches the construction of artificial barriers can be very effective in reducing avalanche damage. There are several types: One kind of barrier (

snow net Avalanche control or avalanche defense activities reduce the hazard avalanches pose to human life, activity, and property.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 Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin th ...

or pre-stressed

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...

. 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 control or avalanche defense activities reduce the hazard avalanches pose to human life, activity, and property.earth mound A mound is a wikt:heaped, heaped pile of soil, earth, gravel, sand, rock (geology), rocks, or debris. Most commonly, mounds are earthen formations such as hills and mountains, particularly if they appear artificial. A mound may be any rounded a ...

s are placed in the avalanche's path to slow it down. Finally, along transportation corridors, large shelters, called

snow shed Avalanche control or avalanche defense activities reduce the hazard avalanches pose to human life, activity, and property.Radar station for avalanche monitoring in ">Zermatt. Modern radar technology enables the monitoring of large areas and the localization of avalanches at any weather condition, by day and by night. Complex alarm systems are able to detect avalanches within a short time in order to close (e.g. roads and rails) or evacuate (e.g. construction sites) endangered areas. An example of such a system is installed on the only access road of Zermatt in Switzerland. Two radars monitor the slope of a mountain above the road. The system automatically closes the road by activating several barriers and traffic lights within seconds such that no people are harmed.

Survival, rescue, and recovery

Avalanche accidents are broadly differentiated into 2 categories: accidents in recreational settings, and accidents in residential, industrial, and transportation settings. This distinction is motivated by the observed difference in the causes of avalanche accidents in the two settings. In the recreational setting most accidents are caused by the people involved in the avalanche. In a 1996 study, Jamieson et al. (pages 7–20) found that 83% of all avalanches in the recreational setting were caused by those who were involved in the accident. In contrast, all of the accidents in the residential, industrial, and transportation settings were due to spontaneous natural avalanches. Because of the difference in the causes of avalanche accidents, and the activities pursued in the two settings, avalanche and disaster management professionals have developed two related preparedness, rescue, and recovery strategies for each of the settings.

Notable avalanches

Two avalanches occurred in March 1910 in the Cascade and Selkirk Mountain ranges; On 1 March the

Wellington avalanche Wellington (later known as Tye) was a small unincorporated railroad community in the northwest United States, on the Great Northern Railway in northeastern King County, Washington. Founded in 1893, it was located in the Cascade Range at the we ...

killed 96 in

Washington state Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...

, United States. Three days later 62 railroad workers were killed in the

Rogers Pass avalanche The 1910 Rogers Pass Avalanche killed 58 men clearing a railroad line just outside of Revelstoke in Rogers Pass through the Selkirk Mountains in British Columbia on March 4, 1910. It is Canada's worst avalanche disaster. Rogers Pass The Canadi ...

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

, Canada. During

World War I World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...

, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Sw ...

at the Austrian-Italian front, many of which were caused by

artillery Artillery is a class of heavy military ranged weapons that launch munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during siege ...

fire. Some 10,000 men, from both sides, lost their lives in avalanches in December 1916. In the

northern hemisphere The Northern Hemisphere is the half of Earth that is north of the Equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the solar system as Earth's Nort ...

winter of 1950–1951 approximately 649 avalanches were recorded in a three-month period throughout the

in Austria, France, Switzerland, Italy and Germany. This series of avalanches killed around 265 people and was termed the

Winter of Terror The Winter of Terror was a three-month period during the winter of 1950–1951 during which an unprecedented number of avalanches took place in the Alps along the Austria–Switzerland border. The series of 649 avalanches killed over 265 people ...

. A mountain climbing camp on Lenin Peak, in what is now Kyrgyzstan, was wiped out in 1990 when an earthquake triggered a large avalanche that overran the camp. Forty-three climbers were killed. In 1993, the Bayburt Üzengili avalanche killed 60 individuals in Üzengili in the province of Bayburt,

Turkey Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a list of transcontinental countries, transcontinental country located mainly on the Anatolia, Anatolian Peninsula in Western Asia, with ...

. A large avalanche in

Montroc, France Montroc is a hamlet in eastern France, located in the territory of the commune of Chamonix. Several houses at Poses 150m North East of Montroc were destroyed on the 9th of February 1999 by a slab avalanche An avalanche is a rapid flow o ...

, in 1999, 300,000 cubic metres of snow slid on a 30° slope, achieving a speed in the region of . It killed 12 people in their chalets under 100,000 tons of snow, deep. The mayor of Chamonix was convicted of second-degree murder for not evacuating the area, but received a suspended sentence. The small Austrian village of

Galtür Galtür is a village and ski resort in the upper Paznaun valley in Austrian state of Tyrol located in the Central Eastern Alps 35 km southwest of Landeck near the border of Vorarlberg and Switzerland. History Galtür was settled by th ...

was hit by the

Galtür avalanche Galtür is a village and ski resort in the upper Paznaun valley in Austrian state of Tyrol located in the Central Eastern Alps 35 km southwest of Landeck near the border of Vorarlberg and Switzerland. History Galtür was settled by the ...

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. On 1 December 2000, the

Glory Bowl Avalanche Glory may refer to: Honor and renown * Glory (honor), high renown, praise, and honor obtained by notable achievements * '' Kleos'', the Greek word for "glory", often translated to "renown" (what others hear about you) Arts and entertainment Fict ...

formed on Mt. Glory which is located within the

Teton Mountain Range The Teton Range is a mountain range of the Rocky Mountains in North America. It extends for approximately in a north–south direction through the U.S. state of Wyoming, east of the Idaho state line. It is south of Yellowstone National Park and ...

in Wyoming, United States. Joel Roof was snowboarding recreationally in this backcountry, bowl-shaped run and triggered the avalanche. He was carried nearly 2,000 feet to the base of the mountain and was not successfully rescued.

Classification

European avalanche risk

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

, 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. 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 Hiking is a long, vigorous walk, usually on trails or footpaths in the countryside. Walking for pleasure developed in Europe during the eighteenth century.AMATO, JOSEPH A. "Mind over Foot: Romantic Walking and Rambling." In ''On Foot: A Histor ...

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:

North American Avalanche Danger Scale

In the United States and Canada, the following avalanche danger scale is used. Descriptors vary depending on country. Danger Scale - English

Avalanche Problems

There are nine different types of avalanche problems: * Storm slab * Wind slab * Wet slab avalanches * Persistent slab * Deep persistent slab * Loose dry avalanches * Loose wet avalanches * Glide avalanches * Cornice fall

Canadian classification for avalanche size

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

United States classification for avalanche size

The size of avalanches are classified using two scales; size relative to destructive force or D-scale and size relative to the avalanche path or R-scale. Both size scales range from 1 to 5 with the D size scale half sizes can be used.

Rutschblock Test

Slab avalanche hazard analysis can be done using the Rutschblock Test. A 2 m wide block of snow is isolated from the rest of the slope and progressively loaded. The result is a rating of slope stability on a seven step scale. (''Rutsch'' means slide in German).

Avalanches and climate change

Avalanche formation and frequency is highly affected by weather patterns and the local climate. Snowpack layers will form differently depending on whether snow is falling in very cold or very warm conditions, and very dry or very humid conditions. Thus, climate change may affect when, where, and how often avalanches occur, and may also change the type of avalanches that are occurring.

Impacts on avalanche type and frequency

Overall, a rising seasonal snow line and a decrease in the number of days with snow cover are predicted. Climate change-caused temperature increases and changes in precipitation patterns will likely differ between the different mountain regions, and the impacts of these changes on avalanches will change at different elevations. In the long term, avalanche frequency at lower elevations is expected to decline corresponding to a decrease in snow cover and depth, and a short-term increase in the number of wet avalanches are predicted. Precipitation is expected to increase, meaning more snow or rain depending on the elevation. Higher elevations predicted to remain above the seasonal snow line will likely see an increase in avalanche activity due to the increases in precipitation during the winter season. Storm precipitation intensity is also expected to increase, which is likely to lead to more days with enough snowfall to cause the snowpack to become unstable. Moderate and high elevations may see an increase in volatile swings from one weather extreme to the other. Predictions also show an increase in the number of rain on snow events, and wet avalanche cycles occurring earlier in the spring during the remainder of this century.

Impacts on burial survival rate

The warm, wet snowpacks that are likely to increase in frequency due to climate change may also make avalanche burials more deadly. Warm snow has a higher moisture content and is therefore denser than colder snow. Denser avalanche debris decreases the ability for a buried person to breath and the amount of time they have before they run out of oxygen. This increases the likelihood of death by asphyxia in the event of a burial. Additionally, the predicted thinner snowpacks may increase the frequency of injuries due to trauma, such as a buried skier striking a rock or tree.

Avalanches on the planet Mars

References

Bibliography

* McClung, David. ''Snow Avalanches as a Non-critical, Punctuated Equilibrium System'': Chapter 24 in Nonlinear Dynamics in Geosciences, A.A. Tsonsis and J.B. Elsner (Eds.), Springer, 2007 * Daffern, Tony: ''Avalanche Safety for Skiers, Climbers and Snowboarders'', Rocky Mountain Books, 1999, * Billman, John: ''Mike Elggren on Surviving an Avalanche''. ''

Skiing Skiing is the use of skis to glide on snow. Variations of purpose include basic transport, a recreational activity, or a competitive winter sport. Many types of competitive skiing events are recognized by the International Olympic Committee (IO ...

'' magazine February 2007: 26. * McClung, David and Shaerer, Peter: ''The Avalanche Handbook'', The Mountaineers: 2006. 978-0-89886-809-8 * Tremper, Bruce: ''Staying Alive in Avalanche Terrain'', The Mountaineers: 2001. * Munter, Werner: ''Drei mal drei (3x3) Lawinen. Risikomanagement im Wintersport'',

Bergverlag Rother Bergverlag Rother is a German publisher with its headquarters in Oberhaching, Upper Bavaria. Since 1950 the company, that formerly went under the name of ''Bergverlag Rudolf Rother'', has published the Alpine Club Guides in cooperation with the Ge ...

, 2002. (partial English translation included in ''PowderGuide: Managing Avalanche Risk'' ) * Michael Falser: ''Historische Lawinenschutzlandschaften: eine Aufgabe für die Kulturlandschafts- und Denkmalpflege'' In: kunsttexte 3/2010, unter
Historische Lawinenschutzlandschaften: eine Aufgabe für die Kulturlandschafts- und Denkmalpflege