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Snow Avalanche: Varieties and Safety

An avalanche (also called a snow slide) is an event that occurs when a cohesive slab of snow lying on a weaker layer of snow breaks and rolls down a steep slope. Avalanches usually occur in the starting zone due to mechanical failure in the snow cover (snow avalanche ) when the forces of the snow exceed its strength, but sometimes only with safe expansion (loose snow avalanche ). Once an avalanche occurs, it usually accelerates quickly and increases in mass and volume as it carries more snow. If an avalanche moves fast enough, some of the snow may mix with the air to form a snow avalanche, which is a type of gravity current.

Slips of rocks or debris that behave like snow are also called avalanches (see Landslide). The rest refers to snow avalanches because they are the ones that look most common in the world.

The load on the snowpack can only be due to gravity, in which case the failure can be caused either by a weakening of the snowpack or by a weakening of the load due to precipitation. Avalanches caused by this process are known as spontaneous avalanches. Avalanches can also be caused by other loads, such as human activity or biologically related activity. Seismic activity (crustal activity associated with tile movement) can also cause snowpack failure and avalanches.

Although large avalanches consist mainly of snow and air, they can carry ice, rocks, trees, and other surface materials. However, they differ from slush flows, which have a higher water content and more laminar flow, mudflows with greater fluidity, rock slides, which are often free of ice, and serac collapses during icefall. Avalanches are not rare or accidental mountain phenomena and are characteristic of any ridge that produces standing snow. Avalanches most often occur in winter or spring, but glacial movement can cause snow and ice avalanches at any time of year. In mountainous terrain, avalanches are among the most serious objective disasters to life and property because of their destructive capacity due to their ability to carry huge masses of snow at high speeds.

There is no generally accepted system for classifying the various forms of avalanches. Avalanches can be described by their size, destructive potential, mechanism of origin, composition and dynamics.

Form

Most avalanches occur spontaneously during storms with increased stress due to snowfall and/or erosion . The second largest cause of natural avalanches is metamorphic changes in the snow cover, such as melting due to solar radiation. Other natural causes including rain, earthquakes, rockfall, and icefall. Man-made avalanche triggers include skiers, snowmobilers, and controlled blasting. Contrary to popular belief, avalanches are not caused by loud sound; sound pressure is several orders of magnitude too low to cause an avalanche.

An avalanche can start at a point where only a small amount of snow is moving at first; this is typical for snow avalanches or avalanches on dry, loose snow. However, if the snow has melted and become a rigid slab overlapping a weak layer, cracks can spread very quickly, so that a large volume of snow, which may amount to thousands of cubic meters, can start moving almost simultaneously.

The snow cover will collapse when the load exceeds the strength. The load is simple; it is the weight of the snow. However, the strength of snow cover is more difficult to determine, and it is heterogeneous. It varies in detail depending on the properties of the snow grains, size, density, morphology, temperature, water content; and the properties of the bonds between the grains. All of these properties can change over time depending on local water vapor flow, temperature, and heat flux. The top of the snow cover is also strongly influenced by incoming radiation and air currents. One goal of avalanche research is to test and validate computer models that can describe the evolution of seasonal snow cover over time. The complicating factor is a complex interaction of terrain and attracting attention to the spatial and temporal variability of depths, formal crystallization, and layering of seasonal snow cover.

Avalanches of slabs

Slab avalanches are formed in snow that has been deposited or re-deposited by the wind. They have the characteristic appearance of a snow block (slab) cut from its surroundings by cracks. The elements of a slab avalanche are as follows: crown cracks at the top of the starting zone, lateral cracks on the sides of the starting zones, and a crack at the bottom, called a stochwall. The top and side cracks will provide vertical walls in the snow, outlining the snow carried away by the avalanche from the remaining snow on the slope. Tiles can vary in thickness from a few centimeters to three meters. Slab avalanches account for 90% of the fatalities associated with recorded trees.

Avalanches

The largest avalanches form turbulent slurry flows, known as snow avalanches or mixed avalanches. They consist of a powder cloud that covers a dense avalanche. They can form from any type of snow or triggering mechanism, but usually arise from fresh dry powder. They can exceed speeds of 300 kilometers per hour (190 miles per hour) and up to a mass of 10,000,000 tons; their flows can travel long distances across their valley bottoms and even rise over short distances.

Avalanches of wet snow

Unlike snow avalanches, wet snow avalanches are a low-velocity suspension of snow and water with flow, a limited path of travel (McClung, first edition 1999, p. 108). The low velocity is due to friction between the sliding surface of the track and the water-saturated stream. Despite their low speed (~10-40 km/h), avalanches in wet snow are capable of generating powerful destructive forces because of their large mass and density. The flow body of a wet snow avalanche can break through soft snow and can also dig through boulders, ground, trees and other vegetation; leaving unprotected and often clogged ground in the path of the avalanche. Wet snow avalanches can be caused by either loose snow dumping or system dumping, and can only be triggered in snowpack that is saturated with water and isothermally equilibrated to the melting point of water. The isothermal characterization of wet snow avalanches has led to the secondary term isothermal landslides, which appears in the literature (e.g., Daffern, 1999, p. 93). In temperate latitudes, wet snow avalanches are associated with climatic avalanche cycles at the end of the winter season, when there is significant diurnal warming.

Avalanche path

As an avalanche moves down a slope, it usually follows a trajectory that depends on how steep the slope is and the volume of snow/ice involved in moving the mass. The beginning of an avalanche is called the starting point and usually occurs on a 30-45 degree slope. The body of the trail is called the Avalanche Trail and usually occurs on a 20-30 degree slope. When the avalanche loses momentum and eventually stops, it reaches the Beat Zone. This usually happens when the slope begins to steepen to less than 20 degrees. These degrees are not always true due to the fact that each avalanche is unique depending on the stability of the snow cover from which it originated, as well as the environment or caused the mass movement.

Death by avalanche

People caught in an avalanche can die from suffocation, injury, or hypothermia. On average, 28 people die in avalanches each winter in the United States. Globally, on average, more than 150 people die each year from avalanches.

Ice Avalanche

An ice avalanche occurs when a large chunk of ice, such as from a serac or calving glacier, falls onto the ice (e.g., the Khumbu Icefall), causing broken pieces of ice to move. The resulting movement is more like a rockfall or landslide than a snow avalanche. They are usually very difficult to predict and almost impossible to reduce.

Terrain, snow cover, weather

Doug Fesler and Jill Fredston developed a conceptual model of the three basic elements of avalanches: topography, weather, and snow cover. Terrain images the location where avalanches are observed, weather that shows meteorological conditions that show snow cover, snow cover, and snow characteristics that make avalanche formation possible.

Landscape

Avalanche formation requires a mechanism gentle enough to accumulate snow, but steep enough for the snow to mogrify, applied through a combination of mechanical damage (snowpack) and gravity movement. The angle of inflation can hold snow, called the angle of natural slope , depends on many factors, such as the shape of the crystals of moisture content. Some forms of drier and colder snow will only hold on gentler slopes, while wet and warm snow can stick to very steep surfaces. In particular, in coastal mountains such as the Cordillera del Paine region of Patagonia , deep snow packs gather on vertical and even overhanging cliffs. The angle of inclination that moving snow can accelerate depends on many factors, such as the shear strength of the snow (which itself depends on the crystals) and the configuration of the layers and interlayer boundaries.

Snow cover on sun-exposed slopes is highly dependent on sunlight . Daily cycles of thawing and refreezing can stabilize the snow cover by promoting subsidence. Strong freeze-thaw cycles result in a surface crust at night and unstable surface snow during the day. Slopes on the leeward side of a ridge or other wind obstacle accumulate more snow and are more likely to include pockets of deep snow and cornices that, if disturbed, can lead to an avalanche. Conversely, the snow cover on a windward slope is much shallower than on a leeward slope.

Avalanches and avalanche paths share common elements: the start area where the avalanche begins, the trail where it flows, and the exit area where the avalanche ends. Debris deposits are the accumulated mass of an avalanche after it has stopped in the area of the avalanche. The image to the left shows that each year in this avalanche path, most of these avalanches do not travel the entire vertical or horizontal length of the path. The frequency with which avalanches form in a given area is known as the recurrence period .

The initial zone must be steep enough to allow snow to accelerate once it starts moving, and convex slopes are less stable than concave slopes because of the mismatch between the tensile strength of the snow layers and their compressive strength . The composition and structure of the ground surface beneath the snow cover affects the stability of the snow cover. Avalanches are unlikely to form in very dense forests, but boulders and rarely create weak widespread areas in the snow cover because of the formation of strong temperature gradients. Full-depth avalanches (avalanches that sweep away virtually no snow cover) are more common on slopes with smooth surfaces such as grass or stone slabs.

Generally speaking, avalanches follow the drainage down the slope, often separating the drainage elements from the summer watersheds. At tree line level and below, avalanche trails through the drainage are clearly vegetation boundaries, called pruning lines , which occur where avalanches blow down trees and prevent the re-growth of large vegetation. Engineering drainage systems such as clipping lines were built to protect people and property by redirecting avalanches. Deep deposits of avalanche debris will accumulate in the watersheds at the final points of exit, such as ravines and river beds.

Slopes below 25 degrees or steeper than 60 degrees typically have fewer avalanches. Human-caused avalanches most often occur when the natural slope angle of the snow is between 35 and 45 degrees; the critical angle at which human-caused avalanches are most frequent is 38 degrees. Increases the scaling of the slope angle when the number of people caused leads to a decrease in scalability for a given direction of impact. Rule of thumb: A slope that is gentle enough to hold snow but steep enough to ski can trigger an avalanche, regardless of the angle.

Structure and characteristics of the snow cover

The snow cover becomes of parallel surfaces that accumulate over the winter. Each layer contains ice that is representative of the different meteorological conditions during which the snow formed and fell. After deposition, the snow layer continues to develop under the meteorological conditions prevailing after deposition.

For an avalanche to occur, the snow cover must have a weak layer (or instability) beneath the layer of cohesive snow. In practice, the formal mechanical and structural factors associated with snow cover instability are not directly observed outside of laboratories, so the more easily observed properties of snow layers (e.g., penetration resistance, grain size, grain type, temperature) are used as index measurements of snow mechanical properties (e.g., tensile strength , friction coefficients , shear strength and toughness ). This leads to two major sources of uncertainty in determining snow stability based on snow structure: first, both the factors affecting snow stability and the specific characteristics of the snow cover vary widely over small areas and time scales, which leads to considerable difficulty in extrapolating point observations of snow. layers at different scales of space and time. Secondly, the relationship between easily observed snow cover characteristics and the critical mechanical properties of the snow cover has not been completely elucidated.

Although the deterministic relationship between snowpack characteristics and snowpack stability is still the subject of ongoing scientific research, there is a growing empirical understanding of snow composition and sediment characteristics that influence the probability of an avalanche. Observations and experience have shown that freshly fallen snow takes time to combine with the layers of snow beneath it, especially if the new snow falls in very cold and arid conditions. If the ambient air temperature is low enough, the fine snow over or around boulders, plants, or other inhomogeneities on the slope is weakened by the rapid crystal growth that occurs when there is 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 are tightly packed into each other. Compound snow is less likely to crumble than loose friable snow or wet isothermal snow; nevertheless, solid snow is a prerequisite for snowpack, and permanent snowpack instability may lurk beneath well-compacted surface layers. The uncertainty associated with an empirical understanding of the factors affecting snow stability leads most professional avalanche workers to recommend conservative use of avalanche terrain over current snow instability.

Weather

Avalanches occur only in standing snow. Usually during winter seasons at high latitudes, at high altitudes, or both, the weather is unstable enough and cold enough for the snowfall to accumulate into a seasonal snow cover. Continentality , due to its potential influence on the extreme meteorological phenomena experienced by snowpack, is an important factor in the development of instability and, consequently, the occurrence of avalanches and the more rapid stabilization of snow cover after storm cycles. Snow cover evolution is critically sensitive to small changes in a narrow range of meteorological conditions that allow for snow accumulation in the snowpack. Among the critical factors controlling snow cover evolution are solar heating, radiative cooling , vertical temperature gradients in standing snow, the amount of snowfall, and snow type. Generally, mild winter weather will settle and stabilize snow cover; conversely, very cold, windy, or hot weather will weaken snow cover.

At temperatures close to the freezing point of water, or during periods of moderate sunshine, a mild freeze-thaw cycle will occur. Melting and re-freezing of water in the snow strengthens the snow cover during the freezing phase and weakens it during the thawing phase. A rapid rise in temperature to a point above the freezing point of water can cause an avalanche at any time of the year.

Constant low temperatures can either prevent new snow from stabilizing or destabilize existing snow cover. Cold air temperatures on the snow surface temperature gradient on the snow because the ground temperature at the base of the snowpack is usually about 0 °C, and the ambient air temperature can be much lower. When the temperature gradient is more than 10° C at the height of the snow for more than a day, angular crystals, called deep frosting , or facets, begin to form in the snow cover because of the rapid transport of moisture along the temperature gradient. . These angular crystals, which do not adhere well to each other and to the surrounding snow, often become a permanent weak link in the snowpack. When a slab overlying a persistent weak layer is loaded with a force greater than the strength of the slab and the persistent layer, the persistent weak layer can collapse and cause an avalanche.

Any light t winds can cause rapid rapid accumulation on sheltered winds. Wind slabs form quickly, and weaker snow underneath may not have time to adapt to the new load. Even on a clear day, the wind can quickly pile snow on a slope, carrying snow from one place to another. Overhead loading occurs when the wind is blowing snow off the top of the slope; Cross-loading, when the wind is depositing snow parallel to the slope. When blowing over the top of a mountain, the leeward or leeward side of the mountain experiences a top down load of that leeward slope. When the wind blows across the ridge leading into the mountain, the leeward side of the ridge is subjected to a cross-slope load. Cross-loaded wind plates are usually identified visually.

Blizzards and downpours are important components of avalanche danger. A heavy snowfall will cause the existing snow cover to become unstable, both because of the extra weight and because the new snow does not have enough time to bond with the underlying layers of snow. Rain has a similar effect.

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