low additional load
large additional load
area within certain altitude ranges (accuracy ± 100 m)
Terrain close to the ridge line, the ridge or the peak. These areas are strongly influenced by wind.
Further explanation: Often this relates to the transition from extremely steep terrain into less steep terrain. Steep terrain and sub-ridges, that are not directly connected to the main ridge, belong to this category. There is no clear boundary between areas adjacent and non-adjacent to a ridge, but it is a transition zone.
Avalanches triggered by external force (e.g. explosives, snow machines or machinery, people, wildlife).
Direction into which a slope faces as indicated by compass direction of the fall line. E.g. a north slope faces to the north direction.
A snow mass with usually a volume greater than 100 m³ and a minimum length of 50 meters, that slides rapidly downhill.
The avalanche advisory gives detailed information on the snow cover and avalanche situation. The avalanche danger is rated according to the 5-level European avalanche hazard scale.
Snow deposited by the avalanche. In the valleys, the deposited avalanche snow often remains visible for longer periods.
Locations where people or objects can be affected by avalanches.
Attention: In the avalanche bulletin it is often used in the following context:
Location, where an avalanche can be triggered by a snow recreationist.
Note: The meaning of the term becomes obvious in the context of the actual advisory.
Extent of the avalanche, classified by destructive potential and runout length.
Area below head wall. Typical features: interface of solid rock to scree slopes, with a discontinuity in slope angle. Below a rock wall the terrain is usually extremely steep or steep.
The surface on which a slab avalanche runs (can be the ground). Not to be confused with the failure layer.
The process of snow being transported by wind suspended high above (2 m) the snow surface (the visibility is sensible reduced).
Areas that are protected from avalanches or other alpine hazards by technical measures (structures, explosive use, closure).
An overhanging mass of snow created by the wind, usually near a sharp terrain break such as a ridge.
Wet spring snow consisting of large grains resulting from repeated melt-freeze cycles.
Steep, narrow gully bound by rocks on either side. Often containing rubble/scree. Terrain that is prone to accumulate wind drifted snow.
The new snow is a load for the old snow cover and hence increases the avalanche danger. The rule of thumb for the critical new snow depth is:
Favourable: low to moderate wind speeds, air temperature close to 0°C, strongly irregular old snow surface, frequently skied slopes.
Unfavourable: high rate of precipitation, strong winds (> 50 km/h, roaring wind), low temperature (below –5 to –10°C ), smooth old snow surface (surface hoar, melt-freeze crust or ice, , very old snow surface,), rarely skied slopes.
Decomposing and fragmented precipitation particles. Partly rounded particles, characteristic shapes of precipitation particles still recognizable. Result of equilibrium growth metamorphism. Characteristic grain size: about 1 to 2 mm.
The number and size of the bonds between the individual ice crystals decrease, such that less load can be taken.
The motion is primarily flowing, sliding, slipping, in contrast to powder cloud avalanche.
Large, hollow crystals with steps and striations on their surfaces The crystal type forms at high growth rates (kinetic growth). Depth hoar is often a weak layer. Characteristic grain size: 2 to 5 mm or larger.
Development of the avalanche danger in the course of the day. During a day the avalanche danger can vary strongly. The spring situation is typical: early in the morning after a clear night the avalanche danger is low, and in the course of the day it increases due to solar radiation and daytime warming.
The process of snow being picked up from the snow surface and transported by wind close to the snow surface (the visibility is not sensible reduced).
Traffic route close to a slope, which is endangered of avalanches
Metamorphic process in dry snow at low temperature gradient: New snow stellar like crystals decompose into small rounded grains (or into more rounded shapes). The process leads to settlement and in general a consolidation of the snowpack.
Part of a road, street, rail road track, ski or sledding slope exposed to avalanche danger. Transportation route in the runout zone of an avalanche path.
Slopes steeper than about 40°, typically near ridges. Particularly avalanche prone terrain.
Solid faceted crystals. The crystal type is the result of kinetic growth or faceting. Characteristic grain size: 0.5 to 3 mm
Metamorphic process in dry snow at large temperature gradient. Grains grow and form flat faces, edges and corners. Finally, large, hollow, cup-shaped crystals form. Layers of facets have usually low strength.
Further explanation: This process is favoured on shady areas, by shallow snow cover, usually early in winter, and in areas with brush vegetation.
Weak layer within the snow cover, that failed and produced a slab avalanche.
Partially compacted granular snow that is the intermediate stage between snow and glacial ice. It is formed under the pressure of the overlying snow by the processes of compaction, recrystallization and melting. These processes take about one year.
In colloquial German also used for a softened surface crust.
Thin, very delicate ice layer that forms over snow by the interaction of solar radiation, melting and radiation cooling. Often during spring, sunny slopes appear with a glazed surface due to the high reflectivity of the firnspiegel.
An avalanche that involves the entire season‘s snowpack and slides on the ground, or on firn snow or on a glacier.
Tensile fracture across the slope which extends to the ground. It occurs when there are different sliding velocities in the downslope direction. Particularly in the presence of melt or rain water, it may result in eventual fracture and sliding of the associated gliding avalanche.
When the gliding movement changes to a rapid movement, a gliding slide or avalanche developes. Gliding avalanches can occur anytime during the day or night.
Not to mistake for full-depth slab avalanche!
Slow gliding movement of the snow cover on smooth or wet ground (long grass, smooth rock slabs). Measured speed: few millimeters to meters per day. Glide cracks may appear.
Wet snow avalanche (often during spring) that erodes soil in the track, with partly contaminated wet deposit.
Usually a steep, elongated, eroded trench where wind transported snow may accumulate.
|1895||Altels (Switzerland)||6 fatalities, 158 cattle died|
|1965||Mattmark (Switzerland)||88 fatalities|
|1970||Huascaran (Peru)||with subsequent debis flow: 18 000 fatalities|
Solid, clear, thin ice layer within the snowpack from refreezing rain or melt-water. No single ice grains are visible.
Downwelling shortwave radiation and long-wave (infrared radiation) radiation emitted from the atmosphere. Shortwave radiation will be mainly (90%) reflected on the snow surface. The transmitted part is absorbed and warms the uppermost layers of the snowpack. Long-wave (infrared radiation) radiation will almost 100% be absorbed at the snow surface.
The number and size of the bonds between individual ice crystals increase, such that more load can be taken.
The number and size of the bonds between individual ice crystals increase, such that more load can be taken.
Areas enclosed by high alpine ridges and therefore poor in precipitation. Typical inneralpine regions in Switzerland are the central Valais, the Engadine and the central Grisons which are situated between the northern alpine ridge and the main alpine ridge. Such regions in Austria are the Ortler-Vinschgau region and the Oetz Valley.
In general, avalanches can be triggered by a group of people on the same slope, but there is a small chance of triggering by a single person.
Note: Term is used for explanations in the avalanche advisory.
Equal or constant temperature throughout the snow cover. Typically found in spring when the whole snowpack has reached 0 °C. Then, the snow cover is often moist or wet and looses strength.
Slope facing down-wind where additional snow is deposited. Snow deposition can exceed several times the average snow depth. It is a terrain that is prone to accumulate wind drifted snow.
Total length of an avalanche measured from the highest point of the fracture line to the lowest point of the deposition.
Distance between the fracture line and the stauchwall, the lower boundary of the slab.
Event with a probability of occurrence exceeding 66%. Further explanations: Background: Russian Roulette. It is "likely" that the player will lose with 4 bullets in the magazine of a revolver with 6 spaces available.
Areas with an extent from slope scale to basin scale. Within one region, locally different avalanche situations may prevail
A type of avalanche (of dry or wet snow with no or low cohesion) starting from a point fanning out downhill and leaving an inverted V-shaped scar.
The snow has no cohesion. The term ?loose snow? is, for example, used with new snow and or depth hoar, however, by definition it also applies to very wet snow. Loose snow can lead to loose snow avalanches.
When snow is warmed to 0°C rounded, coarse grains develop with melt water in-between. This grain structure is weak, but once frozen again forms strong crusts.
Snow at the melting temperature (0°C). Water is not visible and cannot be pressed out of a snow sample. It is easy to press the snow into a ball.
Watershed with several starting zones (or an avalanche path where a number of discrete starting zones lead into a common track and runout zone). Mostly used for large avalanches.
Release of an avalanche without being triggered by a person, explosives, etc.
Newly fallen snow. Hardly decomposed and hardly densified or settled snow layer, from the current or very recent precipitation period. Characteristic grain size: 1 to 3 mm. The avalanche bulletin refers normally to the total new snow depth of a snowfall period.
Previously deposited snowpack with no recent new snow layers. Old snow layers consist of metamorphosed snow crystals.
The snowpack emits longwave radiation to the atmosphere. With clear sky conditions the snow surface cools significantly (up to 20 °C) below the air temperature.
Lowest area within a ridge where the wind speed is highest and snow drift is enhanced.
Event with a probability of occurrence not exceeding 66%.
A dry-snow slab avalanche has developed in a powder avalanche when most of the flowing snow is suspended by turbulence (powder cloud). The dust or powder cloud of airborne particles of snow moves independently from the dense flowing part. Speeds: 100-300 km/h. Associated with strong pressure waves that can cause damage in front of the deposition area.
Energy transport by electro-magnetic waves at different wave lengths: short wave radiation (visible light), long wave radiation (thermal radiation).
Areas with an extent of several valleys. In avalanche advisories, the regions are generally subdivided climatologically or politically.
Release of a snow slab avalanche triggered from outside the starting zone (e.g. by a snowboarder, avalanche, snow machine). Nevertheless, if the trigger, e.g. snowboarder, is in the avalanche path it can be caught and buried by the avalanche.
Long, pronounced mountain ridge.
Further explanation: A ridgeline is an obvious crest that connects peaks of a mountain range.
Elongated erosional ridges on the snow surface, pointing towards the wind direction. Not to mistake for snow dunes.
The probability or chance of death or losses. Snow avalanche risk can be analysed in terms of avalanche frequency (probability of occurrence) (Is an avalanche likely?), the exposure of people or property (Will somebody or something be hit?) and consequences of the avalanche on exposed elements (destructive potential) (What will be the damage?). Further explanations: In the avalanche bulletin, the avalanche danger, and not the avalanche risk is described.
Small, rounded, oblong grains. Result of equilibrium growth metamorphism. Characteristic grain size: 0.2 to 0.5 mm.
Precaution in backcountry traveling by maintaining a distance between persons to minimize exposure to avalanche hazard and hence to reduce risk in avalanche prone terrain. Contrary to spacing out, when practicing safety spacing only one person is exposed to the hazard at a time. Commonly used during descent, when only one person at a time skies a steep slope.
Slow decrease of the snow depth due to rounding, sintering and densification of snow.
Slopes in the shade, not hit by the sun, typically north-facing slopes. Further explanations: With low solar elevation angle during December and January shady slopes are more widespread than in spring. Mountains can cast a shadow on nearby slopes of any aspect, so that not only north-facing slopes are in the shade.
Sudden cracking of the snowpack is a clear sign of stress and instability.
Formation of bonds between snow grains resulting in increased strength.. Sintering is faster at higher snow temperatures. Sintering can be particularly well observed in compacted snow (e.g. snow ball, avalanche snow, old ski tracks).
The simultaneous release of a cohesive snow layer (slab) characterized by a distinct fracture line (or crown fracture) at the top of the avalanche.
Snow is bonded if the particles are sintered to a degree that a carefully isolated block does not collapse. A slab is formed by the influence of wind or by rounding. Next to the weak layer itself, a slab layer is necessary for slab avalanche formation.
Maximum distance between the two lateral boundaries (flanks) of a slab avalanche.
*) The slope angle is measured in the fall line, at the steepest part of the slope, measured on a map scale 1:25000 or estimated in the field.
A slope area where the slope angle becomes significantly steeper. Terrain that is prone to accumulate wind drifted snow.
The property of being a small part of a slope or its fringe, having an areal extent of a few meters to 20 m at most.
The mass per unit volume of a given quantity of snow. Depending on its type and state, snow can have quite variable densities.
|snow type||density [kg/m³]|
|very light new snow||approx. 30|
|new snow||approx. 100|
|decomposing and fragmented precipitation particles||150 - 300|
|rounded snow||250 - 450|
|faceted snow||250 - 400|
|depth hoar||150 - 350|
|wet snow||300 - 500|
|firn||500 - 830|
|glacial ice||approx. 900|
Wind-deposited snow. The result of drifting and blowing snow is usually a dense snow layer deposited on lee slopes. Areas prone to deposition are adjacent to ridge lines, gullies, slope depressions or slope discontinuities. Further explanations: Snow that has been transported by wind. Three main processes operate: rolling, saltation and suspension. During the transport, the snow crystal size decreases to 10 to 20 % of its original size. The small fractured particles are closely packed by the wind which leads to a cohesive snow layer (a dense-cohesive slab or a soft-cohesive slab) on the lee slope.
Size of snow drift accumulations (thickness)
Extent of snow drift accumulations (spatial)
Snow deposition formed by wind transported snow. The flat side is the windward slope and the steep side is the leeward slope. Not to mistake for ripples.
Altitude above sea level at which the precipitation type is snow that deposits on the ground. The snowfall level is usually about 300 m lower than the freezing level. During heavy precipitation or in closed valleys the snowfall limit can be about 600 m below the freezing level.
Stratification of the snowpack. Each layer is characterized by grain shape, grain size, layer hardness, temperature and density
The lower topographic limit of the permanent snow cover. Depending on the slope aspect, the snow line can vary greatly in altitude.
The height of the water column if a snow sample is melted (expressed in millimeter), with reference to the same area. The water equivalent of a 20 cm snow sample with a mean snow density of 100 kg/m³ is 20 mm. With a density of 500 kg/m³ the equivalent of a 20 cm snow sample is 100 mm of water
Precautionary measure in backcountry traveling by maintaining a distance between persons to reduce loading of the snowpack. During the ascent maintain at least 10 m distance, during the descent significantly more.
The strength of a snowpack in regard to (internal or external) disturbances. The stability is the balance between strength and stress within a snow layer.
Terrain with a slope angle greater than 30°, regardless of form and type of the terrain.
Load carrying capacity (resistance to disintegration). The more packed a snow layer is, the stronger it is. Strength depends on the number and size of the bonds between the snow grains (crystals).
Due to the weight and the snow creep that is slightly faster in the upper layers of the snowpack than in the lower layers, stresses form in the snowpack that concentrate in the bonds between snow grains.
Snow deposited during a few days, usually during a storm lasting several days (e.g.: 3-day sum of new snow).
Part of the terrain that is strongly influenced by direct solar radiation. Typical sun exposed slopes are easterly, southeasterly, southerly, southwesterly and westerly slopes, depending on the angle of the sun. Further explanations: Sunny slopes are less frequent in early and mid-winter than in spring when the solar elevation angle is higher.
Crystals, often shaped like feathers, spikes or wedges, that grow upward from the snow surface when the air just above the snow surface is cooled to the dew point (deposition of excess water vapour). Surface hoar grows most often during cold, calm, clear, humid nights. Once buried, layers of surface hoar are weak and may persist for weeks as potential failure planes for slab avalanches.
Slab avalanche with a failure plane within the snowpack and not on the ground (as opposed to a gliding avalanche).
The change in temperature per unit distance of depth , expressed in °C/m. The temperature gradient is measured in the snowpack vertically from the ground to the surface. A "small" temperature gradient is e.g. 1 °C per meter, a "large" temperature gradient is 25 °C per meter.
The elevation up to which trees grow and form forests. Below tree line the snowpack is usually more stable than above. It is determined by climatological and forest historical factors. In Switzerland:
Layer within a snowpack with low strength, where fractures can occur and propagate. Typical weak layers are: buried surface hoar, faceted layers, or buried loose new snow.
Snow at the melting temperature (0°C). Water is visible and can be pressed out of a snow sample.
Avalanche consisting of wet snow. Compared to a dry snow avalanche, the flow speed of a wet snow avalanche is usually slower and therefore the runout distance is usually shorter. However, the impact on obstacles is considerable due to the higher density of wet snow.
Clustered, rounded crystals formed by melt-freeze metamorphism. Characteristic grain size: 0.5 to 3 mm.
Distinctive sound ("whumph" or "whumpf") that occurs when a weak layer below a slab collapses.. The sound usually indicates an unstable situation and can be accompanied by cracking. Repeated whumpfs are a clear alert of avalanche hazard.
Re-deposition of snow occurring at a wind speed greater than about 4 m/sec for loose snow, and greater than 10 m/sec for denser snow. Further explanations: The amount of snow deposited by windincreases with the third power of the wind speed, i.e. double the wind speed results in the eightfold amount of drifted snow. A maximum of snow drift is reached at wind speeds between 50 and 80 km/h. At higher wind speeds snow drift is reduced.