Glossary

Area of slope where snow, transported by wind , accumulates and builds.

Amount of daily snowfall deposited over a limited period, e.g. three days.

Terrain adjacent to ridgeline, crest or summit; highly influenced by wind.

An avalanche triggered by artifically applied force (e.g. explosives, snow machines, people).

Direction in which a slope faces as indicated by compass direction of the fall line; i.e. a north slope faces in a northerly direction.

Rapidly moving snow masses in volumes exceeding 100 m³ and minimum length of 50 meters.

The avalanche bulletin provides detailed information on the snowpack and avalanche situation. Avalanche danger is ranked in accordance with the 5-level European avalanche hazard scale.

Snow deposited by an avalanche. Such snow deposits frequently persist for longer periods on valley floors.

Total length of an avalanche measured from the highest point of the fracture line to the lowest point of the deposition.

See: Typical avalanche problems

New snow

Wind drifted snow

Persistent weak layer

Gliding snow

Wet snow

Locations delineated by aspect or altitude where avalanches can trigger and people or objects are at risk.

Avalanches are categorized among different criteria. The most important distinctions are made regarding

• the manner of starting (loose snow avalanche/ slab avalanche)
• the form of movement (flow avalanche / powder avalanche) and
• the liquid water content in the snow (dry snow/wet snow avalanche).

Additionally there are glide snow avalanches, where the whole snowpack slides on the ground.

Slab avalanche

Loose snow avalanche

flow avalanche

powder avalanche

wet snow avalanche

glide snow avalanches

Snow being transported by wind high above (approximately 2m) snowpack surface (visibility noticeably reduced).

Rounded or elongated concavity; typically tending to accumulate snowdrift.

Surface crust due to wind, sun or air temperature that breaks when stepped upon.

A rounded, elongated shoulder of high terrain.

See: Increasing firmness (of a snow layer)

An overhanging mass of snow created by wind, usually near a sharp terrain break such as a ridge.

A steep gully usually adjacent to ridgelines, flanked by rocks, often containing rubble/scree; tends to accumulate drifted snow.

New fallen snow is a burden on the existing snow cover, can thus increase avalanche danger.
In unfavourable conditions, e.g. poor layering, low temperatures, strong winds, even a few cm can be critical. In favourable conditions, e.g. stable old snowpack, light winds, even 50 cm of snow presents no problem.

Layer of hard-compacted snow resulting from a melt-freeze process or wind.

Conditions, circumstances or processes which can result in damage and/or injury.

Typical avalanche problems and danger patterns have one thing in common: they point to typical, repetitive and usually obvious danger situations.
The difference lies in the level of observation. While avalanche problems give an initial rough overview of possible sources of danger (e.g. new snow), danger patterns immerse deeper into the processes within the snowpack and focus on the causes of the problem (e.g. problem due to excessive new snow load on a weak layer). Danger patterns thus describe different scenarios or processes that lead to the respective avalanche problem.
The goal is clear: dangerous situations should be recognized quickly with the help of the avalanche problems and danger patterns. In that way, recreationsist may adapt their behavior accordingly and thereby may avoid avalanche accidents.
The 10 decisive danger patterns (dp) are presented on this page.

• dp.1 deep persistent waek layer
• dp.2 gliding snow
• dp.3 rain on snow
• dp.4 cold on warm/warm on cold
• dp.5 snowfall after long period of cold
• dp.6 loose snow and wind
• dp.7 shallow snow next to deep snow
• dp.8 buried surface hoar
• dp.9 buried graupel
• dp.10 springtime situation

The avalanche danger is evaluated in the avalanche bulletin of the individual avalanche warning services and describes the avalanche danger with the five-part European avalanche danger level scale.

• 1 – low
• 2 – moderate
• 3 – considerable
• 4 – high
• 5 – very high

The five danger levels are described by three different parameters:

• Probability of avalanche release
• Distribution of hazardous sites
• Size and frequency of expected avalanches

The danger level always applies to a region with an area of >100 km² and not to a specific individual slope. The avalanche danger described on avalanche reports is always a forecast with uncertainties. It should always be checked on site.

More information about the European avalanche danger level scale can be found here.

Evolving avalanche danger over the course of a day. Avalanche danger can vary greatly during the day. Springtime situations are typical: after a clear night, avalanche danger is low early in the morning, then increases over the course of the day due to daytime warming and solar radiation. Also common while heavy snowfall, prolonged wind activity and rain.

Bonding between ice crystals deteriorates or is lost, diminishing overall capacity of crystals to absorb loading.

Avalanche motion which primarily flows, slides, slips, in contrast to powder cloud avalanches.

Large, hollow crystals with edges, striations and facets on the surface, the result of faceting amidst high internal temperature disparities.

Characteristic grain size: 2 to 5 mm or larger

Depth hoar is an accumulation of cup-shaped crystals. Weak layers are rather often made of depth hoar.

See also: www.snowcrystals.it

Amount of snow deposited in the previous 24 hours.

At the onset of winter, slab avalanches are often observed following the second heavy snowfall, usually at high altitudes (>2000 m) and high alpine altitudes (>3000 m) on shady, very steep slopes. This is because in autumn after the first big snowfalls, a lengthy period of stable, highpressure weather or at least precipitation-free weather frequently follows. That is when ideal conditions prevail for the partial metamorphism of snow into loose crystals, which then form a deep persistent weak layer  for subsequent snowfall to be deposited on. During the course of early winter, this can occur in a wide range of altitudes and aspects. In general, deep persistent weak layer formed in early winter are triggered by skiers and freeriders at the beginning of the season, but it is also possible later on in winters with little snowfall. This occurs especially in spring when deep persistent weak layer are weakened further by water.

For further informations watch https://youtu.be/rdYIWWzOWWY

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Snow glides most easily towards the valley across steep, smooth surfaces. While this process is taking place, glide cracks – easily visible, sometimes several-meters deep cracks in the snowpack surface – are formed. Such fissures are now considered to be not the favorable signs which ancient folk knowledge once posited (such traditions are hard to overcome) but instead, utterly unfavorable indicators of potential, sometimes imminent avalanches. A glide crack points to the possibility of a glide-snow avalanche without giving any indication if or when such a glide-snow avalanche will actually release. Regarding their timing, glide-snow avalanches number among the most difficult avalanches of all to predict, since they can trigger literally at any time of day or night, even when the snow situation is at its most stable. That includes the coldest day and the warmest day of the winter. Furthermore, glide-snow avalanches are not triggered by additional loading.

For further informations watch https://youtu.be/Pnb-sSwtccc

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Rain-on-snow is considered the classic alarm signal in snow and avalanche science because, on the one hand, its additional weight burdens the entire snowpack; and on the other, because the firmness of the snowpack is immediately lost. For those reasons, avalanches are predestined. Rain can occur at any time during the winter. The big advantage is that no danger sign could be simpler to recognize.

For further informations watch https://youtu.be/Hegy61JcLsQ

Click to go back to danger patterns.

A theory purporting that huge temperature disparities during snowfall (regardless whether cold on warm or warm on cold) exercise a positive effect on the avalanche situation held sway for much too long in avalanche science. In fact, this is the case only under certain very specific prior conditions. For the most part, such temperature differences have a negative effect, since they reinforce snow metamorphism inside the snowpack. As a rule, a thin, weak layer forms which is highly prone to triggering. This layer is often found on south-facing slopes. It is highly treacherous for the additional reason that the weak layer does not yet exist immediately after the new snow is deposited, it forms only over the course of the following days.

For further informations watch https://youtu.be/OQWqjRIqf3c

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Among avalanche events, this is a classic: following a lengthy period of cold, it begins to snow. In addition, strong winds prevail which transport the new snow accordingly. Within the shortest imaginable time, a treacherous situation arises. This is also the case if, following a lengthy period of cold, “only” strong winds are blowing without snowfall. The problem is that fresh snowdrifts are deposited on wind-protected leeward slopes on top of a loose old snow cover surface which usually contains depth hoar. Wind-drifted snow and persistent weak layers are very poorly bonded with each other. The snowpack is just waiting to be triggered by additional loading.

For further informations watch https://youtu.be/Eo7BJEVRMqc

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“Wind is the architect of avalanches.” This classic proverb from the 1930s by Wilhelm Paulcke remains unchanged even today. Wind influences both snow which is falling and snow which has already been deposited. It is a major factor in the formation of avalanches. When the snow is loose and dry, wind transports it, thereby increasing avalanche danger. The colder the transported snow, the more sensitively it reacts to additional loading, because it is so brittle. It is characteristic of this danger pattern that the weak layer usually consists of new snow and is buried by snowdrifts. Thus, there must have been snowfall at low temperatures without wind impact either just before or in a previous period when the drifting began; or else the snowfall set in without wind influence, but winds arose during the snowfall. This is a pattern which ordinarily can be recognized with ease and lasts only for a short time. The exception is a situation in which the old snowpack surface consisting of loose, faceted crystals is itself transported by wind. When that happens, hard, brittle slabs form which remain prone to triggering for a lengthy period.

For further informations watch https://youtu.be/CXnfPRZIIao

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Areas where the snow is shallow usually have less favorable snowpack layering than where the snow is deep. This is because of amplified transformation processes which go on inside a shallow snowpack. Furthermore, slab avalanches are easier to trigger where the snow is shallow because the weak layers are not as deeply embedded in the snowpack, thus are more liable to trigger through additional loading. For that reason, avalanche triggerings are often observed in transition zones from shallow snow to deep snow. This occurs frequently near terrain edges, and repeatedly on broad ridges.

For further informations watch: https://youtu.be/FIZpZvthr84

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Surface hoar numbers among the most beautiful types of snow grain  in existence. In and of itself, it bears no danger potential. That potential unfolds only when it is buried by fresh, bonded layers of snow. Then it becomes hazardous. In fact, it is considered one of the most critical weak layers in all of snow and avalanche science.

For further informations watch https://youtu.be/AYEEA14IVOk

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A weak layer inside a snowpack is often compared with ball bearings. This image is actually suitable only for graupel: a spherical, granular form of precipitation which is often deposited during showers such as thunderstorms during springtime. It is easy to grasp that snowdrifts accumulating atop of a layer of graupel can only be poorly bonded with it, so that the danger of avalanches increases. Graupel is frequently spread over very small surface areas. Without examining the snowpack itself it is difficult to spot, even for professionals. This is an extremely treacherous phenomenon.

For further informations watch: https://youtu.be/X3vNbaO4lCA

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Springtime presents a particularly delicate challenge to backcountry skiers and boarders, as well as to members of avalanche commissions. Rarely do states of “safe” and “unsafe” lie quite so near in time to each other. Thus, the span of danger levels, which occur within the course of one single day is wide ranging. On the one hand, avalanche danger is never easier to evaluate than in stable corn-snow conditions; on the other, avalanches of such magnitude as in springtime are rarely registered in deep winter. Apart from the snowpack layering, the complex interaction of air temperature, air moisture, solar radiation and wind play a major role. For backcountry skiers and boarders, high discipline in timing and flexibility of route planning are more essential than ever.

For further informations watch: https://youtu.be/MqELgeDEFxY

Click to go back to danger patterns.

Snow being lifted from the snow surface and transported by wind just above the snow surface (visibility not noticeably reduced).

Transportation route near a slope potentially endangered by avalanches.

Exposure to wind, sun, avalanches or other general danger.

Sector of road, railway, or similar infrastructure facility at risk from avalanche danger; often the runout zone of an avalanche path.

Load-bearing capacity (resistance to disintegration)
Depends on extent and quality of snow crystal bonding.

A very thin layer of ice on the snowpack surface formed through interaction of solar radiation, melting, wind impact and outgoing radiation.

Sunny slopes frequently have a glazed surface in springtime due to the high reflectivity of the firn mirror on the snowpack surface.

Slab thickness at the fracture line, measured vertically.

Firmly attached precipitation accumulating on wind-affected side of e.g. trees, electric lines and summit crosses during high humidity and wind. Also known as Rime Ice.

An avalanche which glides over the ground, across firn snow or atop a glacier in the fracture zone, sweeping the entire season’s snowpack with it.

When gliding movements become increasingly rapid, a gliding avalanche develops. Releases can occur anytime during day or night. Gliding avalanches are not induced by a crack in a weak layer.

Back to Avalanche Problems.

Water coming out of the soil, for example lifted through a hydraulic pressure gradient between the soil surface and the overlaying snowpack. The water either gets advected through channels in the soil or is stored as ice in the soil which gets melted. Also springs are called ground water outflows. Both is leading to liquid water at the snow-soil-interface, destabilizing the snowpack.

Usually a steep, elongated, eroded trench; typically prone to snowdrift accumulation.

For the assessment of the hardness of the individual layers, one considers the hand hardness.

in general:  the softer the layer, the lower the strength and the greater the difference in hardness between two adjacent layers, the more likely the layer boundary is a potential fracture surface because the difference in hardness can lead to stress concentration and because differences in hardness favor fracture propagation.
A hardness difference of two or more levels of hand hardness is usually considered critical.

Areas above approximately 3000 m (particularly glaciated areas).

Glacier ice which breaks and plunges over a steep step, sometimes sweeping snow in the avalanche track with it. Often responsible for large-scale disasters.

Thin ice layer inside the snowpack resulting from rain or melt-water refreezing
No single grains are visible.

Bonding between ice crystals improves, increasing overall capacity of crystals to absorb loading.

Glossary

See: Lee slope

Event with a probability of occurrence exceeding 50 %.

A type of avalanche (dry or wet snow with little or no bonding) fanning out downhill and leaving a widening conical scar.

Layer of hard-compacted snow resulting from a melt-freeze process; increases firmness.

Avalanche not caused by external forces, e.g. snowfall or loss in firmness due to weather conditions.

Back to Avalanche Problems.

Back to Avalanche Problems.

A layer with poor bonding of the ice grains (corn snow).

Event with a probability of occurrence not exceeding 50 %.

A snowpack or snow layer that tends to release from additional loading.

Energy transport by electromagnetic waves at varying wave lengths: short wave radiation (visible light), long wave radiation (thermal radiation).

Areas comprising several valleys.

In avalanche bulletins, regions are generally subdivided climatically or geographically.

Remote release of a slab avalanche triggered by additional loading, e.g. skiers or freeriders and other recreationists.

In general the trigger is outside of the avalanche path. However it may occur that persons can be caught and buried, if the avalanche reaches the remote triggering location.

Elongated sub-ridge or ravine in a slope or mountain face.

Narrow mountain crest.

Long mountain ridge silhouette

Further explanation:
A ridgeline is a clearly delineated crest frequently connecting numerous peaks of a mountain range.

Elongated erosional ridges on the snow surface, pointing in the wind direction; not to be mistaken with snow dunes. Dunes are made of snow deposit.

Faceted crystals with rounding facets and corners. Trend to a transitional form reducing its specific surface area; corners and edges of the crystals are rounding off in response to a decreasing temperature gradient.

Areas protected from avalanches and other alpine hazards through technical or temporary measures.

Slow decrease of the snow depth due to every kind of metamorphism (also faceting!) and the influence of weight of the overlaying snow layers; increases firmness and density of snow.

Slopes in shadow, untouched or little struck by sunlight, typically north-facing.

Additional note:
More prevalent in December and January, due to lower solar angles, than in spring. Mountains can cast shadows on surrounding slopes in any aspect; thus, not only north-facing slopes are shady.

Sudden cracking of a brittle snowpack; a clear sign of stress and instability.

Process through which crystals of snow bond together, leading to increased firmness.

At higher snow temperatures, sintering is faster. Sintering is prevalent in compacted snow, e.g. snow ball, avalanche snow, old ski tracks.

Distance between fracture line and lower boundary (stauchwall) of the slab.

The abrupt release of a snow board (slab) on a mountain slope.

After crack initiation and crack propagation in a weak layer the snowcover is divided in three parts: The weak layer, the gliding horizon and the slab. If slope angle is steep enough, the slab will glide down. If the slope is not steep enough, the slab will settle down on the broken weak layer. Possibly a whumpf will follow due to air pressed out of the weak layer.

The fracture is sharply edged.

Slab thickness at the fracture line, measured at a right angle to the slope.

Maximum distance between the two lateral boundaries of a slab avalanche.

A slope area where the slope gradient suddenly becomes significantly steeper; highly prone to accumulate drifted snow masses