European Avalanche Warning Services

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Additional load

Low additional load

High additional load

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Altitudes

Area within certain altitude ranges (accuracy ± 100 m)

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Amount of fresh fallen snow

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

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Area adjacent to the ridgeline, crest or summit

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

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Area distant from ridgelines, wide-open slope

Mountain terrain unconnected to a ridgeline.

Further explanation:
Often refers to transitions from extremely steep to less steep terrain. Steep terrain and stone steps not connected to the main ridge belong in this category. There is no clear boundary line between areas adjacent to or distant from a ridge, it is a transitional zone.

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Artificial avalanche release

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

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Aspect

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

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Avalanche

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

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Avalanche bulletin

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.

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Avalanche deposit

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

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Avalanche length

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

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Avalanche prone location, danger zone

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

For details, see here.

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Avalanche size

Size of avalanche, classified by destructive potential, runout length and dimension.

Size 1: sluff


Size 2:
small avalanche


Size 3:
medium avalanche


Size 4:
large avalanche


Size 5:
very large avalanche

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Base of a rock wall

Lowermost visible area of a rock wall, frequently scree at a highly divergent slope angle, the steepness gradient tending to decrease as it descends.

The terrain below a rock wall is usually extremely steep.

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Bed surface

The surface across which a slab avalanche releases (can be the ground).

Attention:
Not to be confused with the weak layer

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Blowing snow

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

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Bonded snow

Snow is "bonded" if the particles are interlinked (sintered) to such a degree that a carefully isolated block does not collapse upon itself. Bonded snow can be soft or hard.

Forms when wind-transported snow is deposited or through metamorphosis. When adjacent to a weak layer, bonded snow is an important factor in the triggering of a slab avalanche.

 

 

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Bowl

Rounded or elongated concavity; typically tending to accumulate snowdrift

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Breakable crust

Surface crust that breaks when stepped upon.

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Broad ridge

A rounded, elongated shoulder of high terrain.

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Consolidation

See: Increasing firmness (of a snow layer)

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Cornice

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

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Corn snow

Snow accumulated in recent years, mostly atop glaciers, intensely metamorphosed and denser because of melting and refreezing, as well as from pressures of overlying snow masses; usually described as superficially softened melt-freeze crusts in the advanced season

See: Firn

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Couloir

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

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Critical depth of new fallen 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.

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Crust

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

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Danger

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

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Daytime changes, during the course of the day

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.

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Decomposed snow

Irregular, forked particles resulting from snow crystal rounding and/or mechanical influences as e.g. wind drift Fragments of original snow crystals are frequently still recognizable.

Characteristic grain size: about 1 to 2 mm

See also: www.snowcrystals.it

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Decreasing firmness (of a snow layer)

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

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Dense flow avalanche

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

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Depth hoar, cup-shaped crystals

Large, hollow crystals with edges, rims and ripples 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

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Depth of new, fresh fallen snow

Amount of snow deposited in the previous 24 hours.

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Drifting snow

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

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Endangered traffic route

Transportation route near a slope potentially endangered by avalanches.

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Equilibrium metamorphism (snow crystal rounding)

Transformation process of dry snow with little temperature disparity inside the snowpack.

New fallen, dry snow crystals decompose into small, rounded grains, leading to settlement and general consolidation of the snowpack.

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Exposed

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

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Exposed transportation route

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

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Faceted snow crystals, grains

Snow grains with multiple surfaces and sharp edges from faceting metamorphism, usually poorly bonded to one another (fewer contact points), a critical factor in avalanches if a faceted layer is covered with bonded snow.

Typical grain size: 0.5 to 3 mm

See also: www.snowcrystals.it

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Firmness (of snow)

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

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Firn

Snow accumulated in recent years, mostly atop glaciers, intensely metamorphosed and denser because of melting and refreezing, as well as from pressures of overlying snow masses.

Usually described as superficially softened melt-freeze crusts in the advanced season (see: Corn snow).

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Firn mirror

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.

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Fracture depth

Slab thickness at the fracture line, measured vertically.

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Full depth slab avalanche

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.

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Glide crack

A crack visible on the snowpack surface formed by disparate internal movements on steep, and particularly grass-covered slopes.

Areas below slopes with visible glide cracks should be avoided whenever possible due to threat of gliding sluffs or avalanches.

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Gliding sluff, gliding avalanche

When gliding movements become increasingly rapid, a sluff or gliding avalanche develops. Releases can occur anytime during day or night.

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Gliding snow

Slow, gliding movement of the snowpack over smooth or wet ground, e.g. grassy slopes or smooth rock slabs, attaining velocities of a few millimeters to a few meters per day.

Glide cracks may appear as a result of disparate gliding movements.

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Graupel

Special form of precipitation formed in the atmopshere by supercooled water droplets adhering to snow crystals .

Characteristic grain size: ≤5 mm

See also: www.snowcrystals.it

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Ground avalanche

General term for an avalanche, frequently occurring during spring, that sweeps away the soil in its track, and is thus often mixed with earth and debris.

Often gliding avalanches, but sometimes slab avalanches which release when a weak lower layer fails.

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Gully

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

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High alpine regions

Areas above approximately 3000 m (particularly glaciated areas).

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Ice avalanche

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.

yearlocation 
1895Altels (Switzerland)6 fatalities, 158 cattle killed
1965Mattmark (Switzerland)88 fatalities
1970Huascaran (Peru)with subsequent debris flow: 18‘000 fatalities

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Ice lense

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

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Incoming radiation

External radiation which strikes the snowpack.

Shortwave radiation is largely (up to 90 %) reflected from the snow surface, depending on type of snow. The remainder warms the uppermost layers of the snowpack and possibly moistens them.

Long-wave (infrared radiation) radiation is almost completely absorbed by the snow surface.

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Increasing firmness (of a snow layer)

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

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Inneralpine regions

Areas enclosed by high alpine ridges, subject to intensified precipitation.

Typical inneralpine regions are central Valais, Engadine and central Grisons (CH), located between Northern Alpine Ridge and Main Alpine Ridge; Ortler-Vinschgau region (I), Oetz Valley (A).

In France, the following are considered inneralpine regions: Vanoise, Maurienne, Grandes-Rousses and Oisans-Pelvoux, as well as the mountain region near the French-Italian border.

In Spain, the area of Cerdanya (Perafita-Pulgpedrés) in the Catalonian Pyrenees is included.

 

 

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In particular (with high additional loading)

In general, avalanches can be triggered by high additional loading, in isolated cases by minimum additional loading.

Note:
Term is used for avalanche danger scale and in the daily bulletin.

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Isothermal snow cover

Equal, constant temperature through entire depth of the snow cover

Typically found in spring when the whole snowpack reaches and maintains 0 °C. The snow cover is often moist or wet and loses its firmness.

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Kinetic metamorphism (faceting)

Transformation process of dry snow with great temperature disparity inside the snowpack: the  crystals decompose into faceted, hollowed grains; the crystals grow in size, the hollows recede, the bonding decreases, lowering the firmness of the transformed layers. The greater the temperature disparity, the more intense is the transformation.

Additional note:
The process is accelerated in shady terrain with shallow snow cover and great temperature disparities. This process can affect the whole snowpack. On the snow cover surface it preferably develops during clear sky nights and on shady slopes.

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Lee slope

Slope facing downwind

Snow deposits can attain several times average snow depth.

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Leeward

See: Lee slope

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Likely (something is likely)

Event with a probability of occurrence exceeding 50 %.

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Local, from place to place

Areas limited to slopes and basin areas.

Within one region, different local avalanche situations may prevail.

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Loose snow avalanche; point release avalanche

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

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Melt-freeze crust

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

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Melt-freeze metamorphism

When snow is warmed to 0 °C, a mixture of ice crystals and water is created.

The grain structure is weak, but forms strong crusts when refrozen.

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Multiple starting zones

Area in which a number of discrete avalanches originate.

The term is often associated with avalanche sizes 4 to 5.

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Naturally triggered avalanche, natural release

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

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New fallen snow

Freshly fallen snow
A neither transformed, nor densified nor settled snow layer, from current or recent precipitation.

Characteristic grain size: 1 to 3 mm

The avalanche bulletin ordinarily cites period of snowfall.

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Old snow cover, old snowpack

Snow layers deposited from earlier precipitation, prior to fresh fallen snow.

Old snow layers consist of metamorphosed snow crystals.

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Outgoing longwave radiation

The snowpack surface emits longwave radiation into the atmosphere.

If skies are clear, the  surface cools significantly (up to 20°C) below air temperature.

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Pass area

Lowest area on a ridge

Wind velocity is heightened, snowdrift accumulation enhanced

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Poorly bonded layer

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

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Possible (potential)

Event with a probability of occurrence not exceeding 50 %.

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Powder avalanche, powder cloud avalanche

Avalanche (often a slab avalanche) of fine-grained, dry powder in which most of the flowing snow is suspended in the air by turbulence (powder cloud). Speeds: 100-300 km/h

Associated with strong pressure waves which cause damage in front of the deposition area.

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Prone to triggering

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

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Radiation

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

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Region, regional

Areas comprising several valleys.

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

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Remote triggering

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

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.

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Rib

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

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Ridge

Narrow mountain crest.

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Ridgeline

Long mountain ridge silhouette

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

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Ripples

Elongated erosional ridges on the snow surface, pointing in the wind direction; not to be mistaken with snow dunes

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Risk

Likelihood of occurrence combines mathematical probability, risk exposure and possible damages.

Additional note:
In the avalanche bulletin, avalanche danger, not avalanche risk, is determined.

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Round snow grains

Small, spherical grains resulting from snow crystal rounding.

Characteristic grain size: 0.2 to 0.5 mm

See also: www.snowcrystals.it

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Safety spacing

Precautionary backcountry measure: maintaining a distance between persons to minimize exposure to avalanche hazards, i.e. reduce risk in dangerous terrain.

Contrary to spacing out, safety spacing permits only one person at a time being exposed to risky terrain. Commonly used during descents when a steep slope is skied singly.

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Secured areas

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

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Settlement

Slow decrease of the snow depth due to snow crystal rounding and the influence of weight of the overlaying snow layers; increases firmness and density of snow.

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Shady slope

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.

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Shooting crack

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

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Sintering

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.

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Size of the starting zone (for slab avalanches)

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

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Slab avalanche

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

The weakly bonded layers inside the snowpack provide a bed surface for it to slide on. The fracture is sharply edged and breaks at a right angle to the gliding mass.

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Slab thickness

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

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Slab width

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

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Slope discontinuity

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

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Slope gradient

The slope gradient is measured in the fall line at the steepest part of a slope at a scale of 1:25000 or estimated on-site.

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Slopes become bare

Snowpack melts until the ground becomes visible.

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Small-scale

Slope areas or margins ranging in size from a few meters to approximately 20 m.

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Snow base, fundament

Lowermost layers of the snowpack close to the ground.

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Snow cover, snowpack

Snow deposited on the ground in a multiplicity of layers.

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Snow density

The mass per unit volume of a given quantity of snow. Snow can have highly varied densities:

Snow type
Density [kg/m³]
very light new snowapprox. 30
new snowapprox. 100
decomposing and fragmented precipitation particles150 - 300
rounded snow250 - 450
faceted snow250 - 400
depth hoar150 - 350
wet snow300 - 600
firn600 - 830
glacial iceapprox. 900
pure ice917

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Snow depth

Thickness of the snowpack measured vertically.

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Snow depth increase

Increase in snow depth within a specific time frame

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Snowdrift accumulation, snowdrift deposit

The result of snow transport. Drifting and blowing snow usually forms a dense layer deposited on lee slopes, often with brittle, fragile bonding. Areas prone to drifting are gullies, bowls, slope discontinuities and areas adjacent to ridgelines.

Further explanations:
Snow masses transported by wind. Three main processes take place: rolling, saltation and suspension. During transport, snow crystal size decreases considerably, depending of wind speed and duration, up to 10 to 20 % of its original size. The small fractured particles are closely packed by wind, bringing about 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)

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Snow dunes

Snow deposits formed by wind-transported snow.

The flat side is the windward slope, the steep side is the leeward slope. Not to be mistaken for ripples.

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Snowfall level

Altitude above sea level at which precipitation falls as snow which is deposited on the ground.

Snowfall level is usually about 300 m lower than zero-degree altitude. During intense precipitation or in enclosed valleys, snowfall level can be as much as 600 m below the zero-degree altitude.

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Snow layering

Stratification of the snowpack

Each layer is characterized by grain shape, grain size, layer hardness, temperature, water content and density.

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Snow line

Lower topographical limit of permanent snow cover, designated by altitude.

Depending on slope aspect, the snow line can vary greatly.

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Snowmelt

Rounded, generally large-grain crystals formed by melt-freeze metamorphism, frequently in large clusters.

Characteristic grain size: 0.5 to 3 mm
 
See also: www.snowcrystals.it

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Snow metamorphism

The process which changes shape and size of snow grains in the snowpack.

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Snowpack capable of bearing loads

Snow surface layer strong enough to support a person walking on it.

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Snow plume

Snow being lifted by the wind and carried away from a peak or ridge into the air.

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Snow thickness

Thickness of the snowpack measured at right angle to slope.

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Snow water equivalent

The height of the water column if a snow sample is melted (measured in millimeters), 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.

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Spacing distances, maintaining distances

Precautionary measure in outlying terrain: maintaining distances between persons to reduce snowpack loading.

During ascents at least 10 m distance maintained, during descents significantly more.

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Stability, snowpack stability

The strength of a snowpack to withstand internal and external disturbances.

Stability is determined by firmness vs. stress inside a snow layer.

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Stabilized, consolidated

See: Increasing firmness (of a snow layer)

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Starting zone

Sector of terrain where avalanche releases.

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Steep terrain

Terrain with a slope angle exceeding 30°, regardless of form or type of terrain.

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Stress (in the snowpack)

Stresses in the bonding of grains inside a layer of the snow cover, caused by weight and downward creep of the layers of snow atop it.

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Sunny slope

Terrain heavily impacted by direct solar radiation.

Typical sun-exposed slopes are easterly, southeasterly, southerly, southwesterly and westerly slopes, depending on the angle of the sun.

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Surface crust

Encrusted layer on the snowpack surface.

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Surface hoar (hoar frost)

Transparent, frequently flat crystals forming on the snowpack when moist air settles on the surface.

It forms most often during cold, clear, humid nights. Once it is blanketed by fresh fallen snow, surface hoar is one of the most critical weak layers, providing a potential bed surface for slab avalanches.

See also: www.snowcrystals.it

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Surface-layer slab avalanche

Slab avalanche which glides inside the snow cover.

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Temperature gradient

Change in temperature per unit distance of depth, expressed in °C/m or °C/cm.

The temperature gradient is recorded in the snowpack vertically from the ground to the surface. It is determined as the difference between adjacent measurements. For example a “small” temperature gradient is 1 °C per meter, a “large” temperature gradient is 25 °C per meter.

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Thoroughly moist snow

A thoroughly moist snow layer has a temperature of 0 °C; water is not visible and cannot be pressed out; it is easy to form into a snowball.

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Thoroughly wet snow

A thoroughly wet snow layer has a temperature of 0 °C. Water is visible and can be pressed out.

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Transported snow

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.

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Tree line

The border of a forest and climate zone, e.g. in the Pyrenees of Catalonia at 2400m, in SW Poland at 1600m.

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Unbonded snow

Snow lacking cohesion. The term "loose snow" is used for slack new fallen snow, depth hoar or very developed faceted crystals; however, the definition also applies to very wet snow. Loose snow can lead to loose snow avalanches.

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Unstable

Loss in firmness

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Valley avalanche

Large or very large avalanche that runs all the way to the valley floor

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Valley flank

The lateral sides of a valley from valley floor to ridge.

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Very light new snow

New fallen snow with very low density: typically 30 kg/m³ (champagne powder, diamond snow).

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Very steep, extreme terrain

Quite unfavourable slope with regard to angle (steeper than 40 degrees), terrain profile, proximity to ridge, smoothness of underlying ground surface.

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Wafted slope area

Slope area where great masses of snow have been transported by wind.

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Weak layer

Snowpack layer in which the structure of crystals has undergone a break.

Typical weak layers causing avalanches: surface hoar; faceted crystals; hollow crystals; crusts.

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Wet snow avalanche

Avalanche of wet snow masses.

Flowing more slowly than dry avalanches; usually for shorter runout distances, however with greater impact on obstacles due to higher snow density.

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Whumpfing, collapsing sound

Distinctive noise (resembling a "whumpf" sound)  occurring when a weak layer beneath a slab collapses.

The sound usually indicates an unstable situation and can be accompanied by cracking. A whumpf is a clear avalanche alert.

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Wind crust

Layer of hard-compacted snow brought about by wind; often increases firmness of the surface.

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Wind speed

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Windward

See: Windward slope

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Windward slope

Slope facing the wind

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Zero-degree level

Altitude above sea level at which air temperature is 0 °C.

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