Glossary

High additional load

• two or more skiers/snowboarders etc. without good spacing (closer than 10 m apart)
• snow machine
• explosives
• single hiker/climber on foot

Area within certain altitude ranges (accuracy ± 100 m)

• high alpine regions: over 3000 m above sea level
• high altitude: 2000 to 3000 m above sea level
• intermediate altitude: 1000 to 2000 m above sea level
• low altitude: below 1000 m above sea level

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.

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

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

Attention:
Not to be confused with the weak layer!

See: Increasing firmness (of a snow layer)

Typical avalanche problems and danger patterns commonly indicate typical, repetitive and usually obvious danger situations that highlight an avalanche hazard situation.
However it should be noted that while avalanche problems give an initial indication of possible  danger  factors (e.g. new snow), danger patterns provide deeper explanation into the processes within the snowpack and the causes of the problem (e.g. problem due to excessive new snow load on a weak layer). Therefore, danger patterns help to describe different scenarios or processes that signal the development of a particular avalanche problem.  By recognising avalanche problems and danger patterns recreations can be forewarned about the development of dangerous situations and  change their plans accordingly.

The avalanche danger is evaluated in the avalanche bulletin in each of the individual avalanche warning services and describes the avalanche danger using the five-level European avalanche danger 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 in 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.

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.

The gliding snow problem is characterized by gliding of the entire snowpack on the ground. These full depth avalanches release due to failure in the basal layer or failure at the snow soil interface. The presence of liquid water at the glide horizon is crucial for the release. Depending on the origin of the water, they can be classified into warm (melt water or rain is percolating the snowpack) and cold (the warm ground causes melt at the basal layer or groundwater outflow) events. They are difficult to predict, although glide cracks open usually before a release.

For more information see Gliding Snow Problem.

Back to Avalanche Problems.

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

Often gliding avalanches, but sometimes slab avalanches which release when a weak layer on the ground fails.

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.

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.

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.

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.

Transformation process of dry snow with a strong  temperature gradient in the snowpack. Crystals develop into faceted, hollowed grains, growing in size with  hollows receding.  Bonding between grains decreases with a lessening of density in the affected layers. The greater the temperature gradient, the more intense is the rate of change.

The process is accelerated in shady terrain with a shallow snow cover. This process can affect the whole or isolated areas of the snowpack.  Layers of faceted crystals are often found near to crusts. On the snowpack  surfaces the process develops more readily during clear sky nights.

See: Lee slope

Areas limited to slopes and basin areas.

Within one region, different local avalanche situations may prevail.

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

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.

Area in which a number of discrete avalanches originate. The term is often associated with avalanche sizes 4 to 5.

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

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.

See also: www.snowcrystals.it

The new snow problem is related to current or recent snowfall. The additional loading on the existing snowpack or a lack of cohesion in the newly fallen snow can cause avalanche activity. The problem is widely present, often in all aspects and lasts usually until a few days after the snowfall event.

For more information see New Snow Problem.

Back to Avalanche Problems.

Snow layers deposited from earlier precipitation, prior to fresh fallen snow.
Old snow layers consist of metamorphosed snow crystals.

Lowest area on a ridge
Wind velocity is heightened, snowdrift accumualtion enhanced.

The persistent weak layers problem is related to the presence of persistent weak layers in the old snowpack. These weak layers typically include buried surface hoar, depth hoar or faceted crystals. Weak layers can persist for weeks to months; possibly most of the winter season.

For more information see Persistent Weak Layers.

Back to Avalanche Problems.

A weak layer with poor bonding between individual crystals or grains. eg facets or buried surface hoar.

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.

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

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.

Narrow crest line of a mountain.

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

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

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.

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

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 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.

The abrupt release of a slab of snow on a mountain slope.

After crack initiation and crack propagation in a weak layer the released slab  comprises  3 elements: The weak layer, the gliding horizon and the slab. If the slope angle is steep enough, the slab will glide down. If the slope is not steep enough, the slab will settle down on the failed  weak layer. Possibly a whumpf sound will follow due to air pressed out of the weak layer.

The fracture line will be sharp edged.

• moderately steep: less than 30°
• steep: 30° and more
• very steep: 35° and more
• extremely steep: 40° and more

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

A mudflow-like avalanche composed of slush—very saturated snow. Commonly occurring after rainfall and/or intense thawing have produced more water than can drain through the snow. Slush avalanches can occur on very gentle slope angles. They usually occur in Arctic climates on permafrost soil when dry depth hoar becomes rapidly saturated with water in spring.

Lowermost layers of the snowpack close to the ground.

Snow deposited on the ground in a multiplicity of layers.

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

Thickness of the snowpack measured vertically.

Increase in snow depth within a specific time frame.

Snow deposits formed by wind-transported snow. Ripples are small transversal depositional features. Dunes are typically larger in scale and often Barchanoid.

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

Stratification of the snowpack

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

Lower topographical limit of continuous snow cover, designated by altitude.
Depending on slope aspect, the snow line can vary greatly.

The process which changes the shape and size of snow grains in the snowpack. There are 2 main processes:  isothermal and kinetic metamorphism.

Snow profiles are local and temporal points recordings of the snow cover and
snowpack and play a major role in the assessment of avalanche danger. In such a profile, the snowpack is exposed all the way to the ground for a cross-section examination. In this way, the different layers in the snow become visible and can also be further investigated in terms of parameters such as hardness, snow type/grain shape, water content/moisture and snow temperature.
Due to the transformation of the snow, the individual layers are in a constant state of change and thus allow conclusions to be drawn about meteorological influences at the time of the formation of the respective layer (fresh snow, rain, wind, solar radiation, etc.), but also later developments within the snowpack (inherent pressure, slope, etc.).
The snow profile alone only allows potential weak layers to be identified, but their actual stability cannot be assessed. This can be determined by means of a stability test.

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.

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. The colder snow is while forming a deposit, the more brittle the deposit is.

Size of snow drift accumulations (thickness)

• small snow drift accumulations: 5- 20 cm thick
• medium snow drift accumulations: 20 – 50 cm thick
• large snow drift accumulations: thicker than 50 cm

Extent of snow drift accumulations (spatial)

• some snow drift accumulations:
  very little snow drift accumulation with small spatial extent
• extensive snow drift accumulations:
  major snow drift accumulations mostly with large spatial extent on slopes of all aspects

Rounded crystals formed by melt-freeze metamorphism, frequently in large clusters. Can be moist (= 0°C). When frozen, snowmelt is forming a melt-freeze-crust.

Characteristic grain size: 0.5 to 5 mm

See also: www.snowcrystals.it

See: Increasing firmness (of a snow layer)

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 layer.

See also: www.snowcrystals.it

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.

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.

See: Loss in firmness

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

Snowpack layer in which the crystals are poorly bonded and the layer has the potential to fail.

Typical crystals in dry weak layers causing avalanches are : precipitation particles; surface hoar; faceted crystals; hollow crystals; graupel. Generally >1mm in size, loose grains with few bonds  Wet weak layers can be formed by all snow crystal/grain types.

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.

The wet snow problem is related to the weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to high radiation impact (sunshine) which leads to melt or rain (advecting energy into the snowpack leading to melt as well).

For more information see Avalanche Problems – Wet Snow.

Back to Avalanche Problems

Distinctive noise (resembling a “whumpf” sound) occurring when a weak layer beneath a slab collapses. The air of the weak layer is forced outside the snowpack, with the resulting whumpf sound being heard.

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

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

Mechanical transformation of snow crystals by wind in which forkings and other distinct shapes are obliterated.

This can occur in the atmosphere during snowfall or on the ground – snowdrift/windslab results.

See also: Equilibrium metamorphism (snow crystal rounding)

The wind-drifted snow problem is related to wind-drifted snow. The transported snow is typically packed on leeward sides into gullies, bowls and behind ridgelines or other wind-sheltered locations. Also commonly known as “windslab”. The problem is less widespread distributed than the new snow problem.

For more information see: Wind Slab Problem

Back to: Avalanche Problems

See: Windward slope

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