Glossary
Glossary
This is the worlds largest glossary on snow and avalanche terminology. Currently comprising over 100 standardised terms in 9 languages this resource is constantly updated and expanded.
HEADER PICTURE: Wind Signs © Ragnar Ekker, The Norwegian Avalanche Warning Service | EAWS
Glossary
a
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 rocky 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.
Avalanche problems
The five typical avalanche problems as defined by the European Avalanche Warning Services EAWS aim to describe typical situations as they occur in avalanche terrain and to support avalanche professionals and recreationists in their evaluation of the avalanche hazard. They complement the danger level and the danger locations (slope aspect and elevation) and represent the third level in the information pyramid. The linked definitions include a general characterization of the problem including expected avalanche types, a description of the typical spatial distribution and of the position of the weak layer in the snowpack, a characterization of the release mechanism, a description of typical durations and time periods of the problem, and finally some travel advises for recreationists. The main focus thereby is on recreationists traveling in avalanche terrain. However, the typical avalanche problems may also be useful for avalanche safety services.
For more information see: Avalanche Problems
Avalanche size
Size 2: medium avalanche
- could bury, injure or kill a person
- snow avalanche stops typically at the end of a slope
Size 3: large avalanche
- could bury and destroy a car, damage a truck; destroy a small building or break a few trees
- snow avalanche could traverse flat terrain (considerably below 30°) over distances of less than 50 m
Size 4: very large avalanche
- could bury and destroy a railway car, large truck, several buildings or a piece of forest
- snow avalanche traverses flat terrain (considerably below 30°) over distances more than 50 m and can reach valley ground
Size 5: extremely large avalanche
- could gouge the landscape; disastrous damage potential
- snow avalanche reaches valley ground; largest runout distance known
Avalanche types
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.
Glossary
b
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.
Glossary
c
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
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 (10-20) can be critical. In favourable conditions, e.g. stable old snowpack, light winds, 20-30cm can be critical.
Glossary
d
Danger patterns
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.
Danger scale
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.
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. Also common while heavy snowfall, prolonged wind activity and rain.
Depth hoar, cup-shaped crystals
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
Glossary
e
Equilibrium metamorphism, isothermal metamorphism (snow crystal rounding)
Transformation process of dry snow with a minimal temperature gradient in the snowpack.
New dry snow crystals decompose into small, rounded grains, leading to settlement and general consolidation of the snowpack.
Glossary
f
Faceted snow crystals, grains
Snow grains, solid, with multiple flat glassy faces and sharp edges. Developed 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
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).
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.
Glossary
g
Gliding snow problem
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.
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 soil and debris.
Often gliding avalanches, but sometimes slab avalanches which release when a weak layer on the ground fails.
Groundwater outflow
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.
Glossary
h
Hardness (of a snowlayer)
For the assessment of the hardness of the individual layers, one considers the hand hardness.
| 1 | fist | very soft | fresh snow, depth hoar, surface hoar, decomposed, faceted crystals, melting form |
| 2 | 4 fingers | soft | round grains, decomposed, faceted crystals, melting form, rounding faceted particles |
| 3 | 1 fingers | medium hard | round grains, melt-freeze crust, rounding faceted particles |
| 4 | pencil | hard | round grains, melt-freeze crust |
| 5 | knife | very hard | melt-freeze crust |
| 6 | ice | compact | ice lense |
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.
Glossary
i
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.
| year | location | |
| 1895 | Altels (Switzerland) | 6 fatalities, 158 cattle killed |
| 1965 | Mattmark (Switzerland) | 88 fatalities |
| 1970 | Huascaran (Peru) | with subsequent debris flow: 18‘000 fatalities |
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.
Inneralpine regions
Areas enclosed by high alpine ridges, subject to weakened 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.
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.
Glossary
k
Kinetic metamorphism (faceting)
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.
Glossary
n
New snow problem
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.
Glossary
o
Old snow cover, old snowpack
Snow layers deposited from earlier precipitation, prior to fresh fallen snow.
Old snow layers consist of metamorphosed snow crystals.
Glossary
p
Persistent weak layer
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.
Glossary
r
remote triggering
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.
rounding faceted particles
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.
Glossary
s
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.
settlement
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.
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.
slab avalanche
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.
slushflow
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.
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 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 – 600 |
| firn | 600 – 830 |
| glacial ice | approx. 900 |
| pure ice | 917 |
snowdrift accumulations, 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. The colder snow is wh