Glossary
This is the world’s largest glossary on snow and avalanches. Currently with more than 100 standardized terms available in 9 languages. A resource that 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 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.
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
Base of a rock wall
Lowermost visible area of a rock wall, frequently scree at a highly divergent slope angle, the steepness tending to decrease as it descends.
The terrain below a rock wall is usually extremely steep.
Bed surface
The surface across which a slab avalanche releases (can be the ground).
Attention:
Not to be confused with the weak layer!
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 can be critical. In favourable conditions, e.g. stable old snowpack, light winds, even 50 cm of snow presents no problem.
Glossary
D
Danger patterns
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.
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.
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
Depth hoar, cup-shaped crystals
Large, hollow crystals with edges, rims 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
Dp.1 deep persistent weak layer
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
Click to go back to danger patterns.
Dp.2 gliding snow
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
Click to go back to danger patterns.
Dp.3 rain on snow
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.
Dp.4 cold on warm / warm on cold
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
Click to go back to danger patterns.
Dp.5 snowfall after long period of cold
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 for backcountry skiers. 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
Click to go back to danger patterns.
Dp.6 loose snow and wind
“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
Click to go back to danger patterns.
Dp.7 shallow snow next to deep snow
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
Click to go back to danger patterns.
Dp.8 buried surface hoar
Surface hoar numbers among the most beautiful types of snow 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
Click to go back to danger patterns.
Dp.9 buried graupel
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. Fortunately, the problems it creates are short-lived.
For further informations watch: https://youtu.be/X3vNbaO4lCA
Click to go back to danger patterns.
Dp.10 springtime situation
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.
Glossary
E
Equilibrium metamorphism, isothermal 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.
Glossary
F
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
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
J
Glossary
K
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.
The process is accelerated in shady terrain with shallow snow cover. This process can affect the whole snowpack or only some parts of the snowpack. Layers of faceted crystals are often found near to crusts. On the snow cover surface it preferably develops during clear sky nights.
Glossary
L
Lee slope
Slope facing downwind
Snow deposits can attain several times average snow depth.
Glossary
M
Multiple starting zones
Area in which a number of discrete avalanches originate.
The term is often associated with avalanche sizes 4 to 5.
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
Q
Glossary
R
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.
round snow grains
Small, spherical grains resulting from snow crystal rounding. Snow layers of round-grained snow are matt white, not glassy.
Characteristic grain size: 0.2 to 0.5 mm
See also: www.snowcrystals.it
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 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.
slope discontinuity
A slope area where the slope gradient suddenly becomes significantly steeper; highly prone to accumulate drifted snow masses
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 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
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.
