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.
Glossary
A
Additional load
Low additional load
- individual skier/snowboarder, riding softly, not falling
- group with good spacing (minimum 10 m) keeping distances
- snowshoer
High additional load
- two or more skiers/snowboarders etc. without good spacing (or without intervals)
- snow machine
- explosives
- single hiker/climber
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
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 can be critical. In favourable conditions, e.g. stable old snowpack, light winds, even 50 cm of snow presents no problem.
Glossary
D
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, 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
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 chracterized 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
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 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.
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
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
Glossary
P
Persistent weak layers
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
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.
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 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. 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.
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.
Snowmelt
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
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.
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 layer.
See also: www.snowcrystals.it
Glossary
T
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.
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 wind increases 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.
Typical 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
Glossary
U
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.
Glossary
V
Glossary
W
Weak layer
Snowpack layer in which the structure of crystals has undergone a break.
Typical dry weak layers causing avalanches: precipitation particles; surface hoar; faceted crystals; hollow crystals; graupel. Wet weak layers can be formed by every snowcrystalls.
Wet snow problem
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 Wet Snow Problem.
Back to Avalanche Problems.
Whumpfing, collapsing sound
Distinctive noise (resembling a “whumpf” sound) occurring when a weak layer beneath a slab collapses. Thus the air of the weak layer is pressed outside the snowpack, a whumpf follows.
The sound usually indicates an unstable situation and can be accompanied by cracking. A whumpf is a clear avalanche alert.
Wind-drifted snow problem
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. The problem is less widespread distributed than the new snow problem.
For more information see Wind-drifted Snow Problem.
Back to Avalanche Problems.
Glossary
X
Glossary
Y
HEADER PICTURE: Wind Signs © Ragnar Ekker, The Norwegian Avalanche Warning Service | EAWS