Low additional load
- individual skier/snowboarder, riding softly, not falling
- group with good spacing (minimum 10 m) keeping distances
High additional load
- two or more skiers/snowboarders etc. without good spacing (or without intervals)
- snow machine
- single hiker/climber
Mountain terrain unconnected to a ridgeline.
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.
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
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.
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.
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.
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
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.
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.
|1895||Altels (Switzerland)||6 fatalities, 158 cattle killed|
|1965||Mattmark (Switzerland)||88 fatalities|
|1970||Huascaran (Peru)||with subsequent debris flow: 18‘000 fatalities|
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.
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.
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.
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.
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.
Slopes in shadow, untouched or little struck by sunlight, typically north-facing.
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.
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.
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.
The mass per unit volume of a given quantity of snow. Snow can have highly varied densities:
|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|
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.
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
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.
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.
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.
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.
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
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.
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.
HEADER PICTURE: Wind Signs © Ragnar Ekker, The Norwegian Avalanche Warning Service | EAWS