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© Ragnar Ekker, The Norwegian Avalanche Warning Service | EAWS

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.

A
B
C
D
E
F
G
H
I
K
L
M
N
O
P
R
S
T
U
V
W
Z
Glossary

A

Accumulation zone

Area of slope where snow, transported by wind, accumulates and builds.

Wafted slope area Wafted slope area

© Avalanche Warning Service Tyrol

Additional load

Low additional load

  • individual skier/snowboarder, riding softly, not falling
  • group with good spacing (minimum 10 m) keeping distance
  • snowshoer

  • © Avalanche Warning Service Tyrol

  • © Peter Plattner

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

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

Altitudes

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

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

Amount of new snow

Amount of new snow accumulated in a certain period of time, for example, three days.

Area adjacent to the ridgeline, crest or summit

Terrain adjacent to ridgeline, crest or summit; highly influenced by wind.

Area adjacent to the ridge line Area adjacent to the ridge line

© Avalanche Warning Service Tyrol

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.

Area distant from ridgelines Area distant from ridgelines

© Avalanche Warning Service Tyrol

Artifical avalanche release

An avalanche triggered by artifically applied force (e.g. explosives, snow machines, people).

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

Aspect

Direction in which a slope faces as indicated by compass direction of the fall line; i.e. a north slope faces in a northerly direction.

Aspect Aspect

© TIRIS maps

Avalanche

Rapidly moving snow masses in volumes exceeding 100 m³ and minimum length of 50 meters.

Avalanche Avalanche

Avalanche bulletin

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.

  • © Avalanche.report

  • © SLF

  • © NVE

  • © Avalanche Warning Service Bavaria

Avalanche deposit

Snow deposited by an avalanche. Such snow deposits frequently persist for longer periods on valley floors.

Avalanche deposit Avalanche deposit

© Avalanche Warning Service Tyrol

Avalanche length

Total length of an avalanche measured from the highest point of the fracture line to the lowest point of the deposition.

Avalanche length Avalanche length

© Avalanche Warning Service Tyrol

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 prone location, danger zone

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

Avalanche size

Size of avalanche, classified by destructive potential, runout length and dimension.

Size 1: small avalanche (sluff)

  • minimal danger of burying (danger of people being carried/falling)
  • snow relocation typically stops before the end of a slope

Small avalanche (sluff) - size 1 Small avalanche (sluff) - size 1

© Avalanche Warning Service Tyrol

Small avalanche (sluff) – size 1

Size 2: medium avalanche

  • could bury, injure or kill a person
  • snow avalanche stops typically at the end of a slope

Medium avalanche - size 2 Medium avalanche - size 2

© Avalanche Warning Service Tyrol

Medium avalanche – size 2

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

Large avalanche - size 3 Large avalanche - size 3

© Avalanche Warning Service Tyrol

Large avalanche – size 3

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

Very large avalanche - size 4 Very large avalanche - size 4

© Avalanche Warning Service Tyrol

Very large avalanche – size 4

Size 5: extremely large avalanche

  • could gouge the landscape; disastrous damage potential
  • snow avalanche reaches valley ground; largest runout distance known

Extremely large avalanche - size 5 Extremely large avalanche - size 5

Extremely large avalanche – size 5

Avalanche types

Avalanches are categorized among different criteria. The most important distinctions are made regarding:

Additionally there are glide snow avalanches, where the whole snowpack slides on the ground.

Slab avalanche Slab avalanche

© Avalanche Warning Service Tyrol

Loose snow avalanche Loose snow avalanche

© Avalanche Warning Service Tyrol

Dense flow avalanche Dense flow avalanche

© Avalanche Warning Service Tyrol

Wet snow avalanche Wet snow avalanche

© Avalanche Warning Service Tyrol

Gliding avalanche Gliding avalanche

© Michael Auckenthaler

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.

Base of a rock wall Base of a rock wall

© Thomas Mariacher

Bed surface

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

Attention:
Not to be confused with the weak layer!

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

Blowing snow

Snow being transported by wind high above (approximately 2m) snowpack surface (visibility noticeably reduced).

Blowing snow Blowing snow

© Avalanche Warning Service Tyrol

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.

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

Bowl

Rounded or elongated concavity; typically tending to accumulate snowdrift.

Bowl Bowl

© Thomas Mariacher

Breakable crust

Surface crust due to wind, sun or air temperature that breaks when stepped upon.

Breakable crust Breakable crust

© Avalanche Warning Service Tyrol

Broad ridge

A rounded, elongated shoulder of high terrain.

Broad ridge Broad ridge

© Avalanche Warning Service Tyrol

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

Corn snow Corn snow

© Avalanche Warning Service Tyrol

Cornice

An overhanging mass of snow created by wind, usually near a sharp terrain break such as a ridge.

Cornice Cornice

© Avalanche Warning Service Tyrol

Couloir

A steep gully usually adjacent to ridgelines, flanked by rocks, often containing rubble/scree; tends to accumulate drifted snow.

Couloir Couloir

© Thomas Mariacher

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.

Critical depth of new fallen snow Critical depth of new fallen snow

© Avalanche Warning Service Tyrol

Crust

Layer of hard-compacted snow resulting from a melt-freeze process or wind.

Crust Crust

© Avalanche Warning Service Tyrol

Glossary

D

Danger

Conditions, circumstances or processes which can result in damage and/or injury.

Danger Danger

© Avalanche Warning Service Tyrol

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.

Daytime changes Daytime changes

© Avalanche Warning Service Tyrol

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

  • © Lisa Manneh

  • © EAWS

Decreasing firmness (of a snow layer)

Bonding between ice crystals deteriorates or is lost, diminishing overall capacity of crystals to absorb loading.

Decreasing firmness Decreasing firmness

© Avalanche Warning Service Tyrol

Dense flow avalanche

Avalanche motion which primarily flows, slides, slips, in contrast to powder cloud avalanches.

Dense flow avalanche Dense flow avalanche

© Avalanche Warning Service Tyrol

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

Depth hoar Depth hoar

© Lukas Ruetz

Depth of new, fresh fallen snow

Amount of snow deposited in the previous 24 hours.

Depth of new snow Depth of new snow

© Adi Kerber

Drifting snow

Snow being lifted from the snow surface and transported by wind just above the snow surface (visibility not noticeably reduced).

Drifting snow Drifting snow

© Avalanche Warning Service Tyrol

Glossary

E

Endangered traffic route

Transportation route near a slope potentially endangered by avalanches.

Endangered traffic route Endangered traffic route

© Avalanche Warning Service Tyrol

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.

Snow metamorphism Snow metamorphism

© Lisa Manneh

Exposed

Exposure to wind, sun, avalanches or other general danger.

Exposed Exposed

© Avalanche Warning Service Tyrol

Exposed transportation route

Sector of road, railway, or similar infrastructure facility at risk from avalanche danger; often the runout zone of an avalanche path.

Exposed transportation route Exposed transportation route

© Avalanche Warning Service Tyrol

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

Faceted snow crystals Faceted snow crystals

© www.snowcrystals.it

Firmness (of snow)

Load-bearing capacity (resistance to disintegration). Depends on extent and quality of snow crystal bonding.

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 Firn

© Avalanche Warning Service Tyrol

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.

Firn mirror Firn mirror

© Avalanche Warning Service Tyrol

Fracture depth

Slab thickness at the fracture line, measured vertically.

Fracture depth Fracture depth

© Avalanche Warning Service Tyrol

Frost build-up/Riming

Firmly attached precipitation accumulating on wind-affected side of e.g. trees, electric lines and summit crosses during high humidity and wind. Also known as Rime Ice.

Frost buildup Frost buildup

© Avalanche Warning Service Tyrol

Full depth slab avalanche

An avalanche which glides over the ground, across firn snow or atop a glacier in the fracture zone, sweeping the entire season’s snowpack with it.

Full depth slab avalanche Full depth slab avalanche

© Avalanche Warning Service Tyrol

Glossary

G

Glide crack

A crack visible on the snowpack surface formed by snow-gliding on steep, and particularly grass-covered slopes.

Areas below slopes with visible glide cracks should be avoided whenever possible due to threat of gliding sluffs or avalanches.

Glide crack Glide crack

© Avalanche Warning Service Tyrol

Gliding sluff, gliding avalanche

When gliding movements become increasingly rapid, a gliding avalanche develops. Releases can occur anytime during day or night. Gliding avalanches are not induced by a crack in a weak layer.

  • © Avalanche Warning Service Tyrol

  • © Michael Auckenthaler

  • © Avalanche Warning Service Tyrol

Gliding snow

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.

  • © Alex Holaus

  • © Jonathan Flunger

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.

Gliding snow Gliding snow

Back to Avalanche Problems.

Graupel

Special form of precipitation formed in the atmopshere by supercooled water droplets adhering to snow crystals.

Characteristic grain size: ≤5 mm

See also: www.snowcrystals.it

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

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.

  • © Avalanche Warning Service Tyrol

  • © Stefan Wierer

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.

Gully

Usually a steep, elongated, eroded trench; typically prone to snowdrift accumulation.

Gully Gully

© Thomas Mariacher

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 forms
2 4 fingers soft round grains, decomposed, faceted crystals, melting form, rounding faceted particles
3 1 finger 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.

  • © Martin Berner

  • © Avalanche Warning Service Tyrol

High alpine regions

Areas above approximately 3000 m (particularly glaciated areas).

High alpine regions High alpine regions

© Avalanche Warning Service Tyrol

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

Ice avalanche Ice avalanche

© Avalanche Warning Service Tyrol

Ice lense

Thin ice layer inside the snowpack resulting from rain or melt-water refreezing. No single grains are visible.

Ice lense Ice lense

© Martin Berner

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.

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.

Incoming radiation Incoming radiation

© Avalanche Warning Service Tyrol

Increasing firmness (of a snow layer)

Bonding between ice crystals improves, increasing overall capacity of crystals to absorb loading.

Increasing firmness Increasing firmness

© Avalanche Warning Service Tyrol

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.

Inneralpine regions Inneralpine regions

© University of Vienna

Isothermal snow cover

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.

Isothermal snow cover Isothermal snow cover

© Avalanche Warning Service Tyrol

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.

Kinetic metamorphism Kinetic metamorphism

© Lisa Manneh

Glossary

L

Lee slope

Slope facing downwind/ wind sheltered aspect. Snow deposits can attain several times average snow depth.

Lee slope Lee slope

© Avalanche Warning Service Tyrol

Likely (something is likely)

Event with a probability of occurrence exceeding 50%.

Local, from place to place

Areas limited to slopes and basin areas.

Within one region, different local avalanche situations may prevail.

Loose snow avalanche; point release avalanche

A type of avalanche (dry or wet snow with little or no bonding) fanning out downhill and leaving a widening conical scar.

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

Glossary

M

Melt-freeze crust

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

Melt-freeze crust Melt-freeze crust

© Avalanche Warning Service Tyrol

Melt-freeze metamorphism

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.

Multiple starting zones

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

Multiple starting zones Multiple starting zones

© Avalanche Warning Service Tyrol

Glossary

N

Naturally triggered avalanche, natural release

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

Naturally triggered avalanche Naturally triggered avalanche

© Avalanche Warning Service Tyrol

New fallen snow

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

New fallen snow New fallen snow

© Avalanche Warning Service Tyrol

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.

New snow New snow

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.

Old snow Old snow

© Avalanche Warning Service Tyrol

Outgoing longwave radiation

The snowpack surface emits longwave radiation (infrared) into the atmosphere.
If skies are clear, the surface cools significantly (up to 20°C) below air temperature.

Outgoing longwave radiation Outgoing longwave radiation

© foto-webcam.eu

Glossary

P

Pass area

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

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

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.

Persistent weak layers Persistent weak layers

Back to Avalanche Problems.

Poorly bonded layer

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

Poorly bonded layer Poorly bonded layer

© Lisa Manneh

Possible (potential)

Event with a probability of occurrence not exceeding 50%.

Powder avalanche, powder cloud avalanche

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.

Powder avalanche Powder avalanche

Prone to triggering

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

Prone to triggering Prone to triggering

© Avalanche Warning Service Tyrol

Glossary

R

Radiation

Energy transport by electromagnetic waves at varying wave lengths: short wave radiation (visible light), long wave radiation (thermal radiation).

Radiation Radiation

© Lisa Manneh

Region, regional

Areas comprising several valleys.
In avalanche bulletins, regions are generally subdivided climatically or geographically.

Regions Regions

© Avalanche Warning Service Tyrol

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.

Remote triggering Remote triggering

© Avalanche Warning Service Tyrol

Rib

Elongated sub-ridge or ravine in a slope or mountain face.

Rib Rib

© Avalanche Warning Service Tyrol

Ridge

Narrow crest line of a mountain.

Ridge Ridge

© Thomas Mariacher

Ridgeline

Long mountain ridge silhouette

Further explanation:
A ridgeline is a clearly delineated crest frequently connecting numerous peaks of a mountain range.

Ridgeline Ridgeline

© Thomas Mariacher

Risk

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

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

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

Rounded snow grains Rounded snow grains

© www.snowcrystals.it

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.

Safety spacing Safety spacing

© Avalanche Warning Service Tyrol

Sastrugi

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.

  • © AINEVA

  • © AINEVA

  • © AINEVA

Secured areas

Areas protected from avalanches and other alpine hazards through technical or temporary measures.

Secured areas Secured areas

© Avalanche Warning Service Tyrol

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.

Settlement Settlement

© Avalanche Warning Service Tyrol

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.

Shady slope Shady slope

© Avalanche Warning Service Tyrol

Shooting crack

Sudden cracking of a brittle snowpack; a clear sign of instability.

Shooting crack Shooting crack

© Avalanche Warning Service Tyrol

Sintering

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.

Sintering Sintering

© Avalanche Warning Service Tyrol

Size of the starting zone (for slab avalanches)

Distance between fracture line and lower boundary (stauchwall) of the slab.

Size of the starting zone Size of the starting zone

© Avalanche Warning Service Tyrol

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.

Slab avalanche Slab avalanche

© Avalanche Warning Service Tyrol

Slab thickness

Slab thickness at the fracture line, measured at a right angle to the slope.

Slab thickness Slab thickness

© Avalanche Warning Service Tyrol

Slab width

Maximum distance between the two lateral boundaries of a slab avalanche.

Slab width Slab width

© Avalanche Warning Service Tyrol

Slope discontinuity

A slope area where the slope gradient suddenly becomes significantly steeper; highly prone to accumulate drifted snow masses

Slope discontinuity Slope discontinuity

© Avalanche Warning Service Tyrol

Slope gradient

  • moderately steep: less than 30°
  • steep: 30° and more
  • very steep: 35° and more
  • extremly 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.

Slope gradient Slope gradient

© TIRIS maps

Slush

Separated rounded particles completely immersed in water (liquid water content > 15% (volume fraction)).

Slush Slush

© Sam Colbeck

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.

Slushflow Slushflow

© Markus Eckerstorfer

Small scale

Slope areas or margins ranging in size from a few meters to approximately 20 m.

Small scale Small scale

© Avalanche Warning Service Tyrol

Snow base, fundament

Lowermost layers of the snowpack close to the ground.

Snow base Snow base

© Avalanche Warning Service Tyrol

Snow cover, snowpack

Snow deposited on the ground in a multiplicity of layers.

Snow cover Snow cover

© Avalanche Warning Service Tyrol

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


Snow density Snow density

© Avalanche Warning Service Tyrol

Snow depth

Thickness of the snowpack measured vertically.

Snow depth Snow depth

© Gerhard Figl

Snow depth increase

Increase in snow depth within a specific time frame.

Snow dunes, ripples

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.

  • © AINEVA

  • © AINEVA

  • © AINEVA

Snow layering

Stratification of the snowpack

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

Snow layering Snow layering

© Avalanche Warning Service Tyrol

Snow line

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

Snow line Snow line

© Avalanche Warning Service Tyrol

Snow metamorphism

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

  • © Lisa Manneh

  • © Lisa Manneh

Snow plume

Snow being lifted by the wind and carried away from a peak or ridge into the air.

Snow plume Snow plume

© Avalanche Warning Service Tyrol

Snow profile

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.

Snow profile Snow profile

© Avalanche Warning Service Tyrol

Snow thickness

Thickness of the snowpack measured at right angle to slope.

Snow thickness Snow thickness

© Gerhard Figl

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.

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

  • © Avalanche Warning Service Tyrol

  • © Avalanche Warning Service Tyrol

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.

Snow fall level Snow fall level

© Avalanche Warning Service Tyrol

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

  • © www.snowcrystals.it

  • © www.snowcrystals.it

Snowpack capable of bearing loads

Snow surface layer strong enough to support a person walking on it.

Snowpack capable of bearing loads Snowpack capable of bearing loads

© Avalanche Warning Service Tyrol

Spacing distances, maintaining distances

Precautionary measure in outlying terrain: maintaining distances between persons to reduce snowpack loading.

During ascents at least 10 m distance maintained, during descents significantly more.

Spacing distance Spacing distance

© Avalanche Warning Service Tyrol

Stability, snowpack stability

The strength of a snowpack to withstand internal and external disturbances.
Stability is determined by firmness vs. stress inside a snow layer.

Stability Stability

© Avalanche Warning Service Tyrol