Standards

Avalanche Problems

In the every day life we recognize people who we have already met before. This recognition happens in only a few seconds, without deliberation and is usually correct. For persons we have memorized and know well this process of recognition works with even less detailed characteristics, e.g. a persons shape, gait or voice. In other words, pattern recognition is intuitive. We can also utilise this ability to recognize similar situations when making decisions in avalanche terrain. The five typical avalanche problems were defined with this goal in mind. They describe typical situations, how they occur in the terrain and how they can support both avalanche forecasters and backcountry recreationists in their assessment of the avalanche danger. They supplement the description of the danger level and the hazardous terrain (aspect and elevation) and represent the third level of the information pyramid.

The following definitions contain a description of each problem including: the expected avalanche type, its spatial distribution and the position of weak layers within the snow pack. In addition, trigger mechanisms, the typical duration of the relevant danger and, advice on how to travel in avalanche terrain are included. The main focus of this approach is to inform backcountry recreationists who travel in avalanche terrain. Additionally, the description of avalanche problems can also be of significant value and help for local authorities (e.g. avalanche commissions).

Avalanche Problem

New Snow

Icon Avalanche Problems New-Snow Icon © EAWS | EAWS European Avalanche Warning Services

What?

Characteristics

The avalanche problem is related to current or most recent snowfall. The amount of additional loading by new snow onto the existing snowpack is the crucial factor of the new snow problem. How critical the loading is depends on various factors such as temperature or characteristics of the old snow surface.

Expected avalanche types

Where?

Spatial distribution

Generally widely present and often in all aspects.

Position of weak layers in the snowpack

Usually at the transition to the old snow surface, but sometimes in the new snow layers and sometimes also deeper in the old snowpack.

Why?

Release characteristic

Dry-snow slab avalanches: Additional load due to snowfall on existing or newly created weak layers
Dry loose snow avalanches: Lack of cohesion between the new snow particles

When?

Duration

Typically during snowfall and up to a few days after.

How to manage?

Identification of the problem in the field

The new snow problem is fairly easy to recognize. Watch out for new snow amounts and recent avalanche activity. Be aware of slight weather changes (e.g., changes in air humidity) affecting new snow conditions.

Travel advice

Dry-snow slab avalanches: Wait until the snowpack stabilizes.
Dry loose snow avalanches:  Danger of falling is more important than danger of burial. Consider consequences in steep terrain.

Avalanche Problem

Wind-drifted snow

Icon Avalanche Problems Wind drifted Snow © EAWS | EAWS European Avalanche Warning Services

What?

Characteristics

The avalanche problem is related to wind-drifted snow. Snow can be transported by wind with or without a concurrent snowfall.

Expected avalanche types

Where?

Spatial distribution

Highly variable but typically on leeward sides in gullies, bowls, near distinct changes in slope angle, behind ridgelines or other wind-sheltered locations. More common above treeline.

Position of weak layers in the snowpack

Usually at the transition to the old snow surface or within the windslab layer due to variation in wind speed and variation during storm cycle, but occasionally also deeper in the old snow cover.

Why?

Release characteristic

Wind-drifted snow is an additional load on a weak layer and builds a slab which is particularly prone to support crack propagation.

When?

Duration

Wind-drifted snow can evolve very quickly. The problem lasts typically during the snowdrift event, up to a few days at most, depending on snowpack evolution.

How to manage?

Identification of the problem in the field

If not hidden by new snow the wind-drifted snow problem can be recognized with training and good visibility. Consider wind signs and locate deposits. Typical clues: snowdrift deposits, recent avalanche activity and sometimes shooting cracks or whumps. However, it is often hard to determine the age of wind signs and wind signs do not necessarily imply an avalanche problem (e.g., in absence of a weak layer).

Travel advice

Avoid snowdrift deposits in steep terrain, in particular in areas where the snow cover changes from thin to thick or from hard to soft.

Avalanche Problem

Persistent weak layers

Icon Avalanche Problems Persistent weak layer © EAWS | EAWS European Avalanche Warning Services

What?

Characteristics

The avalanche 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.

Expected avalanche types

Where?

Spatial distribution

The avalanche problem can be widespread or quite isolated. It can exist in all aspects, but is more frequent on shady, wind sheltered slopes.

Position of weak layers in the snowpack

Anywhere in the old snowpack, often deep in the snowpack. However, when deeply buried triggering becomes increasingly hard.

Why?

Release characteristic

Release of avalanche when loading exceeds the strength of the weak layer.

When?

Duration

Weak layers can persist for weeks to months; possibly most of the winter season.

How to manage?

Identification of the problem in the field

Persistent weak layers are very challenging to recognize. Signs of instability such as whumps are typical but not necessarily present. Stability tests can be helpful to detect the persistent weak layers. Information on snowpack history is critical and reference to the published avalanche bulletin is important. Crack propagation over long distances is common and remote triggering is possible.

Travel advice

Travel conservatively and avoid large steep slopes. Consider the history of weather and snow cover processes in the area. Be extra cautious in areas with a thin snowpack and at the transition from thin to deep snowpack. This problem is a major cause of recreational avalanche fatalities.

Avalanche Problem

Wet snow

Icon Avalanche Problems Wet snow © EAWS | EAWS European Avalanche Warning Services

What?

Characteristics

The avalanche problem is related to weakening of the snowpack due to the presence of liquid water. Water infiltrates the snowpack due to melt or rain.

Expected avalanche types

Where?

Spatial distribution

When sun is the main cause, distribution of the problem is mostly depending on aspect and elevation. All aspects are affected in the event of rain on snow.

Position of weak layers in the snowpack

Anywhere in the snowpack

Why?

Release characteristic

Wet-snow slab avalanches: Weakening of pre-existing weak layers in the snowpack or ponding at layer interfaces
If rain, there is also additional loading on weak layers
Wet loose snow avalanches: Loss of cohesion between snow crystals

When?

Duration

Hours to days
Rapid loss of stability possible
Especially critical as water infiltrates for the first time deeper down, once the snowpack has warmed up to 0 °C.
Natural avalanches might be more likely at certain times of the day, in particular in the afternoon (unless rain is the dominating factor).

How to manage?

Identification of the problem in the field

The wet snow problem is usually easy to recognize. Onset of rain, snowballing, pin wheeling and small wet slabs or loose wet avalanches are often precursors of natural wet-snow slab avalanche activity. Deep foot-penetration is another sign of increased wetting.

Travel advice

In the presence of a sun crust the conditions after cold nights with clear skies are usually favourable in the morning due to freezing. After warm nights with overcast skies the problem often exists already in the morning. Normally rain on fresh snow creates this problem almost immediately. Good timing and trip planning are important. Consider avalanche runout zones.

Avalanche Problem

Gliding snow

Icon Avalanche Problems Gliding Snow © EAWS | EAWS European Avalanche Warning Services

What?

Characteristics

The entire snowpack is gliding on the ground, typically on smooth ground such as grassy slopes or smooth rock zones. High activity of glide-snow avalanches are typically related to a thick snowpack with no or only few layers. Glide snow avalanches can occur both with a cold dry snowpack and with a warm wet snowpack. The release of a glide-snow avalanche is difficult to predict, although glide cracks open usually before a release.

Expected avalanche types

Where?

Spatial distribution

Predominant on smooth ground and on every aspect, but more often on south-facing slopes.

Position of weak layers in the snowpack

Interface between the ground and overlaying snowpack

Why?

Release characteristic

Glide-snow avalanches are caused by a loss of friction at the snow-ground interface.

When?

Duration

Days to months; possibly entire winter-season. The release can occur at any time during the day. In the spring, gliding avalanches occur mostly in the later part of the day.

How to manage?

Identification of the problem in the field

With the presence of glide cracks the problem can often be localized, however, the presence of glide cracks does not indicate imminent avalanche release, as this is nearly impossible to predict. Avalanche release without pre-existing glide cracks is also common.

Travel advice

Avoid areas close to glide cracks.

HEADER PICTURE: Wet Snow © Jostein Aasen, The Norwegian Avalanche Warning Service | EAWS