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AVALANCHE PROBLEMS

The five typical avalanche problems have been defined by the Association of European Avalanche Warning Services (EAWS) with the aim of describing typical situations as they occur in the terrain and to support both avalanche warning and winter sports enthusiasts in their assessment of the avalanche danger. They complement the description of the danger level and the danger zones (exposure and altitude).

FRESH SNOW PROBLEM

Neuschneeproblem
Neuschneeproblem

SPACIAL DISTRIBUTION

Mostly widespread and in all exposures.

DURATION OF CAUTION

During the snowfall until a few days afterwards.

FORMATION

Current snowfall or fresh snow that has just fallen.

Main influencing factor: Critical amount of fresh snow, which depends on several factors, such as temperature or properties of the old snow surface.

TYPES OF AVALANCHES

  • Dry snow slab avalanches
  • Dry avalanches of loose snow
  • Spontaneous and artificial triggering

IDENTIFICATION

Relatively easy to recognize:

Note the critical amount of fresh snow and fresh avalanches.

Note that small changes in weather (e.g. change in humidity) can significantly change the fresh snow conditions.

TRIGGER MECHANISM

Additional load from snowfall on existing or newly formed weak layers.

Missing connection between the new snow crystals.

POSITION OF THE WEAK LAYER

  • Mostly at the transition to the old snow surface
  • Within the fresh snow layers
  • Deeper in the old snow cover

BEHAVIOUR

Wait until the fresh snow has stabilized.

Pay particular attention to the danger of being carried along and falling in extreme steep terrain.

DRIFT SNOW PROBLEM

Triebschneeproblem
Triebschneeproblem

SPACIAL DISTRIBUTION

  • Pronouncedly irregularly distributed
  • Tends to be in wind-remote areas (lee), in gutters, troughs, behind ground edges and other wind-reduced areas
  • More often above the tree line than below

DURATION OF CAUTION

Drift snow can develop very quickly.

The problem usually lasts during freighting until a few days after the last wind influence. (stratification due to changes in wind speed during a storm period)

FORMATION

Drift snow is caused by the wind-induced transport of snow.

The formation can occur with or without simultaneous snowfall.

TYPES OF AVALANCHES

  • Dry snow slab avalanches
  • Spontaneous and artificial triggering

IDENTIFICATION

With practice and in good visibility conditions, drift snow is relatively easy to detect, unless the drift snow has been covered by new snow. Note wind signs and localize drift snow deposits.

Typical indications are drift snow deposits, crack formation, humming noises and fresh avalanches.

It is often difficult to estimate the age of the drift snow - drift snow does not necessarily have to be a problem (for example, if there is no weak layer).

TRIGGER MECHANISM

Additional load from the drift snow on a weak layer.

Drift snow forms a snow slab that has a special tendency to break.

POSITION OF THE WEAK LAYER

  • Mostly at the transition to the old snow surface or within the drift snow

(stratification by changes in wind speed during a storm period)

  • Occasionally also deeper in the old snow cover

BEHAVIOUR

Avoid drift snow deposits in steep terrain, especially at transitions from little to much snow and from soft to hard snow!

OLD SNOW PROBLEM

Altschneeproblem
Altschneeproblem

SPACIAL DISTRIBUTION

  • Large-scale, but also small-scale
  • Possible in all expositions, but more often in shady, rather wind protected slopes

DURATION OF CAUTION

Weeks to months; sometimes during the whole winter.

FORMATION

Existing weak layers within the old snow cover. Typical

weak layers: snow-covered surface frost, deep frost (also called cup crystals or "floating snow") or angular crystals.

TYPES OF AVALANCHES

  • Dry snow slab avalanches
  • Mostly artificial triggers (e.g. winter athletes, blasting)
  • Spontaneous avalanches are rare
  • Usually in combination with another avalanche problem

IDENTIFICATION

Extremely difficult to detect!

Signs of instability (e.g. humming noises) are typical, but not necessarily present.

Snowpack tests can help to identify the weak layers. Note the information on snowpacks in the avalanche situation report. Fracture propagation usually occurs over long distances (remote triggering is possible).

TRIGGER MECHANISM

Fracture of a weak layer in old snow if the additional load exceeds the strength of the weak layer.

POSITION OF THE WEAK LAYER

  • Somewhere in the old snow, often deep in the snow
  • Triggering becomes more difficult with weak layers, which are covered by thicker, more stable layers

BEHAVIOUR

Avoid large steep slopes. Pay attention to the weather and snow cover development in an area.

Take special care in areas with little snow and at the transition from areas with little snow to areas with a lot of snow.

The old snow problem is a major cause of fatal avalanche accidents among winter sports athletes.

WET SNOW PROBLEM

Nassschneeproblem
Nassscneeproblem

SPACIAL DISTRIBUTION

If solar radiation is the main cause of the problem, its spreading depends mainly on altitude and exposure.

If rain is the cause, all exposures are affected.

DURATION OF CAUTION

Hours to days. Rapid loss of stability possible.

Critical is the first penetration of water deeper into the snowpack as soon as the snow cover is 0°C-isothermal.

Spontaneous avalanches are more likely to occur in the afternoon than in the morning (unless rain is the main cause of the problem).

FORMATION

Increasing weakening of the snow cover due to water ingress, either through melt or rain.

TYPES OF AVALANCHES

  • Wet snow slab avalanches
  • Wet avalanches of loose snow
  • Mostly spontaneous triggers

IDENTIFICATION

Mostly easy to recognize.

Starting rain, formation of snowballs or snow rolls, small wet snow slab or loose snow avalanches often announce wet avalanche activity.

Deep sinking into the snowpack is also a sign of increasing moisture/moisture penetration.

TRIGGER MECHANISM

Weakening and fracture of former weak layers in the snowpack or slipping of layers at water horizons.

Rain also represents an additional load on the snowpack. Loss of bonds between the snow crystals.

POSITION OF THE WEAK LAYER

  • Somewhere in the snowpack

BEHAVIOUR

After a cold, clear night the conditions in the morning are usually favourable.
After warm, overcast nights, the problem often occurs in the morning.
When it rains on a dry snowpack, the problem usually occurs immediately.
Good timing and good route planning are crucial. Pay attention to avalanche runout areas.

GLIDING SNOW PROBLEM

Gleitschneeproblem
Gleitschneeproblem

SPACIAL DISTRIBUTION

Especially on smooth surfaces.

In all exposures, but more often on southern slopes.

At the transition of the snow cover to the ground.

DURATION OF CAUTION

Days to months, triggering possible during the whole winter.

Triggers can occur at any time of day or night.

In spring they usually occur later in the day.

FORMATION

The entire snow cover slides off on smooth ground (for example grass slopes or smooth rocky areas).

Typically associated with a thick snowpack with few or no weak layers.

In dry, cold as well as wet, 0°C isothermal snow cover.

It is almost impossible to predict the point of origin of gliding snow avalanches, although they usually announce themselves by gliding snow cracks (so-called fish mouths).

TYPES OF AVALANCHES

  • gliding snow avalanches dry/ cold
  • gliding snow avalanches wet/ 0°C-isothermal
  • Almost exclusively spontaneous triggers
  • Artificial triggers are unlikely

IDENTIFICATION

Watch out for gliding snow cracks (fish mouths) and note that the time of release can hardly be predicted.

Triggering is also possible without the formation of gliding snow cracks.

An isothermal snow cover is a snow cover that is at a completely uniform temperature. This means a constant temperature through all layers from the ground to the surface of the snow cover.

TRIGGER MECHANISM

Gliding snow avalanches are triggered due to the loss of friction on a water-saturated layer between the snow cover and the ground.

POSITION OF THE WEAK LAYER

  • Mostly at the transition to the old snow surface
  • Within the fresh snow layers
  • Deeper in the old snow cover

BEHAVIOUR

Do not stay near snow cracks (fish mouths).