Avalanche Problems
Avalanche Problems
In every day life we recognize people who we have already met before. The recognition happens in a few seconds, without any long thoughts and usually is correct. For persons we have memorized and know well can, this process of recognition works with less detailed characteristics, e.g. silhouette, gait or voice. In other words, we are made for pattern recognition.
In fact, we can also use 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 part of the information pyramid.
The following definitions contain a description of the problem including the expected avalanche types, the spatial distribution and the position of the weak layers within the snow cover. In addition, the triggermechanisms, typical duration of the danger and final advice on how to travel in avalanche terrain are discussed. The main focus here is on backcountry recreationists who move in avalanche terrain. In addition, the description of avalanche problems can also be helpful for local authorities (e.g. avalanche commissions).
NEW SNOW WIND SLAB PERSISTENT WEAK LAYER WET SNOW GLIDE SNOW
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
- Dry-snow slab avalanches
- Dry loose snow avalanches
- Natural and human triggered avalanches
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.
What?
Characteristics
The avalanche problem is related to Wind slab. Snow can be transported by wind with or without a concurrent snowfall.
Expected avalanche types
- Dry-snow slab avalanches
- Natural and human triggered avalanches
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 layering due to variation in wind speed and variation during storm cycle, but occasionally also deeper in the old snow cover.
Why?
Release characteristics
Wind slab is an additional load on a weak layer and builds a slab which is particularly prone to support crack propagation.
When?
Duration
Wind slab 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 slab 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 snowdrift deposits and snowdrift deposits 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.
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
- Dry-snow slab avalanches
- Mostly human triggered avalanches; natural avalanches are rare, mainly in combination with other avalanche problems
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 characteristics
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 report 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.
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
- Wet-snow slab avalanches
- Wet loose snow avalanches
- Mainly natural avalanches
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 characteristics
- 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.
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 weak 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
- Glide snow avalanches; cold dry or 0°C-isothermal wet snowpack
- Any avalanche release is usually natural. Human and artificial triggering is unlikely.
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 characteristics
Glide-snow avalanches are caused by a loss of friction at the snow-ground interface due to a wet basal snow layer.
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.