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Avalanche Danger Descriptors


Avalanche Problem Essentials

Not all avalanches are the same! Avalanches can be divided into roughly eight types according to their character.

Avalanche character has direct implication about the types of terrain to favor or avoid, the likely size of avalanches, which observations are most useful, and how long the problem is likely to persist.

Loose Dry Avalanches or “Sluffs”:

  • Are usually small, but may gain significant mass on long steep slopes.
  • Are typically limited to steep terrain (40+ degrees).
  • Stabilise soon after a storm, usually within a few days.

Management: Loose dry avalanches are best managed by avoiding terrain traps and large steep slopes until the surface has stabilised. On large steep slopes, occasionally move across the fall line to avoid being caught by your own sluffs from above.

 

Loose Wet Avalanches:

  • Are more powerful than loose dry avalanches due to their higher density.
  • Are often limited to sunny slope aspects.
  • Are commonly confined to the warmest part of the day.
Management: Loose wet avalanches are best managed by avoiding start zones and avalanche paths when snow becomes moist from daytime heating, from rain, or does not freeze overnight.

Wet Slabs:

  • Involve wet layers in the snowpack, typically including deeper layers.
  • Tend to be large.
  • Occur when water forms or penetrates below the surface of the snowpack.
Management: Wet slabs are best managed by avoiding start zones and avalanche paths when the snow becomes wet from daytime heating, rain, or lack of an overnight freeze.
 

Cornices:

  • May trigger large slab avalanches or relatively stable slopes below.
  • Are often associated with recent wind loading and/or temperature changes.
  • Can be triggered from ridges, sometimes breaking surprisingly far back onto ridge tops.
Management: Cornices are best managed by approaching corniced ridges cautiously. Avoid traveling on or near overhanging cornices and limit time spent exposed to slopes below cornices, especially soon after wind events and during periods of warming temperatures.
 

Wind Slabs:

  • Vary in size from small to medium.
  • Occur on steeper lee and cross-loaded portions of slopes (typically 35+ degrees).
  • Are often limited to specific terrain features such as lee ridge-tops.
  • Can often be recognised by appearance of the snow surface, changes in surface snow hardness, hollow, drum-like sounds and/or shooting cracks.
  • Winds that vary in strength and direction can produce complex and unexpected wind slab patterns.
  • Stabilise fairly soon, usually in a few days to a week.
Management: Wind slabs are best managed by recognising and avoiding areas where wind slabs have formed, until they have stabilised.
 

Storm Snow:

  • Vary in size from small to very large.
  • Maybe soft slabs, fooling people into underestimating slab potential.
  • Tend to occur on moderately steep slopes (35+ degrees).
  • Occur in all terrain, but are larger and more frequent in the alpine.
  • Stabilise soon after a storm, usually within a few days.
Management: Storm slabs are best managed by conservative terrain choices during and after storms until the storm snow has stabilised.
 

Persistent Slabs:

  • Slide on buried persistent weak layers, which often form during clear periods and may involve deeper layers from multiple storms.
  • Vary in size from medium to very large and may cross terrain barriers to involve multiple slide paths.
  • May occur on very gentle terrain, even slopes of 20 degrees or less.
  • May be localised to specific elevations, aspects, or regions.
  • There are often no visible signs of persistent slab instability. Lack of avalanche activity and lack of danger signs are NOT reliable indicators of stability.
  • Compression tests and Rutschblock tests may locate persistent weak layers.
  • Stabilise slowly, tending to persist for several weeks or longer.
  • Often have dormant periods, becoming active again when the weather changes.
  • Are prone to lingering pockets of instability that persist long after most areas have stabilised.
  • Tend to release above the trigger, making it difficult to escape.
  • Are often triggered remotely from a long distance away.
Management: Persistent slabs are best managed by very conservative terrain choices. Allow extra time for persistent slabs to stabilise and use a very cautious approach to new terrain. Be especially cautious after storms or during warming periods.
 

Deep Persistent Slabs:

  • Slide on deeply buried persistent weak layers, which often form during clear periods or rain-on-snow events early in the season. Involve thick, hard slabs, sometimes the entire snowpack.
  • Tend to be very large, commonly cross terrain barriers to involve multiple slide paths.
  • Tend to occur on larger slopes of moderate steepness, typically 30-40 degrees.
  • May be localised to specific elevations, aspects, or regions.
  • There are often no visible signs of persistent deep slab instability. Lack of avalanche activity and lack of danger signs are NOT reliable indicators of stability.
  • Stabilise slowly if at all, persisting for months and often the entire season.
  • Dormant persistent deep slab instabilities often become active again when the weather changes, especially after storms or with warm spring weather.
  • Tend to release above the trigger, making it difficult to escape.
  • Are often triggered remotely from a long distance away.

Management: Deep persistent slabs are best managed by very conservative terrain choices and a very cautious approach to new terrain. Be especially cautious after storms or during warming periods.

Be aware that you are often dealing with more than one avalanche problem at the same time. Depending on conditions, an individual avalanche problem might exhibit the combined characteristics of multiple avalanche character types.

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