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Avalanche Awareness

Definitions

• Point release avalanche - small amount of loose snow breaks away and widens as it descends
  • generally occur on steeper slopes (35+ degrees) during or after snowstorms or warming events
  • can trigger larger slab slides
• Slab avalanche - layer(s) of cohesive snow break(s) away as a unit
  • entire snow pack can slide on grass or rock surface that may be lubricated with water
• Cornice collapse - wind generated cornice breaks away
  • can trigger larger slab slides due to the stress placed on the wind-loaded slope below
• Ice avalanche - ice block (serac) breaks away from steep or overhanging part of a glacier
• Terrain trap - v-shaped valley below steep slopes that will collect/concentrate avalanche debris
• Temperature Gradient - difference in temperature over a certain depth of snowpack
  • high TGs occur in thinner snowpacks during cold weather and weaken snowpack
  • low TGs occur in thicker snowpacks during warm weather and strengthen snowpack
• Depth hoar - non-cohesive layer of facets (angular, poorly bonding crystals) within the snowpack
  • classic weak layer; dangerous when under a cohesive layer (slab)
• Surface hoar - winter equivalent of morning dew
  • can cause very thin bed surface layer in snowpack once buried

Contributing Factors

• Terrain
  • slope angle
    • most slabs release from starting zone angles of 30 - 45 degrees
    • steeper slopes shed snow via point release slides, reducing slab development
    • less steep slopes lack gravitational stress, reducing slide potential
  • slope aspect
    • slabs typically form on leeward (wind-loaded) slopes
    • moderate sun warming can strengthen snowpack
    • strong, direct sun warming can weaken snowpack by lubricating grain bonds
    • shaded slopes can contain persistent weak layers due to lack of solar warming
  • slope shape
    • convex slopes: most dangerous; slabs usually fracture just below the "roll-over" but can fracture above roll-over if snow is highly sensitive
    • straight slopes: tricky because slabs can break free anywhere
    • concave slopes: least dangerous; slabs can be triggered from below by disturbing the supporting snow; lower zone may also be a terrain trap
  • slope anchoring
    • anchors (boulders, trees, brush, etc.) can hold snow in place (until buried)
    • anchors can be starting points of fractures that may propagate to other anchors
• Snowpack
  • snowpack changes continuously becoming either more stable or less stable
  • slab avalanche potential exists when:
    • strong cohesive snow overlies weaker snow
    • strong cohesive snow is not bonded well to underlying layer
  • snow grains
    • rounds -round; formed in warm conditions within thick snowpack (lesser temperature gradient) and result in well-bonded cohesive layer (slab)
    • facets - angular; formed in cold (or shaded) conditions within thin snowpack (or just near trees/rocks) (greater temperature gradient) and result in poorly-bonded collapsible layer
    • wet - found in melting/freezing temperatures (or rain) and result in stable snowpack when frozen and unstable snowpack when melted due to free water lubrication
• Weather
  • snowstorms add weight stress to snowpack
    • especially significant following long period of calm cold that caused weak, faceted layer or surface hoar
  • rain adds weight and lubricates/erodes grain bonds
  • wind transports snow from windward slopes to leeward slopes
    • creates compacted, well-bonded layers (slabs) on the leeward side
    • creates a thin snowpack on the windward side (promoting facets)
    • creates cornices
  • temperature considerations
    • warm snowpacks (28
      +) settle (compress), becoming denser and stronger
    • cold/shaded snowpacks can remain unstable for weeks (slowed settlement)
    • rapid, intense warming can cause instability due to weakening of grain bonds
    • storms that begin cold and end warm are more likely to produce slides due to poor bonding to the existing surface layer that cannot support the denser snow deposited as temperatures increase

Warning Signs

• Recent rapid weather changes (snowfall,intense warming,rain,wind)
• Recent slide activity (especially on similar slope angle, aspect, and elevation)
• Swaths of no trees (or small, damaged trees) between forested areas (caused by past slides)
• Shooting cracks (caused by the release of stored elastic energy through cohesive snow)
• Smooth "pillows", cornices, and surface drift patterns (caused by recent wind loading)
• Cupping texture on the surface and small rolling snowballs (sunballs) (caused by warming)
• Whumphing sounds (caused by collapsing weak layers within snowpack)
• Hollow sounds (caused by less consolidated (potentially weak) layer under denser layer)

Safe Travel

• Never travel alone
• Research the route, check weather and avalanche forecasts and identify alternatives
• Plan a preferred route, alternate route, escape route, and emergnecy evacuation route
• Travel with shovel, probe, beacon and be competent with them
• Be aware of slight increases in slope angle and subtle changes in slope aspect
• Be aware of changing weather conditions
• Be aware of consequences of a slide
• Avoid traveling under steep slopes and above terrain traps/cliffs
• Avoid release zones (convex roll-overs, protruding rocks/brush, cornices)
• Favor windward side of ridges
• Minimize time spent in hazards
• Minimize number of travelers simultaneously exposed to hazard
• Stay within sight of each other and watch each other
• Never travel above others on same slope
• Plan your escape route(s) before entering hazard
• Be suspicious of tender spots (near rocks/brush, roll-overs, shaded aspects)
• When ascending slopes:
  • favor gentle angles and slope margins
  • use ridge routes but be mindful of cornices
• When crossing slopes:
  • favor gentle angles or well into runout zones or as high as possible if no hazards below
  • travel from safe spot to safe spot preferably slightly downslope in same track to minimize exposure time and stress on snowpack
• When descending slopes:
  • favor slope margins and slopes that are fully visible with no terrain traps/cliffs below
  • optionally trigger slope before skiing using a ski cut or cornice cut
• If terrain is a red flag:
  • Do not continue on route if snowpack is a red flag
  • Consider alternate route if weather is a red flag
  • Consider alternate route if both snowpack and weather are marginal

Words of Wisdom

• Applying safe travel technique does not justify moving through hazardous terrain.
• Avalanche forecasts are regional, not site specific.
• Summer routes are not necessarily safe winter routes.
• Ski tracks on a slope indicate nothing about the stability of the underlying snowpack.
• Every avalanche cycle must have its first slide.
• Most human-triggered slides occur on the blue sky days after storms.
• A slope that is not prone to slide does not mean it won't.
• If you find yourself feeling reassured because you're wearing a beacon, reconsider you're route.
• Avoid these mistakes: poor planning, poor communication, denial, haste, tunnel vision, laziness, blindly following the leader, peer pressure, indecision, complacency, summit fever, ego, fatigue

Snowpack Tests

• Ski pole test (perform often to get less localized, more comprehensive information)
  • push pole handle slowly into snowpack feeling for relative hardness/thickness of layers
  • provides no information about how well layers are bonded to each other
• Snowpit test (produces only very localized information)
  • isolate a shovel-width, 5-foot tall vertical column of snow and perform the following tests:
    • stratigraphy test - lightly brush side wall to reveal hard/soft layers
    • resistance test - run credit card down side wall noting hard/soft layers
    • hardness test - insert finger(s)/pencil/knife into layers noting relative hardness
    • compression test - place shovel on column top and strike from wrist, elbow, then shoulder until column fractures, noting force required
    • shear test - place shovel (or ski) behind column and pull forward until column fractures, noting force required
• Rutschblock test (produces only very localized information)
  • isolate a ski length wide, ski pole deep, 5-foot tall block, stand on with skis, and perform knee-bends, hops, then jumps until block fractures, noting force required

10 Point Checklist (AVALANCHES pneumonic)

• Angle and shape of slope (30
  - 45
  ? roll over?)
• Vegetation (above tree line? small/damaged/missing trees? anchors or tender spots?)
• Aspect of slope (shaded? sun-baked? leeward? corniced?)
• Local weather effects (snow-loaded? wind-loaded? sun-baked? rain-soaked? prolonged cold?)
• Avalanche report (compare local report to slope angle, aspect, altitude)
• Noticed avalanche activity (slides on similar slopes? shooting cracks? collapsing snowpack?)
• Consequences of avalanche (cliff or gully below? significant vertical feet?)
• Hollow sounds underfoot (drum-like/whumphing sounds?)
• Escape/return routes (route out if it slides? route back if it doesn't)
• Snowpack stability tests (snow pit? hand pit? pole probe? ski cut? cornice cut?)

Photos Backcountry Huts Journals Vail to Aspen Ouray to Telluride Alaska Documents Avalanche Awareness Gear Checklist