Avalanche Awareness

An avalanche is an uncontrolled movement of snow, and can happen anywhere with significant snowfall and steep slopes especially in areas where skiing, snowboarding and snowmobile are done.

They are triggered when stress applied to a snow pack exceeds the amount that snow can absorb.   This can happen from either excessive loading or weakening of the snow pack.

Figure 1 Typical avalanche plan.   B. The three parts of an avalanche path:   Starting zone, track, and runout zone.

It is essential for winter sportsmen going to avalanche areas to become avalanche aware.

  Many travel into avalanche prone areas without appreciating the risk they are at.

Avalanches are natural occurrences and may be anticipated under certain circumstances.

Ingredients for avalanches include:

  Steep terrains (slopes greater than 30 o )

Smooth slopes

Exposure to wind and sun, heavy wind, rain or snow

Rapid temperature change

Poor bonds or layers in the snow

  Prolonged periods of very cold or very warm temperatures

Loose release (aka point release or sluffs ) avalanches.

Start with a small amount of snow that picks up as the collapse spreads down a slope, leaving a fan shaped path.

Loose Avalanche

Slab Avalanches

Occur when snow slabs release as cohesive forces between layers of snow are weakened.   Slabs form usually at a 30-50 degree slope but most occur near 38 degrees.

Slab avalanches cause most of avalanche deaths.

  Wind erodes loose snow from windward slopes and deposits it onto downward or leeward slopes into dense cohesive layers.

  This breaks up the usual crystalline snowflake structure.

Broken flakes are then densely packed. Knowing which way the wind blows helps mountaineers recognize the leeward sides of mountains where slabs will tend to accumulate.

Slabs will fail or release when the slab exceeds the bonding strength between layers.   This can be from a rapid buildup of stress or internal weakening of the supporting layers.

When a slab fails at its weakest point it will fracture along a crown line.   With very hard slabs this crack will resemble a rifle shot .

Slab Avalanche

Forces Involving Avalanche Triggers

The amount of force or action that upsets the snow mass stability (or trigger) will depend on how close to instability the snowmass is.

Extra forces may be applied to the snowmass or pre-existent stability may decline.

The balance of forces within a snowpack will affect its stability.

Compressive forces are greatest at the bottom of a slope or where the slope angle decreases.

Tension forces reflect the elastic strength of the pack or layer.   In warm weather a layer may be elastic (and accommodate elastic deformation) but at colder temperatures more brittle (more likely to fracture).

  The tension and force of gravity are greatest at the top of the snowmass and where the slope angle decreases. Snow is able to remain at sloped areas because of the elastic bonding strength of individual crystals resisting gravity.

Friction forces exist between different layers as a result of tension and compressive forces. When the shear strength of the bond between layer s is overwhelmed (coefficient of friction) the avalanche will start.   Because of the tremendous forces released during an avalanche the snow will behave like a liquid (allowing the victim to swim) and then rapidly gel to a solid, totally trapping the victim.

Avalanche triggers include the weight of the person, heavy new snow, earthquakes, intense warming from sun, rain, rock falls, rapid cooling, and the collapse of a cornice (a hardened ridge crest).

Direct action avalanche triggers occur when a slab fails just after new snowfall where the weight of new snow overwhelms the snowpack ability to absorb stress. Usually the victim starts this. A snowpack will readjust to stress within 48 hrs. after snowfall.   Travel during or just after a snowstorm is the most dangerous time.

Indirect action avalanche triggers are when a slab is weakened and destabilized.   Very cold temperatures may predispose to formation of ice crystals, depth hoarwhich weakens the cohesiveness of internal layers.

Avalanche Awareness

Before venturing into avalanche country the most important thing to prepare is having a good understanding of the avalanche risk. Groups should choose a leader and establish group goals, have a pre-agreed rescue plan, be proficient in performing an avalanche search, and be wary of avalanche warning signs.  

Mountaineers are trained to read the terrain for physical signs of avalanche danger.   Snow dumping quickly from tree branches is assign of very recent snow settlement.

Signs of unstable snow also include cracks shooting underneath skis, a hollow sound of snow while moving over it (a whoomph sound).

This indicates tension in the snow pack or a hidden weak layer.

Evidence of Past or Recent Avalanches

  Trees without branches

  Chutes through forested areas

  Steep slopes grater than 30 degrees

  Other geographical features include changes in slope angle; narrow gullies, and leeward slopes.

Avalanche Terrain

The worst avalanche prone slopes (38 degrees) also coincide with black diamond and double black diamond ski runs.   Widely spaced trees are considered more risky than dense clumps of trees.

Trees may also be “avalanche flagged” where half of their branches are missing from previous avalanche passage.

Aspens and willows are opportunistic and invasive species and their presence may be used as a clue that avalanches are frequent on a slope since more slow growing trees would not be as successful.   Buried bushes or small trees may contribute to the weakening of a snow slab even if they are not seen.

Avalanche Flagged Tree

Leeward slopes (facing away from prevailing winds) are more prone to be stacked with snow.

South facing slopes receive more sunlight and tend to stabilize more quickly.

Sunballs are snowballs that are released off a slope by warmed snow or small wet sluffs and are signs of destabilization.

North facing slopes are slower to stabilize and are prone to the weakening effects of cold temperatures in mid winter. Shady north faces collect ice crystals easily.

Terrain traps like confined gullies and bowls are frequent sites of avalanche fatalities.

Weather Conditions

Warning weather signs include: heavy snowfall, strong winds and drifting snow, long periods of hot or cold temperatures, rain, warm winds and rapid changes in temperature

New snow at greater than 1 inch per hour or 12 inches in 24 hrs. are high risk.

New snow at greater than 2 inches per hour or 24 inches in 24 hrs. are extreme risks.

Rain at 3 inches in 24 hrs also is extreme risk.

Avalanche Equipment

Basic avalanche equipment that should be carried by everyone in the backcountry:

Avalanche Transceivers (beacons) are beacons that locate buried victims. Travelers should switch their beacon to transmit while in avalanche country.  

The various types Display a signal based on the distance to other beacons, but are not directional.   Rescuers using their beacons on reception have to follow a disciplined search patter in order to find buried victims in time.

Proper search drills are necessary to become efficient in this.

1. Sturdy light shovels (may be collapsible).   They should be strong, lightweight and not bulky.   Those made with treated aluminum or Lexum are good.

2. Collapsible avalanche probe to help locate victim.

3.  Slope measurement device (clinometer).   The clinometer is a card with a free hanging arrow that allows an estimate of slope.   It uses gravity as a plumb line as a reference.   This makes a slope evaluation more subjective since a person may be standing on a slope while looking at another.

4.   Compass

5. First aid winter wilderness survival kit.

6. Hand lens are used to examine snow for the presence of ice crystals.

Testing Snow Stability

Different tests have been devised to grade the risk of an avalanche.   Avalanche courses teach these skills.

Safe Travel in Avalanche Country

1. Avoid steep terrain greater than 30 degrees.

2.   Avoid trigger points.

3. Use safe terrain to your advantage like dense forests and broad ridge tops while avoiding confined gullies, leeward slopes and areas of steep slope changes.

4. Consider the possibility of avalanches and have all transceivers set to transmit.

5.   Have a leader and a follow through plan if an accident occurs.

6. Travel one at a time with everyone watching the lead person.   Remember even if one person passes it still might not be safe.

7.   Move quickly towards areas of safety and don’t linger.

8. Wait 48 hrs. if possible after a snowstorm before traveling.

9. Tighten zippers, gloves and loose clothing.   If wearing a pack release the waist so it can be removed quickly.   Remove ski pole straps and loosen the bindings on skis and snowboards.

Surviving and Avalanche

As soon as the snow starts to move each person must escape to the side or grab a tree.

People should shout, (to alert others), then immediately close their mouth to avoid inhaling snow.

Next, if knocked down people should try to swim.

Discard packs and equipment including ski poles and skis.

If pushed forward, attempting a breaststroke will help.

If feet first, the person shall try to roll onto the back and attempt to skull with arms and legs similar to treading water.

When the flow seems to be slowing, the victim should be alert that the snow will start to harden in the next 1-2 seconds. This is the best time to reach for the surface and thrust forward with swimming motions.

Getting a head, leg or, arm to the surface will help improve survival, as this will aid rescuers.

Use one hand to clear a space around the mouth.

Those who fight the hardest while being buried do the best . A victim may be dead from the physical force alone and survivors must be carefully checked for signs of severe trauma including neck injuries. Victims who have not had lethal injuries may succumb unless rescued quickly so every effort must be made to ensure a fast rescue.

If the last seen point is known, mark it immediately. Usually it is safe to run onto the snow surface (unless another slab is still hanging above where the fracture occurred). Investigations should be thorough. Snow should be kicked or turned to look for clues. Using probes will help. Often victims are found in uphill sides of trees and rocks or bends in the slope where the snow slows.

Self-Rescue devices:

The avalung www.avalung.com , is a vest device that allows victims to breath through a larger air space provided by the vest prolonging avoidance of asphyxiation.

The ABS www.abssystem.com , attempts to provide immediate floatation for those caught in an avalanche, preventing burial and increasing survival.

Natural forces we cannot control form avalanches. Avalanches constantly occur in the wilderness but only become problems when they involve human activity. Respecting the danger and reacting quickly is the best way to avoid problems.

This pamphlet is no substitution for an avalanche course.

References :

  Cyberspace snow and Avalanche Center, www.csac.org

  West side Avalanche Network www.avalanche.org

  Nova online http://www.pbs.org/wgbh/nova/avalanche/

  Avalanche Awareness http://nsidc.org/snow/avalanche/

  Produced by the Skylark Medical Clinic

264 Tache Ave.

Winnipeg, MB   R2H 1Z9

ph: 453-9107 fax: 453-9115

www.skylarkmedicalclinic.com