Parts Of An Avalanche

a. Starting zone. It is usually steeper than 30 degrees and receives large amounts of snow. This is where the unstable snow breaks loose and starts to slide.

b. Avalanche Track. Refers to the path under the starting zone and above the run out zone. They can be channeled or unchanneled. The track is the slope or channel down which snow moves.

(1) Channeled tracks are confined areas such as gullies and couloirs. Unconfined tracks are on open slopes. Some may have trees present.

(2) An avalanche track may have several branches or several small tracks having separate starting zones that may feed into one big track. It is important to remember that multi-branch tracks may run several times in quick succession. A number of rescuers have been killed when working a run out zone and a second avalanche ran down within hours of the first avalanche.

(3) Wet snow avalanches tend to follow the track boundary, whereas dry snow avalanches can easily jump terrain barriers.

c. Run out zones. This is the area at the bottom of the path where debris piles-up. Variation in weather patterns from one year to the next will influence the position of the run out zone. This is where the snow and debris slows down and comes to rest

4. AVALANCHE HAZARD EVALUATION PROCESS. The evaluation process is the interaction of four critical variables, which helps determine whether or not, an avalanche is possible. They are snow pack, weather, terrain, and the human factor.

a. The Snow Pack. Is the snow capable of sliding? As each storm passes, a new layer of snow is added, some by the wind some not, and every layer of snow has it's own texture and strength. Some layers will be strong while some will be weak. Some will bond well, and some won't. Although the study of snow metamorphism is a science, Marines must determine if a weak bond in the snow pack exists. Additionally, one must attempt to estimate the amount of snow that could be potentially released, if triggered.

b. The Weather. Is the weather contributing to instability? It is an observed fact that all natural avalanches occur during or shortly after a storm. Why? The snow pack can't handle the new weight being added. This new weight alters the balance in strength and stress. The three main contributing factors are the precipitation, wind, and temperature.

(1) Signs of Instability.

(a) Recent avalanche activity on similar slopes and small avalanches underfoot.

(b) Booming. The audible collapse of snow layers.

(c) Visible cracks shooting out from underfoot.

(d) Sluffing debris, which is evidence of avalanche activity occurring.

(e) Sunballing, which is caused by rapid warming of the snow surface.

(f) Weather patterns.

1. Heavy amount of snow loading in a short period of time. (1 in/hr for 24 hr period).

2. Heavy rains which warms and weakens the snow pack.

3. Significant wind loading causing leeward slopes to possibly become overloaded.

4. Long, cold, clear, calm period followed by heavy precipitation or wind loading.

5. Rapid temperature rises to above freezing after long a cold period.

6. Prolonged periods (e.g. more than 24 hrs) of above-freezing temperatures.

7. Cold snow temperatures (equal to or less than 25F) slow down the settlement or strengthening process, thus allowing unstable snow conditions to persist longer.

(2) Signs of Stability.

(a) Snow cones or settlement cones form around trees and other obstacles and indicate the snow around the object is settling.

(b) Creep and Glide. Creep is the internal deformation of the snow pack. Glide is slippage of the snow layer with respect to the ground. Evidence of these two properties on the snow pack is a ripple effect at the bottom of a slope. It is an indication that the snow is gaining equilibrium and strength through this type of settlement process.

(c) Absence of wind during storms which is indicated by snow accumulation in the trees.

(d) Snow temperatures remaining between 25 & 32F ordinarily settles snow rapidly, creating a denser and stronger snow pack.

c. The Terrain. Being able to recognize avalanche terrain is a critical step in the evaluation process. Assuming that avalanches occur on only big slopes is a very common mistake. Avalanches can occur on any slope.

(1) Slope Angle. Slope angle should always be factored when planning movements in snow covered mountainous terrain.

(a) As the slope angle increases, so does the stress on the boundary regions of a slab.

(b) Most slab avalanches release on slopes with angles between 35-40 degrees.

(c) Loose snow avalanches occur on high angle slopes 60 degrees and above.

(2) Slope Orientation.

(a) Leeward, wind-loaded slopes tend to increase the stress on the snow pack.

(b) Snow packs moderately hit by the sun can strengthen and stabilize the snow pack.

(c) Direct sunlight has the opposite effect by weakening and lubricating the bonds between grains.

(d) Weak layers are often well developed or persist on shaded slopes due to the colder conditions and absence of solar warming during the winter. Suspect instability on these slopes.

(3) Terrain Roughness (Anchoring). Slopes with anchors are less likely to avalanche than open slopes.

(4) Vegetation. The most convincing evidence of past avalanche activity is a path of fallen trees, aligned in the same direction and sheared at the height above the ground.

(a) Trees void of branches on the uphill side, which are called "flagged" trees.

(b) Cleared strips of trees in a dense forest.

(5) Seasonal. Once an avalanche path has begun to slide in a season, other avalanches may occur a long the same path.

(6) Elevation. Temperature, wind and precipitation often vary significantly with elevation. Common differences include rain at lower elevations or differences in precipitation amounts, or wind speed with elevation. Never assume that conditions on a slope at a particular elevation reflect those of a slope at a different elevation.

(7) Local Population. A good source of information but beware of short-term observations, i.e., 10 years.

d. The Human Factor. What are your alternatives and their possible consequences?

e. Hazard Evaluation. This should be an on going process, and should start before the mission even begins.

(1) Before. Gather information on the weather such as new snowfall, high winds, snow advisories, and topography of the terrain. Find out any past history of the area and recent or past avalanche activity.

(2) During. During your movement, try and fine-tune any information that may help in your decision-making process and support the fact that there may be an avalanche hazard.

(3) Be Objective. We don't have options when assigned missions. You have to look at the overall mission with the present avalanche hazard to determine whether you should continue on or choose an alternate route.

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