Avalanche ABCs for Snowshoers
by Tom Janisch
with illustrations from Allen & Mike's Avalanche Book by Allen O'Bannon and Mike Clelland (Falcon Guides)
Chester Marler Photo: Point release slides on the side of Lichtenberg Mountain near Stevens Pass.
WHAT IS AN AVALANCHE?
A snow avalanche is a mass of snow moving down slope which may also contain ice, soil, rocks, trees or other debris.
Some sobering statistics to consider:
- Over 90% of victims triggered the avalanche that killed them.
- Almost all avalanche fatalities involve recreationists.
- Over the past 20 years about 500 people have died in avalanches in the US.
- Over 90% of avalanche fatalities are male.
- The vast majority of avalanche incidents occur in the backcountry.
- 2/3 of avalanche victims were not carrying basic rescue gear-similar to the percentage of drowning victims who did not wear personal flotation devices.
- In almost 40% of avalanche fatalities the victim was wearing a beacon.
- 75% of avalanche victims did not consult the avalanche advisory.
TYPES OF AVALANCHES
1) Loose Snow:
- Also called point releases, start with a small amount of cohesionless snow and typically pick up more snow as they descend.
- Can sweep into rough terrain, e.g., over cliffs or into trees.
- Loose snow releases occur most often on slopes above 35 degrees during or shortly after a snowstorm or during warming events caused by rain, rising temperatures, or solar radiation.
- Can also trigger larger and deeper slab release.
Illustration from Allen & Mike's Avalanche Book by Allen O'Bannon and Mike Clelland (Falcon Guides).
3) Cornice Collapse:
- Slab avalanches occur when one or more layers of cohesive snow break away as a unit. As the slab travels down slope, it splits up into smaller blocks or clods.
- Slab thickness and width can vary greatly.
- They move faster and more powerfully than you can move, and even a small slab can kill or critically injure.
- Slabs may be hard, soft, wet or dry.
- The upper boundary is called the fracture line or crown face.
- The area immediately above this is referred to as the crown.
- The sides of the slab are called flanks.
- The bottom boundary is called the stauchwall.
- The main sliding surface under the slab is called the bed surface.
- The bed surface is a critical boundary because it typically has a surface area roughly 100 times greater than the surface area of all of the other boundary regions combined.
4) Ice Avalanche:
- Cornices form when windblown snow builds out horizontally at sharp terrain breaks such as ridge crests or sides of gullies.
- Cornices can break off well back from their edges.
- A cornice fall can trigger an avalanche.
- Caused by the collapse of unstable ice blocks in steep part of glacier.
EVALUATING AVALANCHE HAZARD
Four critical variables:
- Terrain. Is the terrain capable of producing avalanches?
- Snowpack. Could the snow slide?
- Weather. Is the weather contributing to instability?
- People. What are the alternatives and possible consequences?
1. Avalanche path: the area in which an avalanche runs.
2. Starting zone: where the snow breaks loose and starts to slide.
3. Track: the slope down which the snow moves at a fairly uniform speed.
4. Run out/deposition zone: where the snow slows down and comes to rest.
- In large avalanches the run out zone can include a powder blast zone that extends far beyond the area of the snow deposition.
- Any steeper snow-filled slope is a potential avalanche path.
- Always consider slopes above you. A valley bottom may not be safe.
1. Slope angle is an important terrain variable determining whether or not it is possible for a given slope to avalanche. As the slope angle increases so does the stress exerted on all of the boundary regions of the slab.
2. Most slab avalanches release on slopes with starting zone angles between 35 and 40 degrees. Statistically the most common bed surface slope angle is 37-38 degrees.
3. Continually ask yourself if the slope you are on/connected to is steep enough to slide.
4. Practice with an inclinometer.
5. Use this very useful map system from CalTopo to show you the slope angles along your entire route. This mapping system overlays slope angles over all the 7.5-minute USGS maps in the country (use the 'Search' function to find a landmark near your route).
1. Which direction is the slope facing relative to both recent winds and the sun?
2. Be suspicious of leeward, wind-loaded slopes because the deposition of wind-transported snow increases the stress on the snowpack and enhances slab formation.
3. Moderate warming by the sun can help strengthen and stabilize the snowpack.
4. Intense, direct sun can have the opposite effect by weakening and lubricating the bonds between grains.
5. On shaded slopes weak layers often persist or are more developed because of generally colder conditions.
1. Terrain Anchoring.
2. Slope Shape.
- Boulders, trees, and ledges act as anchors and help hold snow in place until the anchors are buried.
- A grassy slope might avalanche with only 1 foot of snow.
- Convex, or rollovers, while fun to ski, are most likely to fracture just below the bulge where the stresses are the greatest.
- Concave slopes are capable of avalanching and it is often difficult to predict where the fracture will initiate.
Vegetation can indicate an avalanche zone. Look for:
- Swaths of open slope;
- Trees that are bent, broken, missing branches on the uphill side;
- Presence of disaster species such as alders and willows.
Temperature, wind and precipitation often vary significantly with elevation. Never assume that conditions at one site will reflect those at a different elevation.
SNOW STABILITY EVALUATION
Snowpack Layering and Bonding:
The snowpack accumulates layer by layer with each new snow or wind event. These layers are subject to change throughout the winter. Many combinations of strong and weak layers can exist within the snowpack. Strong doesn’t necessary mean stable.
Slab avalanche potential exists when relatively strong, cohesive snow overlies weaker snow or is not bonded well to the underlying layer.
Illustration above and below from Allen & Mike's Avalanche Book by Allen O'Bannon and Mike Clelland (Falcon Guides)
- Faceted grains develop when a significant temperature gradient exists within or between layers.
- Faceted grains are sometimes referred to as sugar snow.
- It is particularly sensitive as a weak layer when subjected to significant loading of new or wind transported snow.
- Favorable conditions for the development of faceted grains are cold weather and/or a thin snowpack.
- Can be a persistent weak layer.
Surface hoar or hoar frost:
Upside down snow:
- The wintertime equivalent of summertime dew, is formed at the snow surface during cold, clear weather.
- The crystals are loose, feathery, and poorly bonded.
- Can be a persistent weak layer.
- Lighter dry snow that is followed by wetter denser snow.
- In the melt phase, wet grains weaken rapidly and are lubricated by the presence of free water. This can be the slush slides in the spring.
- Rounded ice pellets that act like little ball bearings when loaded.
- Often rolls down slope and can collect in depressions or at the bottom of cliff bands.
Avalanche warning signs:
1. Recent Avalanche Activity on Similar Slopes.
2. Whumping Noises.
3. Shooting Cracks.
4. Recent Wind Loading.
5. Changing weather.
Avalanche tests while Snowshoeing or Ski Touring:
1. Approach small but steep hills from the top and jump.
2. Ski pole test (limited).
3. Ski cuts (caution!).
4. What happens to the track on uphill turns (may just be surface)
5. Hand shears (may just be surface).
Examples of Weather Patterns Resulting in Unstable Snow Conditions:
1. A lot of new snow in a short time period.
2. Heavy rain adding weight but no strength.
3. Long, cold clear, calm followed by heavy precipitation and/or wind loading.
4. Storms that start out cold and end warm.
5. Rapid warming to near or above freezing after prolonged cold weather
6. Intense solar radiation and can happen with thin layers of clouds.
7. Strong winds with snow transport.
HUMAN DECISION-MAKING FACTORS LEADING TO AVALANCHE ACCIDENTS
1. Attitude (High Acceptance of Risk).
2. Ego (Some excellent, skilled skiers thinks they can out ski any avalanche).
3. Poor planning (Not checking weather report; avalanche forecast; not planning route).
4. Laziness (e.g., shorter to cut across avalanche slope rather than climb the longer, but safer route around it).
5. Peer Pressure (statistically among males).
6. Poor Communication (not looking out for partner or discussing observations).
7. Slope Familiarity (generally slopes are stable most of the time).
8. Summit Fever (and taking risk because of it).
9. Fatigue (diminishing judgment).
Remember: ALWAYS UTILIZE AVALANCHE FORECASTS AND CONDITION REPORTS!
EXTRA INFORMATION NOT COVERED IN THE SESSION
Sample Field Pit Tests:
1. Isolate a column about the size of a shovel (1ft X 1ft).
2. Dig 3-4 feet deep (don’t want to miss a weak layer skiers or snowshoers could trigger)
3. Tap 10 times wrist (Easy); 10 times elbow (Medium); 10 times shoulder (Hard).
4. Number at failure is CT and total number of taps.
1. On a safe test slope of at least 30
degrees, completely isolate a block of snow about a ski length across
and a ski pole up slope 3-4 feet deep.
2. Step onto the upper
third of the block while wearing skis and jump progressively harder
until the block fails. The rating is as follows:
Level of Activity and Corresponding Degree of Stability
1. Fails while excavating: Extremely unstable.
2. Fails while approaching: Extremely unstable.
3. Fails while flexing knees: Extremely unstable.
4. Fails with one jump: Unstable.
5. Fails with two jumps: Potentially unstable.
6. Fails after repeated hard jumps: Relatively stable.
7. Doesn’t fail after repeated hard jumps: Stable or very stable
Quality of the Shears
1. Breaks on a clean, smooth break like spring-loaded: Q1
2. Breaks on a smooth plane, but not spring-loaded: Q2
3. Breaks on a rough or broken plane: Q3
Hardness Test for Layers
1. Fist: Very Soft
2. Four Fingers: Soft
3. One Finger: Medium
4. Pencil: Hard
5. Knife: Very Hard
NOTE: FIELD TESTS MAY NOT BE REPRESENTATIVE OF THE WHOLE SLOPE;
MAY BE IMPROPERLY DONE; MAY BE MISREPORTED OR MISUNDERSTOOD
- Allen & Mike's Avalanche Book by Allen O'Bannon and Mike Clelland (Falcon Guides).
The illustrations in this article are from this thin but very useful book. Unlike many avalanche books, this one is an easy read, and the illustrations are considerably more valuable than photos. Despite the brevity and apparent simplicity of this book, it arms you with the practical information needed to keep you safe in avalanche country.
- Arguably the fastest and easiest
research resource for analyzing the the snow conditions, avalanche
forecasts, and coming weather for Central Washington is to use this matrix. From one place you can quickly link to all the regional resources.
Our thanks to Cascade Subaru for helping sponsor this series of classes and for helping get more people outside enjoying winter.