Avalanche Information

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The following article provides very detailed information about avalanches, the potential hazards and dangers they create, how to predict and avoid them, and how to deal with them if they occur. We recommend that the article be printed off and studied in depth. This article provides very useful information in conjunction with an approved avalanche preparation and safety course.

Avalanches are one of the greatest hazards facing winter travellers in the mountains. Every year people die from avalanches that they trigger and then get buried by. The victims only hope for survival is a fast and organized rescue from the rest of the party. Learning to avoid avalanche terrain is essential to safe winter travel in the mountains. There are several good books listed at the end of this site, but nothing can replace a good course about avalanche awareness and experience in the field.

Avalanches require three factors

1. Snow
2. Sufficient slope
3. Instability within the snowpack

Unless all of these conditions exist there is no risk of an avalanche.

There are two types of avalanches, loose snow and slab avalanches. Loose snow avalanches begin from a single point and expand as they descend. The slide path looks much like an upside down V. Loose snow avalanches are usually fairly minor, but in exposed climbing situations, or late in the spring they can be very serious.

Slab avalanches are the greatest threat to skiers, climbers, snowmobilers, etc. A slab is a cohesive layer of snow that has not bonded well with the layer below it. As a result it is under stress, supporting its own weight on a slope. When the stress within the snow layer exceeds the strength of the snow the slab releases much like a pane of glass when it shatters under its own weight. The trigger may be another storm, a change in temperature, or the weight of a person.

Recognizing Avalanche Terrain

Slope angle, orientation to wind and sun and the presence of trees all influence avalanche potential, and are topics covered in an avalanche course. Field tests which identify snow instabilities and the presence of slabs are also taught in avalanche courses.

The Avalanche Danger Scale

These guidelines describing avalanche probability should be used with your judgement, experience and local knowledge. The criteria as shown in the table below are taught in avalanche courses. Always enquire of local experts and guides, and carefully observe conditions.In Canada a standard hazard rating is used to describe the avalanche risk for an area. This system appears to work well, but must be used only as a general assessment. Local conditions and micro-terrain may be very different and requires constant attention.

Avalanche Hazard Rating (Danger Scale)

Danger Level and Colour	Avalanche Probability and Trigger Size	Recommended Action
Low (green)	Natural slab avalanches highly unlikely; human triggered releases unlikely; sluffs possible.	Travel is generally safe; normal caution advised.
Moderate (yellow)	Natural slab avalanches unlikely; loose or human triggered slab releases possible.	Use caution in steeper terrain on certain aspects.
Considerable (orange)	Natural slab or loose avalanches possible; human triggered slabs probable.	Use increasing caution. Be aware of potentially dangerous areas.
High (red)	Natural, human triggered slab or loose avalanche likely.	Travel in avalanche terrain not recommended; safest travel on windward ridges or lower angle slopes without steeper terrain above.
Extreme (red with black border)	Numerous natural avalanches certain and slabs easily triggered by humans.	Travel in and near avalanche terrain should be avoided; travel only in low angle terrain well away from avalanche path routes.

Precautions to take in Avalanche Terrain

Avoid large steep, lee bowls, gullies and cornices. Choose safer routes: ridge tops, valley floors, dense timber and low angle slopes.

If you choose to ski, board or ride an avalanche slope, minimize your risk.

Never have more than one person on a slope at any time. Don't stop in the middle of the slope, move to a safe place. Watch each person on the slope. Stay in voice contact. Have a plan: know who is going to go first, where to stop and where is the escape route.

A note to snowmobilers who play highmark: To increase your chances of keeping a game from turning into a nightmare, pick windward slopes rather than lee slopes. Start on the edge of a slope at lower angles. Don't "centre-punch" the slope! Make sure that only one machine is on the slope at any time, even if your buddy gets stuck. Nature itself provides many clues to the potential of avalanches. We refer to these as Nature's Billboards - Do not ignore them. They are nature's way of telling you when conditions are unstable.

Nature's Billboard	Instability Message
Recent Avalanches on Similar Slopes	These is no better clue! Avoid slopes of similar angles, aspects and elevation which have not yet released.
Whumping(dull thudding) Noises	Whumps happen when a weak layer collapses within the snowpack. Nature is screaming at you! Avoid avalanche prone slopes and runouts.
Cracks	Cracks in the snow indicate that you can trigger slab avalanches.
Recent Wind Loading	Wind loaded snow forms slabs. Evidence of wind loading includes cornices, smooth pillows of snow and drift patterns.
A Sudden Warming Trend	A rapid prolonged rise in temperature, particularly above freezing, may cause avalanches as the snow weakens.
Hollow Sounds	Drum-like hollow sounds from the snowpack as you travel indicate a potentially weak layer is underneath a denser layer.
New Snow	Most avalanches occur during or soon after periods of prolonged or heavy snowfall.

Remember: Weather is the architect of avalanches. Coastal, Columbia and Rocky Mountain weather conditions and snowpacks vary greatly, knowledge of one area does not necessarily work in another

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Slope

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Slope Angle

Avalanches can occur on slopes ranging from 10-60 degrees, but are most common between 30 to 45 degrees. As a rule of thumb, if it is steep enough for good skiing, it can avalanche.

10-25	Wet snow and slush avalanches can occur but are uncommon
25-30	Wet loose snow avalanche, slab avalanche ( not common, but when it occurs this is often a very large slide)
30-45	Slab avalanches very common
45-55	Small slab avalanche common
30-60	Loose snow avalanche (spin drift and larger)

Type of Slope

With all other factors being equal, the shape of the slope can increase the probability of an avalanche. Convex slopes are less supported and more likely to avalanche, especially near the top of the slope. Concave slopes are better supported making them generally more stable.

Orientation to Wind

Lee slopes are a potential hazard because wind deposited snow creates unstable slabs. Windward slopes generally have less snow, which is compacted, and are usually more stable. In the absence of wind, slopes of any aspect may avalanche.

Ground Cover

The nature of the surface of the slope can have a great effect on the stability of the area. Smooth slabs and grassy meadows are good sliding layers for an avalanche. Boulder fields, shrubs, and small trees can be good anchors for the snow until they become buried, but must be assessed cautiously. Shrubs can bend over time due to snow creep and become much less effective later in the season. Boulders can change the temperature gradient in a snowpack and weaken the base. These anchors must be close together and numerous to anchor the slope. Trees offer good anchors and protection in avalanche terrain only if they are close together. In general if you can comfortably downhill ski through the forest, then it is open enough to potentially avalanche. On rare occasions a large slab avalanche can destroy well forested areas.

The Human Factor in Avalanches

Attitude - People sometimes ignore Nature's Billboard due to pride, ego, machismo or ambition.

Pressures of Time - Avalanches don't care that you have to work on Monday. Be prepared to wait for safer conditions.

Blue Sky - Sunny weather draws people into serious terrain, sometimes too soon after a storm.

Herding Instinct - People are more ambitious and bolder in larger groups. Large numbers increase the avalanche hazard. Speak up if you are unsure.

Be willing to turn around and come back another day.

Stability/Hazard Evaluation

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Without a weakness in the snowpack there can be no avalanche. Hard slabs above softer layers, buried surface hoar, faceted layers, and buried crusts are all common examples of an instability. The presence of these features should indicate caution and further investigation about the potential avalanche hazard.

Determining the hazard of an area begins before the trip starts. Information can be gathered from many sources:

• Parks Canada
• Canadian Avalanche Association
• Local Ski Operations
• Local Guides
• Individuals recently in the area

Weather information for the area is also very useful. You can determine if the hazard may soon change, and use the records to forecast what the snowpack may look like.

Field Observation

One of the most critical tools is to observe conditions traveling to and skiing in the area. Look for:

Recent Avalanche Activity	Have there been recent avalanches in the area? How large are they? On what aspects are the sliding, and how frequently? At what elevation are they starting? How deep are they? What do they appear to be sliding on? This is an excellent indicator of snow stability and where the most sensitive areas may be.
Rapid Settlement or "Whumping"	Do you feel the snowpack collapse under your feet? Do you hear the snow go "whump" when you ski on it? Does the snow feel or sound hollow beneath your feet? Any of these conditions suggests that the snowpack is very unstable and the avalanche hazard is high.
Cracks radiating out from your skis	Are cracks rapidly spreading out from your skis like stress fractures? This indicates the presence of a wind slab and potential instability.
Weather Changes	Is the temperature rising quickly? Are the winds transporting snow and loading lee slopes? Is it snowing heavily? Is it raining? These conditions all suggest that the avalanche hazard may be increasing.
Recent Snow	Has 30 cm or more new snow fallen within a short period of time? How long has it had to settle? How warm is it? How much wind was there when it fell? New snow needs time to settle and bond. The more snow that has fallen in a short period of time, the greater the risk. Mild winter temperatures (-1 to -10) will promote rapid settlement.
Wind	How strong is the wind, in the valley and above treeline? Is there evidence of scouring, cornices, and or fat pillows? What direction is the prevailing wind? Wind loading can produce dangerous loading and slabs on lee slopes.

Snow Profile and Stability Tests

If you want to find out what's happening within the snowpack, dig in to it. A snow profile can give you very good information about the layers within the snowpack. Stability tests in conjunction with a profile will help identify weak bonding layers and potential instability.

Snow Profile

For most field purposes digging a quick, hasty pit is sufficient to determine major and important layers within the snowpack. The pit should be dug in a safe area that represents the slope in question as closely as possible. Snow in the area should not be disturbed, or have any rocks, or vegetation interfering with the profile. The pit should be dug to the bottom of the snowpack or at least 2 metres deep, with the front wall at least 1 metre wide, 1.5 is preferred. The front wall and whichever side wall is in better shade are cut vertically and brushed with a shovel or mitt to help define the various layers. Use a card, finger, or thermometer case to probe the sidewall and help identify each layer.

Shovel Shear Test

This test is used to identify potentially weak layers and their location in the snowpack. Isolate a column of undisturbed snow 0.35-0.4 metres deep by 0.25 metres across. Remove soft and very soft snow from the top of the column (fist and four finger snow). Cut the back of the column with a snow saw, but do not cut deeper than 0.7 metres from the top of the column. Insert the shovel at the back of the column, and with both hands pull the shovel towards you in an even, increasingly stronger pull. If the column breaks in a clean smooth manner, record the height of the failure, force required, and observe the crystals at that site. If the column fails to break, or produces an uneven shear, level the column above the height of the 0.7 metre back cut, saw to 0.7 metres from the new top and repeat.

Term	Description
Very Easy	Fails during cutting or insertion of shovel
Easy	Fails with minimum pressure
Moderate	Fails with moderate pressure
Hard	Fails with firm sustained pressure
Collapse	Block fails when cut
No Shear	No shear failure observed

The shovel shear test must be completed several times to obtain useful information. A single test is not sufficient to provide good information.

Compression Test

This test also identifies weak layers in the snowpack, especially near the surface of a snowpack. The compression test is particularly useful in the Canadian Rockies or where shallow snowpacks are common. Isolate a column as explained in the shovel shear test, but do not remove the surface snow layers, and cut the column to ground ( If column collapses cut a second column as far as possible without collapse). Place the shovel flat on top of the column. Tap the blade and observe the snow for failures. Ratings

Term	Description
Very Easy	Fails during cutting
Easy	Fails with 5 to 10 light taps using fingers only
Moderate	Fails with 5 to 10 moderate taps from the elbow using finger tips
Hard	Fails with 5 to 10 firm taps from whole arm using palm or fist
No Failure	Does not fail

Rutschblock Test

This is much like a giant compression test that give good information about the stability of a suspect slope. Isolate a block of snow on a representative slope in a safe location, 1.5 metres upslope, and 2 metres across. The slope angle should be at least 25 degrees steep on undisturbed snow with no natural anchors and an even snow pack. The back of the column can be cut with a ski, pole, large snow saw, or cord. If the lower snowpack is very solid with no weak layers ( as determined by a shovel shear test ) then it is not necessary to cut the block to ground. Once this is completed, a skier (with skis on) of average weight approaches from above the column and gently steps on to the top centre of the block. Stability is rated by how much force it takes for a layer of snow to fail.

Score	Loading Step Producing a Clean Shear Failure
1	The block slides during digging or cutting
2	The skier approaches the block from above and gently steps down onto the upper part of the block
3	Without lifting the heels, the skier drops from straight leg to bent knee position, pushing downwards and compacting the surface layers.
4	The skier jumps up and lands in the same compacted spot.
5	The skier jumps again on to the same compacted spot.
6	For hard or deep slabs, remove skis and jump on the same spot. For soft slabs or thin slabs where jumping without skis might penetrate through the slab, keep the skis on, step down another 35 cm., almost to mid-block and push once then jump three times.
7	None of the loading steps produced a smooth slope parallel failure.

Interpreting the Rutschblock

While this test does give fairly good results it is important to realize that it alone can not determine slope stability. Results can vary dramatically depending on where on the slope the test was done. This is only one more piece of information in determining the avalanche hazard.

1,2,or 3	The slope is unstable. It is likely that a similar slope would release under the weight of a skier.
4,5	The slope stability is suspect. It is possible that a similar slope would produce a skier triggered avalanche
6,7	A similar slope is unlikely to avalanche with the additional load of several skiers

Test standards were taken directly from:

Observation Guidelines and Recording Standards for Weather, Snowpack and Avalanches
Prepared by the Canadian Avalanche Association 1995
ISBN 0-9699758-0-5

Avalanche Size

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In Canada avalanches are defined by the size of the avalanche and amount of material within it. They are classed from 1 to 5. A class 1 could knock a person over but not completly bury them; a class 5 would destroy a town. Like earthquakes each avalanche class is 10 times as powerful as the previous size. A class 2 is 10 times stronger than a class 1, and a class 3 is 10 times more powerful than a class 2 and 100 times more powerful than a class 1.

Size	Avalanche Destructive Potential	Path
1	Relatively harmless to people	10m
2	Could bury, injure, or kill a person	100m
3	Could bury and destroy a car, damage a truck, destroy a small building, or break a few trees	1000m
4	Could destroy a railway car, large truck, several buildings, or a forest area up to 4 hectares (10 acres)	2000m
5	Largest avalanche known. Could destroy a village or a forest of 40 hectares (100 acres)	3000m

If caught in an Avalanche:

• Shout so your group knows you need help.
• Try to discard equipment: skis, poles, board.
• Swim to stay on the surface. Work your way to the side.

As the Avalanche Slows:

• Fight to reach the surface.
• Try to get a hand above the surface so that it can be seen.
• Make an air space in front of your face with your other arm.
• Try to relax to conserve energy

Searching for a Victim: The most experienced person should:

• Take a moment to organize the search party.
• Consider the possibility of another avalanche before attempting a rescue. Set an escape route and post a lookout if necessary.
• Mark the last seen point of the victim(s).

Search with Beacons:

• Ensure all rescuers' beacons are switched to receive.
• Spread out at 30 meter intervals to cover the debris.
• Look for surface clues: a hand, hat, glove, etc.
• Don't litter the slope with rescuers' belongings.
• Search around and downhill for any clues.
• When a signal is picked up assign one or two to locate it while the others continue the search.
• Pinpoint the signal to a small radius, then probe to find the victim.
• Without removing the probe, quickly dig out the victim.
• Turn the victim's beacon to receive if others are still buried.

Searching Without Beacons:

• Have all rescuers vigorously search for surface clues.
• Probe the most likely areas: around clues, in line below the last seen point; around trees, rocks. etc.
• Concentrate on the last few meters of the debris and other areas of deposition.
• After a thorough search by random probing, set up an organized probe line. Start at the base of the debris and work your way up the most likely trajectory.
• There is a 60% chance of finding someone with a probe line. If unsuccessful, cover the area again.

Practice your rescue plan before you need it. Over 50% of people completely buried in avalanches do not survive after 20 minutes. You do not have time to go for help. YOU ARE THE HELP.

References and Courses

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These references were used to check the accuracy of the material in this website. They are also recommended reading for more information. Avalanche Awareness courses through Rocky Mountain Hiking will be posted.

For general information regarding avalanche conditions in Western Canada contact:

• Avalanche Information 1-800-667-1105

• Avalanche Safety for Skiers and Climbers Second Editon
  Tony Daffern, Rocky Mountain Books 1992
  ISBN 0-921102-15-1
  
  
• The Avalanche Handbook
  David McClung and Peter Shearer, 1993
  The Mountaineers, Seattle Washington
  
  
• Canadian Avalanche Association
  Training Schools Avalanche Course Manual
  1995
  
  
• Observation Guidelines and Recording Standards for Weather, Snowpack and Avalanches
  Prepared by the Canadian Avalanche Association1995
  ISBN 0-9699758-0-5
  
  
• Canadian Avalanche Association
  Box 2759 Revelstoke, BC
  V0E 2S0
  Phone # 604-837-2435
  Fax 604-837-4624
  Email Canav@mindlink.bc.ca
  
  

For up to date avalanche information, please see http://www.avalanche.ca/snow/