Avalanches

Avalanches can happen wherever there is snow lying on ground of sufficient angle. Accidents in recent years in most mountain areas, demonstrate the truth of this statement. The vastly increased popularity of winter climbing and hill walking, along with the growth of interest in ski touring means that greater numbers are at hazard.  Sadly, each year adds to the list of injuries or fatalities.  Many of these accidents would have been avoidable, given greater care or knowledge, or if the victims had even paused to consider that avalanche hazard might be present.

In making practical assessments of avalanche hazard, there is no substitute for the instinctive feeling for snow conditions which can be gained only by years of experience. However, no-one is born with such experience and the novice or the less frequent winter mountain user, may still enjoy a safe day out if some basic principles are learned and acted upon.

Avalanches Can Happen To You.

Having accepted this, you have greatly reduced your chance of ever being involved in an avalanche.  Remember that experience in itself is no antidote to avalanches and that "the avalanche does not know you are an expert!"

What Is An Avalanche?

Snow is deposited in successive layers as the winter progresses.  These layers may have dissimilar physical properties and an avalanche occurs when one layer slides on another (Surface Avalanche), or the whole snow cover slides on the ground (Full-Depth).  An avalanche may be Dry or Wet, according to whether free water is present in the snow.  It may be of Loose Snow, when the avalanche starts at a single point or a Slab Avalanche which occurs when an area of more cohesive snow separates from the surrounding snow and slides out.  In practice, any snow slide big enough to carry a person down is important.

Weather

This is the most important factor in determining whether avalanches are likely, and the evolution of the snowpack is entirely dependent on this.  However, as the mountaineer can study both of these, it is useful to do so.

Many weather variables affect avalanche release and information can often be gained before setting out.  Readouts from summit weather stations can be beneficial.  The information provided on temperature, wind speed and direction often enables useful predictions to be made before leaving home.   For instance, if a SW wind of 25mph is indicated with freezing temperatures and snow known to be lying, then it may be assumed that some avalanche hazard will be building on NE - facing slopes.

Local advice can often be obtained regarding recent weather, while forecasts are always available. Remember that mountain weather is particularly difficult to predict and the likely influence of unexpected changes in weather, both on your own expectation as to snow stability should be considered.

Snowpack

When visibility is adequate, snowpack observation can begin from the roadside.  Evidence of recent avalanche activity, main snow accumulation zones, fresh loading by new snow and drifting, can often be noted from below.

Observations can continue on the approach, noting such details as depth of foot penetration, cornice build up, ease of release of small slabs and the effect which localized wind patterns may have had on slab formation.

Any suspect slopes which must be negotiated (bearing in mind that the safest course is to avoid them) may be tested by digging a snowpit.  Pits should not initially be dug on the main suspect slope, but on small, safe slopes of similar orientation.

There's no need to dig to ground level, but only down to the first reasonably thick layer of nevé (old re-frozen snow).  The snow layers may then be identified by smoothing the back wall of the pit and probing with a finger all the way down.  This will help assess the hardness of the layers.   The following features should be looked for:

  • Adjacent layers of different hardness. (difference of more than 2 on a scale of 5).
  • Water drops squeezed out of a snowball made from any layer.
  • Layers of ice.
  • Layers of graupel (rounded, heavily rimmed pellets).  These act like a layer of ball bearings in the snowpack.
  • Feathery or faceted crystals.
  • Layers of loose, uncohesive grains.
  • Air space.
  • Very soft layers. (fist penetrates easily)

Any of the above might be the source of a dangerous weakness in the snowpack.

These observations may be supplemented by a shovel test (see Fig 1).  For this, a shovel is not necessary.  Your ice axe and gloved hands will suffice.

Having made the snowpit observations, isolate a wedge shaped block, cutting down to the top of the next identified layer.  If the top layer then slides spontaneously, clearly a very poor bond exists between the layers.   If it does not, then try to rate the ease with which you can pull the block off by inserting your shovel/axes/hands behind the block and pulling.  Do this for each suspect layer in your pit. Performing this test many times will help you to build up a "feeling" for the stability of the layers.

Techniques such as this should enable you to make an educated hazard assessment. Remember that your snowpit observations will hold good only for slopes of similar orientation and altitude to your test pit.  You will need to extrapolate for situations higher up, for instance below cornices, where surface windslab layers may be much thicker, etc.

An attempt should be made to rate the slope Safe, Marginal, or Unsafe.  Even if a slope is Marginal or Unsafe, it may be possible to choose a safe route by careful selection.

Cornices

Many avalanches are cornice-triggered.   In general, climbing below cornices should be avoided:

  • During snow storms or heavy drifting
  • Immediately (24-48 hours) after these.
  • During heavy thaw or sudden temperature rise.

When walking above cornices, take care to give them a wide berth.  Fig. 2 below, shows the possible fracture line.

Terrain

On most hills, avalanche hazard can be avoided by sensible choice of route.

Slope angle.  Most large slab avalanches run on slopes between 25 and 45 degrees.  This range includes the average angle of coire backwalls and approach slopes to crags.

Ground surface.  Smooth ground such as rock slab is pre-disposed to full-depth avalanches. Rough ground such as large boulders will tend to anchor base layers in position, making avalanches less likely.  Once these boulders are covered, however, surface avalanche activity is unhindered.

Slope profile.  Convex slopes are generally more hazardous than uniform or concave slopes.  The point of maximum convexity is a frequent site of tension fractures, with the release of slab avalanches. (Fig. 3)

Ridges or Buttresses are better choices than open slopes and gullies when avalanche conditions prevail.  The crests of main mountain ridges are usually protected from avalanche, while in climbing situations, rock belays on ribs and buttresses can often provide security.

Lee Slopes should be avoided after storms or heavy drifting. Their location will obviously vary according to wind direction, but will include the sheltered side of ridges and plateau rims.

 

AVALANCHE CHECKLIST
TOP SIX FACTORS

  1. Visible avalanche activity. If you see avalanche activity on a slope where you intend to go, go somewhere else.
  2. New snow build-up.  More than 2 cm/hr may produce unstable conditions.  More than 30cm continuous build-up is regarded as very hazardous.  90% OF ALL AVALANCHES OCCUR DURING SNOWSTORMS.
  3. Slab lying on ice or neve, with or without aggravating factors such as thaw.
  4. Discontinuity between layers, usually caused by loose graupel pellets or airspace.
  5. Sudden temperature rise.  The nearer this brings the snow temperature to 0 degrees C, the higher the hazard, even if thaw does not occur.
  6. Feels unsafe.  The "seat of the pants" feeling of the experienced observer deserves respect.

Travel In Hazard Areas

It is rarely essential to negotiate an avalanche-prone slope.   It is usually possible to find another way, or retreat.   90% OF ALL AVALANCHES INVOLVING HUMAN SUBJECTS ARE TRIGGERED BY THEIR VICTIMS.    If it is essential to proceed, the following should be borne in mind:

  • Solo travelers in avalanche terrain run particularly grave hazards.
  • Skiers are in greater danger than walkers - the lateral cutting action of skis readily releases unstable snow. All off-piste skiers should use avalanche transceivers and have them SWITCHED ON before leaving base.   They should carry collapsible probes and shovels.  Climbers and walkers should also consider the use of these items.
  • Direct descent or ascent is safer than traversing.
  • Go one at a time - the others should closely observe the progress of the person on the suspect slope.
  • Close up clothing.  Wrap scarf or other item around mouth and nose.
  • Belay if possible.  This is rarely feasible on wide, open slopes.

If Caught

In most avalanche situations, any defensive action is very difficult.  Movement relative to the debris is often impossible.   However, some of the following may be useful.

  • Try to delay departure by plunging ice axe into the undersurface. This may help to keep you near the top of the slide.
  • Shout.  Others may see you.
  • Try to run to the side, or jump up slope above the fracture.
  • If hard slab, try to remain on the top of a block.
  • Get rid of gear, sacks, skis etc.
  • Try to roll like a log off the debris.
  • Swimming motions sometimes help.
  • As the avalanche slows down, you may be able to get some purchase on the debris. Make a desperate effort to get to the surface, or at least get a hand through.

If Buried

  • Keep one hand in front of your face and try to clear/maintain an air space.
  • Try to maintain space for chest expansion by taking and holding a deep breath.
  • Try to avoid panic and conserve energy. Your companions are probably searching for you.

Avalanche Rescue

  If you witness an avalanche burial:

  • Observe the victim's progress and if possible mark the point of entry and point at which last seen.
  • Check for further avalanche danger.
  • Make a QUICK SEARCH of the debris surface.
    - LOOK for any signs of the victim.
    - LISTEN for any sounds.
    - PROBE the most likely burial spots.
  • Make a SYSTEMATIC SEARCH, probing the debris with axes or poles.
  • Send for help.
  • KEEP SEARCHING until help arrives.
  • REMEMBER, YOU ARE THE BURIED VICTIM'S ONLY REAL CHANCE OF LIVE RESCUE. Although survival chances decline rapidly with duration of burial, they do not reach zero for a long time.

Why Worry About Avalanches?

Snow avalanches are a natural process, occurring perhaps 1,000,000 times per year, world-wide.  They are one way for snow on an incline to adjust to the pull of gravity. The vast majority of these slides are not a problem, because an avalanche, in and of itself, is not a hazard.  A person (or a person's stuff) has to get involved in order for there to be a problem.

The fact is, that avalanches don't drop from the peaks onto the heads of unsuspecting innocents with the unpredictability of a plummeting meteorite.  95% of people who are caught in avalanches are caught by a slide that was triggered by themselves or a member of their party.  If it is our behavior that is creating the hazard, then we can change our behavior to avoid problems.

Avalanche Characteristics

There are many different types of avalanches, but the one that worries us the most is the "slab" avalanche, in which a mass of cohesive snow releases as a unit.  This type of avalanche is usually easily recognized by it's distinct crown and flanks.  Slab and other avalanches can be hard or soft, wet or dry and can be triggered naturally or artificially.

Spotting Avalanche Hazard - The Avalanche Triangle

Some place have avalanches: Switzerland's Alps, Utah's LaSals, Nepal's Himalayas.  Some places don't:  South Viet Nam, the Mile-High Stadium, your living room (the distinctions are not always so obvious, unfortunately).

      Why is this?

Avalanches are formed by a combination of 3 things that together are known as the "Avalanche Triangle".  These 3 ingredients may be present in one location but absent 10 feet away.  The 3 legs of the triangle are Snowpack, Terrain and Weather.

Avalanche victims are almost exclusively backcountry recreationists--snowmobilers, climbers, snowboarders, snowshoers, skiers and hikers. Snowmobilers lead the list with twice the number of fatalities as any other activity.

  

  

More In-depth Information And Tips...

How You Can Judge Dangerous Avalanche Terrain

  • Steepness.  Most avalanches occur on slopes between 35 and 45 degrees.  Slopes less than 30 degrees seldom produce avalanches and slopes steeper than about 50 degrees sluff so often that they tend not to build up into slabs.  So it's the intermediate slope steepness that produces most of the avalanches.  But the bad news is that exactly the kind of slopes we like to ski, snowboard or snowmobile usually produce most of the avalanches.  A black diamond slope at a ski resort is usually around 35 degrees--prime steepness for producing avalanches.
  • Anchors.  Trees and rocks that stick up through the snowpack can help to hold the snowpack in place.  But the anchors need to be quite thick to be effective.  For instance a thick, mature grove of evergreen trees anchor the slab quite effectively while a sparse grove of aspen trees have very little effect.
  • Aspect with respect to wind.  Recently wind-loaded, steep slopes are almost always very dangerous while recently wind-eroded slopes are usually fairly safe.
  • Aspect with respect to sun.  In the Northern Hemisphere as temperate latitudes, the direction a slope faces (aspect) is very important.  For instance, north facing (shady) slopes usually produce more avalanches and more persistent avalanche hazard in mid winter.  On the other hand, in the spring when wet avalanches occur from strong sun, south facing slopes produce more wet avalanches.  At equatorial or Arctic latitudes, the aspect with respect to the sun has very little effect.
  • Consequences.  What will happen to you if the slope slides?  It's very difficult to survive an avalanche if it strains you through thick trees or dumps you over a large cliff or deposits you into a crevasse or dumps you into a narrow gully (creating a very deep burial).  On the other hand you have a fairly good chance of survival on a small avalanche path, without obstacles and a gentle run-out.
  • The best sign of avalanches are other avalanches.  You can't get much more obvious than that.  But it's surprising how often people miss this clue.
  • Collapsing snow.  When you hear the snowpack collapse catastrophically with a giant "whoomph", that's the sound of the snowpack screaming in your ear that it's extremely unstable.  Stay off of steep slopes and stay out from underneath steep slopes.
  • Cracking snow.  Recent wind loading, especially, creates cracking snow.  The longer the crack, the more dangerous.   Stay off of steep slopes.
  • Avalanche weather. Just like people, avalanches do not like RAPID changes.
    • Recent rapid loading of new or windblown snow
    • Recent rapid warming
    • Recent rapid melting
    • Rain on new snow

     

  • Never put everyone on the slope at once.    There always needs to be someone left behind to do the rescue.  Someone always needs to remain in a safe place while their friend(s) are on the dangerous part of the slope.  With large groups, split them in half and stay in visual and voice contact.
  • Have an escape route planned.  Always think avalanche.  What will you do if the slope slides.  Have a plan first.
  • Use slope cuts.  Keep your speed up and cut across the starting zone, so that if the slope slides, your momentum can carry you off the moving slab into safer terrain.  You can do this on skis, snowboards or on snowmobiles.
  • Watch out for cornices.  They always break farther back than you think.  Always give them a wide berth.  NEVER, NEVER walk out to the edge of a drop-off without first checking it out.  Many people have died this way.
  • If it looks too dangerous, find a safer alternative.  Use terrain to your advantage.  Follow ridges, thick trees and slopes with safer consequences.  You can almost always go back the way you came.   The route got you there, it will most likely get you back as well.
  • If there's no other choice, go underground. You can almost always weather out a bad storm or bad avalanche conditions by digging a snow cave or seeking the shelter of a crevasse.  You may be uncomfortable but you will be alive.

Tips On Building A Snowcave

1. Despite tales of grizzled old mountain men surviving by digging holes in the snow with their bare hands, you must have a snowshovel.   This is a prime reason snowcaves aren't a viable emergency shelter because when was the last time you carried a snowshovel on a routine winter outing?  So, if you can, carry a small snowshovel.

2. It is usually takes 2 to 6 hours of exhausting digging and excavating to build a snowcave but when completed is well worth the effort and can mean the difference between life and death!

3. It's virtually impossible to stay dry building a snowcave.  If you manage not to soak yourself while groveling about on your hands and knees, then all that digging surely will soak your clothes with sweat.   Having a change of clothes is extremely beneficial.

4. Snowcaves aren't as warm as legend holds.   If it gets too warn inside, the walls run with water and the ceiling can collapse.   Still, a snowcave can keep you warmer than a tent if you manage to build it without soaking yourself - thereby avoiding hypothermia - and you build it correctly.

5. Dig tag-team fashion.  One person rests while the other digs.  This reduces fatigue, as well as sweat buildup.

6. Try to dig from a standing position.   This also reduces fatigue and keeps you drier.

7. In soft snow, a big aluminum grain shovel is ideal.  In hard snow, the nylon/plastic/aluminum avalanche safety shovels are better.  With any shovel, avoid burying it and then prying back on the handle, otherwise you'll snap the blade.

8. A snow saw is useful for cutting uniform snowblocks.

9. Take extra care to make the sleeping platform perfectly smooth and level. During the night, your body heat will help turn it into a bed of ice and any errant lumps will be quite apparent and uncomfortable.

 

Snowcave entrance.jpg (30862 bytes)

Entrance To Snow Cave

US Danger Scale 

United States Avalanche Danger Descriptors

Danger Level Avalanche Probability and Avalanche Trigger Degree and Distribution of Avalanche Danger Recommended Action in the Backcountry

...WHAT...

...WHY...

...WHERE...

...WHAT TO DO...

LOW

 

Natural avalanches very unlikely. Human triggered avalanches unlikely. Generally stable snow. Isolated areas of instability. Travel is generally safe. Normal caution is advised.
MODERATE

 

Natural avalanches unlikely. Human triggered avalanches possible. Unstable slabs possible on steep terrain. Use caution in steeper terrain on certain aspects (defined in accompanying statement).
CONSIDERABLE

 

Natural avalanches possible. Human triggered avalanches probable. Unstable slabs probable on steep terrain. Be increasingly cautious in steeper terrain.
HIGH

 

Natural and human triggered avalanches likely. Unstable slabs likely on a variety of aspects and slope angles. Travel in avalanche terrain is not recommended. Safest travel on windward ridges of lower angle slopes without steeper terrain above.
EXTREME

 

Widespread natural or human triggered avalanches certain. Extremely unstable slabs certain on most aspects and slope angles. Large, destructive avalanches possible. Travel in avalanche terrain should be avoided and travel confined to low angle terrain well away from avalanche path run-outs.

AVALANCHE SAFETY BASICS

Avalanches don't happen by accident, and most human involvement is a matter of choice, not chance.   Most avalanche accidents are caused by slab avalanches which are triggered by the victim or a member of the victim's party.  However, any avalanche may cause injury or death and even small slides may be dangerous.  Hence, always practice safe route finding skills, be aware of changing conditions, and carry avalanche rescue gear.   Learn and apply avalanche terrain analysis and snow stability evaluation techniques to help minimize your risk. Remember that avalanche danger rating levels are only general guidelines.  Distinctions between geographic areas, elevations, slope aspects and slope angles are approximate and transition zones between dangers exist.  No matter what the current avalanche danger there are avalanche-safe areas in the mountains.