RAVEN SKI CLUB
AVALANCHES
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About avalanches

Please note that this information has been taken from a leaflet produced by the Scottish Mountain Safety Group.

Avalanches can happen wherever there is snow lying on ground of sufficient angle. In recent years there have been accidents in most Scottish mountain areas, as well as the English Lakes, the Cheviots, the Pennines and Wales.

The vastly increased popularity of winter climbing and hill walking along with the growth of interest in ski touring and off piste skiing, means that greater numbers are at risk. 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 an avalanche might be possible.

In making practical assessments of avalanche risk, there is no substitute for knowledge of snow conditions which comes from years of experience. The novice or the less frequent winter mountain user will 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 "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. An avalanche occurs when one layer slides on another, or the whole snow cover slides on the ground. 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 form the surrounding snow and slide out.

In practice, any snow slide big enough to carry a person down is important. All avalanche configurations are illustrated in the SAIS "Avalanche Recording Form".

How do avalanches occur?

Three main factors determine whether avalanches are likely to occur - the weather, the snowpack and the terrain.

Weather

This is the most important factor in determining whether avalanches are likely, and the evaluation 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 such as the one on Cairngorm are available on Ceefax. The information provided on temperature, wind speed and direction often enables us to make useful predictions before leaving home. For instance, if a SW wind of 25mph is indicated with freezing temperatures and snow is know to be lying, then it may be assumed that some avalanche risk will be building on NE facing slopes.

Local advice can often be obtained regarding weather. 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 and on the SAIS published avalanche risk outlook, 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 localised 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 slope, but on a small, safe slope of similar orientation.

There's no need to dig to ground level, only down to the first reasonably thick layer of neve (refrozen old snow). The snow layer may then be identified by smoothing the back wall of the pit and probing with a finger all the way down.


The following features should be looked for:

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

These observations may be supplemented by a shovel test. Despite the name, a shovel is not necessary. Your ice axe and gloved hands will suffice.

To perform the test, isolate a wedge shaped block, cutting down to the top of the next identified layer. If the top layer slides spontaneously, a very poor bond exists between the layers. If it does, then try to rate the ease with which you can pull the snow block off by inserting your shovel/ice axe/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. As you climb, digging stances, cutting steps or placing deadmen, all give you an opportunity to make a quick check on surface layers.

These techniques should enable you to make an educated risk assessment. Remember that your snowpit observations will hold good only for slopes of similar orientation 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.

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

Many avalanches are cornice triggered and in general, climbing below cornices should be avoided:

When walking above cornices, take care to give them a wide berth. The figure shows the possible fracture line.

Terrain

On most hills in Britain, avalanche risk 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 predisposed 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.

Ridges or buttresses are better choices than open slopes and gullies when avalanche conditions prevail. The crests of mountain ridges are usually protected from avalanches, 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 check list six top factors

  1. Visible avalanche activity. If you see avalanche activity on a slope where you intend to go, go somewhere else.
  2. Rapid slab/new snow build up (more than 2cm/hr) may produce unstable conditions. More than 30cm continuous build up is regarded as very hazardous.
  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 the higher the risk, even if thaw does not occur.
  6. Feels unsafe. The "seat of the pants" feeling of the experienced observer deserves respect.

Travelling in risk areas

It is rarely essential to negotiate an avalanche prone slope. It is usually possible to find another way, or retreat. If you must proceed, the following should be borne in mind.

If caught

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

If buried

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

Avalanche rescue

If you witness an avalanche burial:

SAIS snow and avalanche reports

These are prepared every day mid-December to mid-April for the Glencoe, Lochaber and Cairngorm areas. Glencoe includes Glen Coe and Glen Etive hills. Cairngorm includes all Northern Corries area. Note: Reports are for areas outside developed ski areas.

Avalanche recording

SAIS keeps records of avalanche occurrences in Scotland and elsewhere in Britain. We would appreciate hearing from you if you are involved in an incident or if you witness any avalanches. Avalanche recording forms are available from club secretaries or direct from:

SAIS Coordinator,
Freepost, Glenmore Lodge,
Aviemore, Inverness-shire PH22 1BR


Recommended reading

Acknowledgments

The Scottish Avalanche Information Service is administered and funded by the Scottish Sports Council. The assistance of the following is gratefully acknowledged: Cairngorm Chairlift Company, Aviemore. Nevis Range Development Company, Fort William. White Corries Ltd, Glencoe. Association of Chief Police Officers Scotland.

The Scottish Sports Council,
Caledonia House, South Gyle,
Edinburgh EH12 9D.


 

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