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About Avalanches
Please note that this information has been taken from a leaflet produced for the Scottish Mountain Safety Forum.
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17th December 2001.
IntroductionAvalanches can happen wherever there is snow lying on ground of sufficient angle. Accidents in recent years in most Scottish mountain areas, as well as the English Lakes, the Cheviots, the Pennines and Wales, demonstrate the truth of this in the UK context. 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 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. Avalanche configurations are illustrated in the SAIS "Avalanche Recording Form".WeatherThis 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 such as that on Cairngorm are available on Ceefax. 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, whilst forecast is available from the telephone numbers listed below. 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 hazard outlook, should be considered. SnowpackWhen 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 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:
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. 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 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. 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. CornicesMany avalanches are cornice-triggered. In general, climbing below cornices should be avoided:
When walking above cornices, take care to give them a wide berth. Fig. 2 shows the possible fracture line. TerrainOn most hills in Britain, 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.
Travel in hazard areasIt 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
If caughtIn 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.
If buried
Avalanche rescueIf you witness an avalanche burial:
Avalanche recordingSAIS 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, SAIS snow and avalanche reportsThese are issued every day mid-December to mid-April for the Glencoe, Lochaber, Creag Meagaidh Northern Cairngorms and Southern Cairngorms areas.GLENCOE includes Glen Coe and Glen Etive hills. LOCHABER includes Ben Nevis range to Aonach Mor NORTHERN CAIRNGORMS includes Northern Corries and Loch Avon basin CREAG MEAGAIDH includes the north Laggan hills westward to Beinn a' Chaoruinn SOUTHERN CAIRNGORMS includes Lochnagar and Glen Shee Note: Reports are for areas OUTSIDE developed ski areas. SAIS/Nevisport Reports Freephone 0800 0960 007 Snow and Avalanche Reports are also available from:
Further InformationRECOMMENDED READING
AcknowledgmentsThe SportScotland Avalanche Information Service isadministered and funded by the Scottish Sports Council. The assistance of the following is gratefully acknowledged:
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