This page could stop an Avalanche
Avalanches can happen wherever there is snow lying on ground at a 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".
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. 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 25 mph 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 forecast information 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.
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 it 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 is no need to dig to ground level, but only down to the first reasonably thick layer of nevé (old refrozen 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 it. This will help assess the hardnesses of the layers. The following features should be looked for:
- Adjacent layers of different hardness (difference of more than 2 on scale of 5)
- Very soft layers (fist penetrates easily)
- Water drops squeezed out of a snowball made from any layer
- Layers of ice
- Layers of graupel (rounded, heavily rimed pellets) - these act like a layer of ball-bearings in the snowpack
- Feathery or faceted crystals
- Layers of loose, uncohesive grains
- Air space
Any of the above may 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.
Many avalanches are cornice-triggered. In general, climbing below cornices should be avoided:
- During snowstorms or heavy drifting
- Immediately (24-48 hrs.) after the above
- During heavy thaw or sudden temperature rise
When walking above cornices, take care to give them a wide berth. Fig. 2 shows the possible fracture line.
On most hills in Britain, avalanche hazard can be avoided completely 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 slabs will predispose to full-depth avalanches. Rough ground has 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 fracture, with 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 storm 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 Areas
It is rarely essential to negotiate an avalanche-prone slope. It is usually possible to find another way, or retreat. 90% OF AVALANCHES INVOLVING HUMAN SUBJECTS ARE TRIGGERED BY THEIR VICTIMS. If it is essential to proceed, the following should be borne in mind:
- Solo travellers 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 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 round mouth and nose.
- Belay if possible. This is rarely feasible on wide, open slopes.
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 an 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 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.
- Keep one hand in front of you 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.
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 victims.
- 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.
Avalance Checklist - Top 6 Factors
- Visible avalanche activity. If you see avalanche activity on a slope where you intend to go, go somewhere else.
- New snow build-up. More than 2 cm/hr may produce unstable conditions. More than 30 cm. continuous build-up is regarded as very hazardous. 90% OF ALL AVALANCHES OCCUR DURING SNOWSTORMS.
- Slab lying on ice or nevé, with or without aggravating factors such as thaw.
- Discontinuity between layers, usually caused by loose graupel pellets or airspace.
- Sudden temperature rise. The nearer this brings the snow temperature to zero degrees C, the higher the hazard, even if thaw does not occur.
- Feels unsafe. The "seat of the pants" feeling of the experienced observer deserves respect.
SAIS keeps records of avalanche occurrences in Scotland and elsewhere in Britain and would appreciate hearing from you if you are involved in an incident or if you witness any avalanches. To report an avalanche click here.
Snow and Avalanche Reports
These are issued every day mid-December to mid-April for the Glencoe, Lochaber; Creag Meagaidh, Northern Cairngorms and Southern Cairngorms; Torridon is also covered during a reduced period.
sportscotland Avalanche Information Service (SAIS) Website: www.sais.gov.uk
Note: Reports are for areas OUTSIDE developed ski areas.
Setting the Map
Ticking off features
Taking & Following a Compass Bearing
Estimating Distance Travelled
Aiming Off, Attack Points, Handrails
Symbols and Grid References
Safety and skills information is provided courtesy of Mountaineering Scotland