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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:
- Adjacent layers of different hardness. (difference of more than 2 on a 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 rimmed pellets). These act like a layer of ball bearings in the snowpack.
- Feathery or faceted crystals
- Layers of loose, uncohesive grains
- Airspace.
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:
- 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. 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.
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