This guide, aimed at winter sports enthusiasts and professionals, can help you learn the skill of avalanche prediction. It will help you understand and predict avalanches, evaluate the risks and reduce the dangers. This practical, pocket-sized guide is for use on the mountain and when planning a trip.

(Extract from Avalanches - Classification)

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Powder-snow Avalanches

A powder-snow avalanche is the flow of a snow layer of poor cohesion (settling). So these avalanches involve loose, dry snow, with low mass per volume. In other words, it is the kind of snow which will not form a snowball, which is very easy to ski and which crumbles into your tracks.

Powder-snow avalanches generally release from a single point. The avalanche rapidly grows in size as it runs, metre by metre. Seen from a distance, the track of these avalanches is often pear-shaped, particularly when they happen on a wide slope.

If the slope is steep and long, an airborne powder avalanche may develop. Otherwise the flow will take place on the surface: we call this a new snow slide.

The conditions which produce powder-snow avalanches are fairly typical. They are likely to take place on steep slopes and are usually seen during or immediately after snowfalls when the temperature is particularly low. They are much more frequent when the new snow is deposited on ‘good’ sliding bases, such as ice crusts.

As long as the speed of flow and the mass of snow are not too great, then this type of avalanche does not pose a real threat to the skier. However, an insignificant slide may become much bigger after a few dozen metres. If an airborne powder avalanche forms, the speed of the front of the avalanche often exceeds 200km/hr, which leaves the skier little chance of escape, particularly as the avalanche can travel over shallow-angled slopes or even, sometimes, climb the opposite slope over a distance of several hundred metres!

Although the deposits of snow debris in the run-out zones are rarely very big, airborne powder avalanches can cause great damage over considerable areas, mainly because of their notorious ‘air-blast’.

Slab Avalanches

These avalanches, which are common at higher altitudes, are those which kill most winter mountain users.

A slab is a snow layer with relatively good cohesion, so that a fracture may spread over a whole mountain-side. In some ways, a slab release resembles a window breaking: in a fraction of a second, the whole slab breaks off, then disintegrates as it slides downhill.

The geometry of a slab varies widely. Its depth may vary from 10 centimetres to several metres; the width from a few metres to several kilometres. The characteristics of the snow also vary. Some slabs are soft and brittle, while others are hard and compact; some are of dry snow, while others are composed of damp or wet snow. So, on the mountains, slabs are difficult to recognise and locate.

A slab avalanche, as a rule, leaves little chance of escape for the person who triggers it. Very often, the fracture occurs well above the victim and the whole slope starts sliding at once.

Depending on size, the type of snow involved and the topography of the slide path, the slab breaks up more or less as it falls.

When the slab is very brittle and snow is dry, an airborne-powder avalanche may develop. When the slab is hard, the snow blocks slide or roll on the surface at moderate speeds (a few tens of km/hr) to the bottom of the slope, where they pile up in a chaotic mess.

Snow which may appear powdery can produce a slab avalanche. This avalanche triggered spontaneously in the winter of 1987 on the Grand Sablat glacier (Grande Rousses range, France), on a south-east aspect after a major wind transport episode.

Wet-snow Avalanches

These avalanches often occur in the spring time, but are possible at any time during the winter. They follow on from thaws, rain or prolonged strong sunshine.

These weather conditions warm the snow and cause it to melt, thus moistening the snowpack. When the volume of water contained in the snowpack passes a certain threshold, its stability decreases considerably. The water promotes the melting of the bonds between the snow crystals on the one hand and lubricates the possible slide planes on the other.

Wet-snow avalanches typically exhibit a single-point release. The avalanche often begins as a small surface slide which grows in size little by little to potentially end up being of gigantic proportions.

The snow involved in these avalanches is dense (its mass per volume varies from 200 to 500kg/m³) and often very wet. Certain wet-snow avalanches show many of the characteristics of mud slides; the snow may therefore be quite easily channelled by terrain features, and as a result it is generally fairly easy to predict their path. On the other hand, it is very difficult to predict the exact moment of their release: a snowpack full of water may remain in equilibrium for several days before sudden failure.