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Types of Avalanches



PRINCIPAL TYPES OF AVALANCHES

Powder snow avalanche: It is generally a winter phenomenon occurring on steep slopes after a fresh fall of snow. There has to be some violent shock for it to become detached, such as very strong wind, the blast of another avalanche or the fall of a cornice or of a climber.

It comes down in eddying clouds and is extremely fast. It is preceded by a strong gust of wind, and makes a deafening noise. An avalanche of powder snow is capable of flattening quite a large tract of forest because of its blast. It may even cause destruction on the opposite slope. It can kill a mountaineer by suffocation, because the snow enters the bronchial tubes. If caught in its path, the climber should turn his back to the oncoming avalanche and cover his nose and mouth with a cloth.

Avalanche of fresh wet snow: This is often a surface avalanche, but its weight may make it a ground avalanche. It is caused by high atmospheric temperature and is the type of avalanche which occurs during warm periods when the snow softens. It moves a little slower than a powder snow avalanche. It rolls but makes a powerful blast none the less. Its consequences are the same as those of the powder-snow variety but it has, in addition, a crushing effect because of its weight. On coming to a halt it hardens immediately like plaster.

A climber caught in an avalanche of this type should lie on his back, head uphill, try to stay on the surface by making back stroke swimming movements. He should make all efforts to disengage himself before the snow solidifies

Wet snow avalanche: This is predominantly a springtime avalanche and nearly always a ground avalanche. It may also occur in winter when there is rain. The sun, fog, the rain or contraction at the time of refreezing, any of these may be the cause of its breaking away. It is fairly predictable, usually occurring in the same places each year.

It flows slowly but its force is considerable; it flattens and destroys everything. The climber should avoid the gullies down which it habitually descends. One cubic meter of powder snow weighs about a kilogram while the same volume of wet snow might weigh several hundred times more.

Snow-slab avalanche: This a dangerous avalanche because it is often unforeseeable. A snow-slab is made of snow which is superficially compressed but has not adhered to the underlying layer from which it remains separated by a layer of air. It is usually found on one side of a ridge, often when the ridge is surmounted by a cornice. Snow-slabs are particularly to be feared after a fall of fresh snow. For then they are no longer visible. They are in any case difficult to recognise. Snow slabs are of a dull, yellowish white colour with a dense consistency.

Snow slab avalanches are noisy. They make a hollow sound because they form kind of a vault over the under-layer and the air gap between. They are especially dangerous in winter. In spring, they become gradually compressed and finally stick to the under layer. What sets them in motion is a break down of mechanical equilibrium.

Snow slab avalanches slide. The climber should be vigilant in avalanche prone places particularly after a fresh snowfall.

Cornice avalanche: It is largely in winter and spring that cornices constitute a danger. In summer they are more stabilized, though always somewhat precarious.

Serac avalanche: They are caused by the movements of the glacier and may occur at any time. The climber should move very quickly when crossing an exposed area.

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Published on: 2005-06-02 (1456 reads)

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