If natural disasters were ranked in terms of loss of life throughout the world, avalanches would be listed well below volcanoes and on a par with landslides. The reason has little to do with any rarity of the phenomenon; it is because the vast majority occur in uninhabited areas, and for every one that hits the news, a thousand others cascade down their own secluded mountains incognito. Perhaps it is the sheer unluckiness of the victims that underlines their tragedy.
The roots of the instability are to be found in subtle differences in the cohesive strength of successive layers of snow. If an accumulation of thick, heavy snow lies on top of a layer that is soft and weak and crumbly, any slight stimulus may be sufficient to cause the upper slab to lose its grip, and slide in an accelerating torrent down the mountainside. The texture of the various layers depends in turn on the shape of the crystals that make up each fall of snow. To understand, it is necessary to analyse the structure of the individual snowflakes.
Each flake comprises thousands of tiny hexagonal crystals of ice, that combine into a myriad different shapes to form the snowflakes. In this sense, each snowflake is unique, but the crystals from a particular fall have common characteristics, and it is the shape of the crystals that determines whether the snow is strong or weak.
If the crystals are "spiky", they and the resulting flakes will lock together giving good cohesion: if they are smooth and without any jagged edges, the snow has no more strength than an equivalent number of grains of desert sand.
Very often the "weakest" snow on a mountainside is the result of a thin feathery layer of hoar frost having formed on top of an existing accumulation. If this "sugar snow", as it is sometimes called, is subsequently covered by a thick layer of what we might describe as "normal" snow, the weakness remains, sandwiched some feet below the surface. It is at this surface of discontinuity that the upper slab of snow is apt to shear away, tumbling down the mountain as an avalanche.
The slide of snow may be anything from 10 feet to over a mile in width, and the distance travelled varies depending on the fall of ground. Dry snow may move at speeds of up to 200 m.p.h.; wet snow travels more slowly, so that an expert skier with a fair start might hope to outrun it. But the wet snow causes greater damage to anything in its path because it is much heavier.