What causes an avalanche?

Avalanches are frequent killers of backcountry skiers and snowmobilers, and even the most experienced mountain climbers can get caught up in them.

According to the International Commission for Alpine Rescue, which has 17 member countries, there are approximately 150 avalanche fatalities reported every year.

Eight people died and several others including a cardiologist from Quebec are still missing after an avalanche on Sept. 23 on Mount Manaslu in northern Nepal.

What causes an avalanche?

An avalanche occurs when the snowpack or the layers of accumulated snow on the side of a mountain is in some way disturbed, leading to a fracturing of the top layer and a downward torrent of a large mass of the white stuff.

What sets off the disturbance?

A number of factors can set off a disturbance. They include natural factors like new precipitation (be it rain or additional snow), a sudden warming, wind, ice fall or rock fall, as well as so-called "artificial" factors like skiers and snowmobilers. The long-held notion that avalanches can be triggered by the human voice has been largely debunked.

According to the Utah Avalanche Center, 90 per cent of avalanche fatalities are triggered by the weight of one of the victims in the group.In other words, the pressureexerted by human movement caused a fracture in the snowpack, loosening the top layer and unleashing a torrent ofsnow.

Most avalanches occur 24 hours after a rapid, heavy snowfall.

Are there different kinds of avalanches?

There are two main types. A "loose snow avalanche" typically occurs on steep terrain and is a teardrop-shaped mass that gathers volume and intensity as it travels down the mountain. A loose snow avalanche usually occurs with freshly fallen, low-density snow or old snow thats been softened by extended sunlight.

The second type is a "slab avalanche," in which a layer of snow separates, shatters like glass and comes hurtling down a mountain. Slab avalanches account for about 90 per cent of avalanche deaths.

What makes an avalanche so deadly?

Most people assume that an avalanche takes its victims by submerging them in snow and asphyxiating them. This does happen, of course, but an unlucky skier or mountain climber is just as likely to be killed by the sheer blunt force trauma of a wall of snow as being buried alive. The most powerful avalanches can reach speeds of over 300 km/h.

Sometimes, even if a skier manages to avoid being submerged, the sheer volume of precipitation in the air in the wake of an avalanche can flood a persons mouth and lungs and effectively drown them.