An avalanche is defined as a large mass of snow, which plunges down a mountainside. Most of us initially think that this occurs during heavy snowfalls, but this is not necessarily the case. Many can occur in much quieter conditions.

For most of us in the United Kingdom seeing snow falling is a wonderful sight. It transforms the countryside as we watch and gives great fun for all ages, but it can be a killer.

Those of us that love skiing or snowboarding eagerly await the first snowfalls of the winter in the Alps or across north America. But it is often this first fall that dictates how bad the avalanches will be throughout the winter and into next spring.

There are two accepted types of avalanche, loose snow and slab. The loose snow avalanche is generally not as deadly as the slab type and forms when powder snow falls on the mountainside and just cascades down the slope. Powder snow normally falls from the clouds when the temperatures are well below freezing.

Slab avalanches on the other hand can be much more deadly as they rush down the mountainsides carrying everything before them including trees and rocks.

Even in this country, most of the rain we get actually starts in the clouds as snow, but melts before reaching the ground. In colder weather, the pretty snowflakes we see falling from the sky change appearance and texture after hitting the ground.

We are used to the snow being compacted by vehicles or people walking on it and turning it into dangerous ice. In the high mountains the heavy snowfalls do exactly the same thing. So what fell as fluffy snowflakes compact into hard ice.

Certain weather conditions then either help the packed snow to bond together, or weaken the bond between the flakes. At the beginning of the winter season the first snowfalls cover the mountains. And because the ground on which it falls is still relatively warm, the heat rises up through the snow and sometimes melts the top layers a little. It freezes again, but leaves the top layer of snow with less cohesion or sticking power for the subsequent snow to fall on. Then at anytime during the winter, or spring as the temperatures start to rise or the snow showers turn to rain, the top slabs of snow become unstable and break away as avalanches.

It is worth noting that the vast majority of avalanches are caused by people climbing, skiing or snowboarding. But watch out for the weather patterns such as; a sudden warming from southerly winds coming from the Mediterranean Sea, or the Föhn effect as air descends on the lee side of the mountain.

Related Links:

- Snow Flakes
- Of Snow and Men!
- World Skiing Report
- Ski Club of Great Britain