Avalanches

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 avalanches occur in much quieter conditions.

Apart from snow, two further ingredients are needed for an avalanche to begin; a slope and instability. The most likely slope angle would be around 35 degrees. Instability is tested by safety officers such as the SAIS (Scottish Avalanche Information Service). They dig snow trenches and observe the snow profile looking at the different layers and textures of snow. If the conditions are there for an avalanche, they will probably blast the snow using explosives, obviously clearing the area beforehand.

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 when temperatures are well below freezing.

We are used to snowfall being compacted by vehicles or people walking on it, turning it into dangerous ice. In high mountains heavy snowfalls do exactly the same thing; so what fell as fluffy snowflakes compacts into hard ice. Certain weather conditions then either help the packed fallen 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. When temperatures start to rise, or the snow showers turn to rain, the top slabs of snow become unstable and break away as avalanches.

In the UK, avalanches are a concern - especially in the popular ski resorts and mountains of Scotland. A number of peaks there are over 4000 feet and as storms pile in from the Atlantic, heavy snowfall and high winds increase the avalanche risk. Fresh snow collects in the lee side of a slope, driven in by the winds, and with the prevailing winds - snow is more likely on the north, northeast or east facing slopes in the Scottish mountains.

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.

If you have plans to be out in mountainous terrain, then for your own safety and your families safety do watch the local forecasts and always read the avalanche reports. Remember they are there to help.