Severe weather conditions in many mountain areas mean that avalanche risk remains of concern

February 2015

Severe weather conditions in many mountain areas mean that avalanche risk remains of concern
Deep Snow on The Glyders
Snowdonia National Park
Photo: George White

We appear to be in midst of an extended period of cold weather and the risk of avalanche in high mountain areas remains a concern.

Mountain Rescue teams in areas such as the Lake District have recently issued warnings to walkers and climbers.

So what is an avalanche and why do they happen?

An avalanche is a large mass of snow moving down a hill or mountain side.

The likelihood of snow to avalanche is determined by the stability of the snow and by the existence of a slope.

The stability of the snow pack is determined by the formation of the snow crystals in the atmosphere and subsequent changes in conditions that affect the strength of the bonds formed between individual crystals.

For example, during winter, when the air temperature drops, there will be a temperature difference between the bottom of the snow-pack that is in contact with the warmer ground and the temperature at its surface. Under these conditions, water vapour will move upwards through the snow, re-crystallising at points within the pack. These layers of 'hoar' are extremely unstable and may produce avalanches.

Later on, as air temperatures rise during spring days, the snow-pack will melt and re-thaw repeatedly. Water from the melting phase may steep down to the base of the snow pack completely undermining the anchorage of the snow to its base.

With experience it is possible to dig down a short way into the snow pack to assess the profile of the snow and to make a judgment as to the risk of avalanche. Experts from the Scottish Avalanche Information Service (SAIS) do this as part of their routine risk assessment.

The other 'requirement' for an avalanche is a slope - which must be sufficiently steep to allow the snow to slide - the steeper the slope the more likely is a slide. However, on very steep slopes (>60 degrees ) it's unlikely that snow will be able to accumulate. So the most dangerous inclines are around 25-45 degrees.

There are two different types of avalanche - loose snow and slab.

Loose snow avalanches occur where there is powder, usually have a single point of release and can occur several days after snow fall.

Slab avalanches, in which the slab is attached to the layer below, are the most dangerous and are more likely to form in high wind. These are likely to be released from 'lee' slopes during and after snowfall.

Assessing the risk

Do you have experience of winter walking and the complete equipment (including an ice-axe and crampons) needed? If not - then consider seriously if you should be going.

Winter walking is a fantastic experience but can be extremely hazardous for the inexperienced. Many organisations now run winter walking skills training courses - and attending one may be a really smart investment. Going without the necessary equipment and experience in challenging winter conditions is really putting yourself at great risk even before you take the climate and the mountain into consideration

1. Before starting out gather as much information as you can locally about conditions. If you're intending to walk in Scotland then use the Scottish Avalanche information Service (SAIS)

2. Find out what the weather has been in the previous week or so.

Determine what this tells you about the likely snow conditions on your intended route.

Avalanches often occur after a snow-fall and there's evidence to suggest that the likelihood of avalanche is determine by both the amount AND the rate at which it accumulates. 2 cms/hour of accumulation is considered dangerous. The new snow adds to the pressure on the underlying, unstable layers and so the avalanche happens.

3. Check local weather forecasts.

4. If you're not familiar with the area, discuss your intended route with someone who is. Are there any known avalanche paths that you should plan to avoid?

During your walk

5. As you approach the start of your walk, check for any evidence of avalanche activity. If there are fresh accumulations of snow or drifting - then consider changing your route and go to another slope.

6. Constantly adjust your assessment as your walk progresses. You should be aware of snowfall and changes in wind and temperature.

7. Most avalanches are triggered by their victims so whenever possible avoid walking in areas of high risk - even if you have to abandon the walk. However - if this is not possible:

8. If the worst happens and you are caught in an avalanche.

9. If you survive an avalanche

For more information on avalanches see here.


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