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Severe weather conditions in many mountain areas mean that avalanche risk remains of concern
February 2009
Deep Snow on The Glyders
Snowdonia National Park
Photo: George White
As the UK experiences some of the heaviest snow seen for the best part of 20 years with more forecast, the risk of avalanche in high mountain areas remains a concern.
The Scottish Avalanche Information Service (SAIS) has identified the risk as 'Considerable' week ending 6/2/2009 in all the areas they monitor.
There is no equivalent system for the rest of the UK, so the high levels of snowfall in high mountain areas especially in Snowdonia and the Lake District, mean that walkers need to be extremely vigilant.
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.
- Has there been heavy snowfall and/or strong winds?
- Have there been variations in temperature?
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.
- New snow to a depth of more than 25cms will increase the risk of avalanche significantly. The faster it accumulates the greater the risk and snow falling at a rate greater than 2cms/hour is considered dangerous.
- Wind is the greatest contributor to slab formation and slabs are the greatest avalanche risk.
- A sudden temperature rise after a snow fall can increase the instability of the snow pack.
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:
- avoid the lee side of slopes
- avoid slopes with an incline between 25-45 degrees
- avoid bulging convex slopes
- walk along ridges or cross suspect slopes as high as possible
- avoid small gullies and valleys with steep side walls
- avoid cornices; don't walk below them or on them. If walking around a corniced ridge or cliff, stay well to the windward side and well away (5m (15ft) or more) from the edge.
8. If the worst happens and you are caught in an avalanche.
- get rid of your backpack
- shout to the rest of your party and try to jump to the side or above the moving snow
- use your ice-axe to slow/delay being swept along in the debris
- if possible, use swimming movements (on your back, head up) to reduce the depth to which you sink
- if it's a hard slab, try to remain on top of the blocks
- if buried, try to maintain a space in front of your face and take a deep breath to expand your chest so creating room to breathe. Try to get a hand through the surface so that your position in visible.
- conserve energy and try not to panic while your companions locate you.
9. If you survive an avalanche
- try to remember then mark the spot where your companion was engulfed and the point where he/she was last seen. The line connecting these two spots can provide a pointer to the location of the victim.
- search the area as thoroughly as possible for any sign of the victim using an ice axe or a pole as a probe.
- obviously summon help as quickly as possible but not instead of the initial search. The likelihood of survival is dependent on length of time someone is buried. Carry on the search until help arrives - people can survive after quite long periods of burial.
For more information on avalanches see here.
The views expressed by contributors to this discussion are not necessarily those held by go4awalk.com.
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