Avalanches
Avalanches do happen in Scotland and sadly people are killed and injured by them. It is a risk that should be taken into account before you go out in the hills. The first step is to get the avalanche report for the area you are visiting from the Scottish Avalanche Information Service (SAIS). The SAIS collects information on Scottish snow conditions and posts reports for the five main climbing areas. There is a five point scale of hazard warning and where the forecast is three or above extreme care should be exercised in backcountry touring. Fatalities can occur even at one and two.
It is important to assess the risk carefully and to proceed with extreme caution if the warning signs are present. Remember 90% of avalanches which involve people are initiated by their own activity on the slope.
The propensity to avalanche is determined by a number of factors:
"By the time you set foot on the snow you should have made a pretty good prediction of what conditions will be like, what level of hazard exists and where it is likely to be located. Your field observations modify and update this prediction and apply it to the specific case." (2000, p55)
But they start the chapter on improving the odds with the reminder from Andre Roch:
"Remember, the avalanche does not know you are an expert".
Snow Pack
The safest conditions are where the snow has fused into a single compact layer as a result of rising temperatures throughout the snow pack. Melting and re-freezing can produce stable snow with a hard surface crust. From a skier's point of view it may not be ideal, as it makes control more difficult and can be particularly tedious if the crust is too thin and the skier keeps breaking through. The danger comes when the temperature through the snow pack is uneven and the snow lies in layers, the problem is compounded if the surfaces between layers is formed of ice crystals. In Scotland there is a particular risk of wind rimed cystals which form into ice balls, a little like small ball bearings and known as graupel, getting buried below new snow and providing a very weak bond between the snow above and the surfaces below. Less frequently surface hoar frosts caused by low overnight temperatures but then buried by new snow fall can have similar effects. Careful skiers dig snow pits to examine the bonding of the snow whenever they come to areas where other factors suggest there might be risk of avalanche. The book references below provide access to instruction on how to dig and interpret a snow pit, and this advice is reproduced on the Mountaineering Scotland avalanche page. The risk of the snowpack failing to bond layers is also greater where the temperature gradient through the snow pack is greater. As the ground remains at or near freezing point when covered with an insulating covering of snow, the colder the surface of the snow the steeper the gradient. For this, and a number of other reasons which relate to slowing of the fusion of the snow into a homogeneous mass at low temperatures, extremely cold weather can create situations which make avalanches easier to trigger.
Amount and timing of snow falls
The majority of avalanches occur when it is snowing/just snowed and the rate of fall is/was 2 cm or more per hour. In these circumstances there can be large accumulations of powder snow which has not formed any attachment to the layers below. In Scotland it is important to remember that wind borne snow, particularly where it accumulates under lee slopes, needs time to settle before it becomes stable. The greatest risk is during or within 48 hours of heavy snow or wind blown accumulations. Depressions and gullies which accumulate greater depths of snow can be particularly hazardous.
The angle of slope
Avalanches are most prevalent on slopes between 30 and 45 degrees, or between 1 in 1 and 1 in 3. These are obviously steep slopes, but it is easy to forget that a traverse on a steep slope brings down the angle of descent but not the angle of slope itself.
The shape of slope
Where the hill is concave the snow is less likely to avalanche than where it is convex. Recognising convex and concave slopes is a crucial navigation skill anyway, but in winter it adds to the need to treat convex slopes with care. A concave slope is one that starts more steeply at the top and eases out as it descends, practically speaking it is also a slope which you can see in its entirety from a vantage point at its top. A convex slope is one where the gradient increases as the slope descends, where the change is pronounced it produces a brow in the hill over which you can't see the rest of the slope. A pronounced brow may well create a weakness in the snow that will allow the snow on the lower slopes to breakaway from the snow higher up.
Air temperature and wind
These are important factors that help determine where snow lies and whether it has bonded together. Prolonged cold weather and high winds do nothing to promote the conditions in which the snow pack bonds together. Heavy rain can also undermine the adhesion between the snow and the ground and produces avalanches which tend to set like concrete as soon as the snow stops moving.
Cornices
Another wind created feature which is dangerous in itself and can create avalanches is the cornice, the area of snow hanging over the edge of ridge or plateau and unsupported by the slope below. Skiers need to be careful when approaching edges of the hills where cornices form , particularly at times in the season when thaw may have weakened their adhesion to the ground.
Physical clues from the snow-pack
Further information on avalanches just type 'avalanche training' in Google where you will find some of the following:
Online training tools
Books:
© Copyright Edinburgh Ski Touring Club 2022.
Avalanches do happen in Scotland and sadly people are killed and injured by them. It is a risk that should be taken into account before you go out in the hills. The first step is to get the avalanche report for the area you are visiting from the Scottish Avalanche Information Service (SAIS). The SAIS collects information on Scottish snow conditions and posts reports for the five main climbing areas. There is a five point scale of hazard warning and where the forecast is three or above extreme care should be exercised in backcountry touring. Fatalities can occur even at one and two.
It is important to assess the risk carefully and to proceed with extreme caution if the warning signs are present. Remember 90% of avalanches which involve people are initiated by their own activity on the slope.
The propensity to avalanche is determined by a number of factors:
- Changes in temperature in the snow pack and presence or absence of unstable layers
- The recency and amount of snowfall
- The angle of slope
- The shape of slope
- Air Temperature and Wind
"By the time you set foot on the snow you should have made a pretty good prediction of what conditions will be like, what level of hazard exists and where it is likely to be located. Your field observations modify and update this prediction and apply it to the specific case." (2000, p55)
But they start the chapter on improving the odds with the reminder from Andre Roch:
"Remember, the avalanche does not know you are an expert".
Snow Pack
The safest conditions are where the snow has fused into a single compact layer as a result of rising temperatures throughout the snow pack. Melting and re-freezing can produce stable snow with a hard surface crust. From a skier's point of view it may not be ideal, as it makes control more difficult and can be particularly tedious if the crust is too thin and the skier keeps breaking through. The danger comes when the temperature through the snow pack is uneven and the snow lies in layers, the problem is compounded if the surfaces between layers is formed of ice crystals. In Scotland there is a particular risk of wind rimed cystals which form into ice balls, a little like small ball bearings and known as graupel, getting buried below new snow and providing a very weak bond between the snow above and the surfaces below. Less frequently surface hoar frosts caused by low overnight temperatures but then buried by new snow fall can have similar effects. Careful skiers dig snow pits to examine the bonding of the snow whenever they come to areas where other factors suggest there might be risk of avalanche. The book references below provide access to instruction on how to dig and interpret a snow pit, and this advice is reproduced on the Mountaineering Scotland avalanche page. The risk of the snowpack failing to bond layers is also greater where the temperature gradient through the snow pack is greater. As the ground remains at or near freezing point when covered with an insulating covering of snow, the colder the surface of the snow the steeper the gradient. For this, and a number of other reasons which relate to slowing of the fusion of the snow into a homogeneous mass at low temperatures, extremely cold weather can create situations which make avalanches easier to trigger.
Amount and timing of snow falls
The majority of avalanches occur when it is snowing/just snowed and the rate of fall is/was 2 cm or more per hour. In these circumstances there can be large accumulations of powder snow which has not formed any attachment to the layers below. In Scotland it is important to remember that wind borne snow, particularly where it accumulates under lee slopes, needs time to settle before it becomes stable. The greatest risk is during or within 48 hours of heavy snow or wind blown accumulations. Depressions and gullies which accumulate greater depths of snow can be particularly hazardous.
The angle of slope
Avalanches are most prevalent on slopes between 30 and 45 degrees, or between 1 in 1 and 1 in 3. These are obviously steep slopes, but it is easy to forget that a traverse on a steep slope brings down the angle of descent but not the angle of slope itself.
The shape of slope
Where the hill is concave the snow is less likely to avalanche than where it is convex. Recognising convex and concave slopes is a crucial navigation skill anyway, but in winter it adds to the need to treat convex slopes with care. A concave slope is one that starts more steeply at the top and eases out as it descends, practically speaking it is also a slope which you can see in its entirety from a vantage point at its top. A convex slope is one where the gradient increases as the slope descends, where the change is pronounced it produces a brow in the hill over which you can't see the rest of the slope. A pronounced brow may well create a weakness in the snow that will allow the snow on the lower slopes to breakaway from the snow higher up.
Air temperature and wind
These are important factors that help determine where snow lies and whether it has bonded together. Prolonged cold weather and high winds do nothing to promote the conditions in which the snow pack bonds together. Heavy rain can also undermine the adhesion between the snow and the ground and produces avalanches which tend to set like concrete as soon as the snow stops moving.
Cornices
Another wind created feature which is dangerous in itself and can create avalanches is the cornice, the area of snow hanging over the edge of ridge or plateau and unsupported by the slope below. Skiers need to be careful when approaching edges of the hills where cornices form , particularly at times in the season when thaw may have weakened their adhesion to the ground.
Physical clues from the snow-pack
- Existing Avalanche activity
- Evidence of debris from previous falls
- Snow cracking under foot and sliding away in slabs
- Hollow noises on hard crusted snow, especially on lee slopes, which indicate weaknesses in the snow pack below.
Further information on avalanches just type 'avalanche training' in Google where you will find some of the following:
Online training tools
- Glenmore lodge training quiz
- Back Country Access training pages
- Basic page about snow profile interpretation from SAIS
Books:
- Tremper, Bruce (2013) Avalanche Essentials: A Step by Step System for Safety and Survival
- Barton B and Wright B (2000) A Chance in a Million? Scottish Mountaineering Club, Second Edition.
© Copyright Edinburgh Ski Touring Club 2022.