Backcountry safety and Avalanche risk assessment.
Today the Backcountry is more accessible than ever before. What was once the domain of the winter mountaineer and experienced Backcountry specialist has today become the playground of the masses. As more and more people follow the ‘ever increasing’ number of tracks that lead away from the relative safety of the groomed pistes (or trails), the boundaries of the norm become pushed further and further into the untamed regions of the high mountains.
Today even the most inexperienced rider can very quickly find themselves exposed to an untamed world of Snow, Rock and Ice that they know so little about.
When the sun is shinning, the sky is clear and the snow lies deep and fresh the beauty of the high mountains can all to easily mask the dangers that are hidden within. As the search for the mountains greatest terrain races on, it is becoming all to easy for us to forget, ignore, or underestimate the risks involved in riding the untamed natural terrain of the high mountains.
Safe passage in the Backcountry is all about good decision making. Once you leave the patrolled ski area ‘what you do’, ‘where you go’ and ‘how you go’ suddenly become decisions that control, not only your immediate destiny but possibly your life!
Important decisions like this can only be made if you are aware of the dangers, heed the warnings, and recognize the clues and signs that the mountains provide.
There are many objective dangers hidden in the high mountains. Most obvious are well documented and can be easily avoided with a little bit of research, insight and more often than not ‘common sense’.
Most of the dangers in the backcountry come from either above our heads or below our feet. Common dangers include.
1. Rock, Snow and Ice fall from above.
2. Crevasses, cliffs and *terrain traps below.
3. The snow pack upon which we ride.
Whilst the first two common dangers listed here can more often than not, be detected and avoided, the third is an often ‘unknown quantity’ literally hidden beneath our feet.
The Snowpack upon which we ride!
In the Backcountry, there is no greater threat than that posed by Avalanche!
The ‘snowpack upon which we ride’ with out any doubt, possesses the greatest threat to both the experienced and inexperienced rider whilst riding Off piste or in the Backcountry and statistics show that 90% of avalanche victims triggered the Avalanche in which they were caught, themselves!
Safety in the Backcountry is all about knowledge, awareness and good decision making and whilst the dangers ‘under our feet’ are more often than not hidden from view, there are many signs, clues and warnings that we can use to make the right choice at the right time.
By learning more about Avalanches and what makes them tick we can learn how to assess the slopes and terrain in which we play and hike and ride in safety each and everyday!
(*terrain traps include gullies, bowls, drops which will cause an obstruction to your ride or more importantly would channel and increase the concentration and depth of snow in an avalanche.)
There are two common types of avalanche, the ‘Loose snow avalanche’ and the ‘Slab avalanche’.
Whilst both types of Avalanche can have equally devastating consequences, the Slab avalanche is the one most commonly ‘triggered’ by Skiers and Snowboarders and is our number one cause of concern when riding in the Backcountry!
Here, we take a look at both types of Avalanche, look at what makes them tick and hopefully gain some insight into ‘how to avoid them’.
The Loose snow Avalanche.
The Loose snow avalanche is often overlooked as the lesser evil of the Avalanche world.
Over recent years however, with the increase in numbers heading out into the Backcountry during or immediately following a major snowstorm or touring and riding in the warm spring conditions, the increase in numbers of skiers and Snowboarders getting caught in ‘Loose Snow Avalanche’s has risen dramatically.
Loose snow avalanches start at a single point of in-cohesion and rapidly grow in both velocity and mass as they flow down the mountain.
Easily recognised by their triangular shape, Loose snow avalanches are most common during or immediately after fresh snowfall as snow sloughs off steep terrain (Powder Avalanches) or in the spring when warmer temperatures melt the snowpack and release heavy wet snow avalanches on all gradients.
Loose snow Avalanches occur in a snow pack that has very little internal cohesion such as fresh powder or spring corn snow (the 2 most popular types of snow for riding!). This fragile cohesion can be easily ruptured by natural phenomena such as overloading due to continual snow fall, snow falling from rocks (which heat up quickly in sunlight) or temperature increases causing the mass of the snow on the slope to increase and fall prey to the pull of gravity.
Powder snow Avalanches can travel at speed exceeding 100km/h and can push a destructive shock wave of air ahead of them. If unobstructed a powder snow avalanche can travel a huge distance and the cone shaped debris can be thinly spread over a wide area. If channelled, a Powder Avalanche can posses devastating force!
Wet snow avalanches are very common during the spring melts and move much slower than Powder Avalanches. Wet snow avalanches may in general pose less risk than powder avalanches, but the snow in a wet snow avalanche is heavy and with momentum on a steep slope can be equally as destructive and powerful. On coming to rest the snow in a wet snow slide is often quite deep and rapidly sets like concrete. (With extreme temperature fluctuations becoming common, the risk of getting caught by a heavy wet Loose snow slide is on the increase during the spring touring season).
Loose snow Avalanche risk assessment!
There are many factors to take into account when determining the risk posed by loose snow avalanches.
Slope aspect, altitude and slope angle plus the quantity of snow fallen are the most obvious indicators to look at.
The quantity of snow falling will play an important role in any avalanche risk assessment. Snowfall of 30 to 60 cm ‘without the influence of wind’ will make only the steepest slopes at risk where as quantities superior to this can make even moderate slopes avalanche prone and of substantial risk.
The intensity of the snowfall can also play an important role. 60cm of snow falling over 10 to 12 hours will pose a greater threat than the same amount falling over 2 to 3 days (over time the snow crystals will start to bond and stabilize the snowpack).
The temperature of the air during the snowfall can also help with loose Powder snow Avalanche risk assessment. Snow falling in miler temperatures of around 0 degrees will bond much more strongly and quickly than snow falling in colder conditions of around –10 degrees.
The next most obvious factor to take into account is the steepness of the terrain.
Angle of slope!
The angle at which snow slides varies greatly depending on the type of crystal in its make up. Wet slushy granular snow has very little cohesion in relation to its mass and can slide on slopes as low in angle as 15 degrees! More stellar like lighter crystals that fall in cold conditions with out wind might build up on slopes up to and above 50 degrees before their mass suffers the pull of Gravity.
Loose snow slides are most common on steep slopes of 35 degrees and above and for the experienced Backcountry rider the greatest risk from ‘Loose snow avalanche’ is often that posed by getting caught in a self released slough whilst riding steep terrain.
Slope aspect and temperature.
Slope aspect relates to which way a slope faces. The direction in which a slope is orientated will affect how much sun light and warming it will receive.
The more sun light radiation a slope receives the quicker it will begin the process of settlement and eventually stabilization. During the initial period of change in the first 24 hours after a storm, slopes receiving strong sunlight will be the first to show signs of shedding and loose snow avalanche activity.
In mid winter, slopes receiving sunlight will benefit from the suns weakened radiation and will stabilize through a process known as Equi temperature metamorphism. Towards the spring time however, slopes receiving strong sunlight be affected by massive temperature change and a greatly increased risk of wet snow Avalanche due to an increased water vapour content in the snowpack.
Like wise, in mid winter, slopes sheltered from the suns rays will transform at a much slower rate and can remain unstable for much longer. The intensified cold conditions on these sheltered slopes can crystallize the snow even more as the water vapour content in the snow layers decreases and creates further instability in the snow pack hence increasing the risk of loose powder snow avalanche.
Towards the spring as the ambient temperature rises, these more sheltered slopes will be only moderately affected by the sun and temperatures hitting the sun facing slopes and might now prove to offer the most stable conditions.
Altitude gain means colder temperatures! Hence it follows that all the processes of change mentioned here are delayed with altitude. Also, slopes higher up the mountain often tend to be steeper than those lower down.
In the first 24 hours after a snowfall it is common to see loose snow avalanche activity slowly work its way up the mountain side with wetter snowballing type slides lower down and powder sloughs evident higher up.
Loose snow avalanche risk conclusion.
Loose snow avalanche risk can mostly be judged through simple observation as the tell tale signs are plain to see.
Lots of snow means increased risk on steep slopes during and immediately after a snowstorm.
Current activity spells high risk, at least on slopes of similar aspect and altitude!
Sunlight and slight temperature increases indicate an increased risk in the 24 to 48 hour period after a fresh snowfall.
Large increases in temperature, as in the spring indicate a massive increase in wet snow slide risk.
In recent years due to an increased number of people touring and riding in the Backcountry of the high mountains, the number of people caught in loose snow avalanches has risen.
This is due in part to simply more people being ‘out there’ but also due in part to the every increasing race for first tracks immediately following a storm. In days gone by the law of the mountains was ‘let the mountain lay for a day!’
The spring touring season has also become increasingly popular and often now extends well into the month of May were massive temperature fluctuations can cause huge wet snow avalanches on all aspects and angle of slope and the conditions between the top of the mountain and the bottom can be of two different extremes.
Loose snow avalanches are now back in the game so be aware and be safe!
The Slab Avalanche.
Slab avalanches occur when a large well bonded slab of snow releases en-mass from a weaker layer buried in the snow pack and are easily recognized by their prominent features, a linear crown wall, a smooth often featureless fracture layer and a large area of debris composing of blocks of snow of varying sizes.
Whilst both the ‘Loose snow avalanche’ and the ‘slab avalanche’ can have equally devastating consequences the Slab avalanche is the harder of the two to detect and avoid, it is the one most commonly ‘triggered’ by Skiers and Snowboarders and is the number one cause of concern when riding in the Backcountry!
The snow beneath your feet is made up of many layers that tell the history of the changing winter conditions, snowfall, wind speed, wind direction and rising and falling temperatures, all which greatly affect the composition of the snow crystals and density of the layers within the snow pack.
These different snow crystals and layers don’t always bond well together creating weaknesses that can affect the stability of the snow pack throughout the winter.
Snow slab is a dense strongly bonded layer that is formed as tiny damaged grains of windblown snow come to rest on a sheltered slope. These tiny grains become compressed very tightly together by the increasing weight of the snow deposited on top and by a natural metamorphism (sintering) that rapidly forms a strong bond between each individual crystal.
Snow slab is dull in appearance (the tiny particles reflect very little light) and is smooth and compact with a slightly hollow feel (slightly similar in texture to polystyrene).
It is important to understand that snow slab is most often formed by the action of the wind on both fresh falling snow and older in-situ snow and like the wind snow slab conditions can change day by day.
The wind in the mountains frequently changes direction and is channelled by the terrain, moving snow from one place to another, creating new slab and further loading existing ones.
The stronger the wind, the more snow transported and deposited and the deeper and firmer the slab.
In its self, snow slab can be a very strong stable layer covering a large area of a slope. If however, this slab doesn’t bond to the layers underneath, (which is most common) or if there are weaknesses buried in the snowpack that can’t sustain the mass of the slab on top this can create highly unstable conditions and with the right trigger the whole slab can be released to slide en masse.
Unstable snow slab is easily triggered and has devastating consequences. The amount of snow that can be pulled in a large slab release is incredible and the force with which it slides unthinkable.
Unstable Snow slab is the greatest cause of Avalanche fatality in the high mountains!
The hidden dangers.
Unlike with the ‘Loose snow avalanche’ the dangers of the Slab avalanche are very difficult to detect and hidden in the snowpack beneath our feet.
For this reason it is important that we learn to recognise the symptoms and signs that indicate the ‘possible dangers’ and make our decisions based upon ‘what we see’, ‘what we know and discover’ and ‘what the terrain tells us’.
The more we know about where and how snow slabs are formed and also the formation of the weaker layers that can be hidden in the snowpack, the better prepared we are to make the right decision at the right time.
Statistics show that, when it comes to assessing Slab avalanche risk, there is no such thing as 100% certainty and both the experienced and inexperienced can be equally at risk.
Whilst the more experienced rider might be armed with more knowledge with which to make the correct decisions, by their very nature they are more likely to be riding in areas less frequented and of greater risk.
On the plus side, they are quite likely riding with friends of equal experience to help with the decision making process and to aid and offer assistance if the need arises.
In effect, they are the masters of their own destiny, the risks are increased but can be controlled somewhat through good decision making.
The inexperienced rider, by their nature is more likely to be riding on slopes that are already tracked and of lesser angle. They are more often than not unaware of the dangers and most likely ill prepared and with out Transceiver, Shovel and Probe.
As their confidence grows and their need for fresher lines builds so do the risks involved and suddenly they are to be found riding on steeper terrain and un-ridden snow without the equipment and knowledge to back up their actions!
90% of all avalanche victims trigger the slide that catches them and in 90% of cases this slide will come in the form of a Slab Avalanche!
What causes a Slab release?
An unstable slab of snow will release ‘en-masse’ when the mass of snow within the slab becomes to great for the adhesive properties of the weak layers within the snow pack or the ground on which it lies.
Weaknesses within the snowpack occur when two or more adjacent layers don’t or can’t form a bond between them. The greater the difference in density between 2 layers the less likely they are to form a bond. As snow slab is dense by nature it is common for freshly formed snow slab to be weakly bonded to the layers surrounding it and so highly unstable.
An unstable snow slab needs a trigger to break its bond and release it into gravities pull. The most common triggers for a slab avalanche are an increase in mass due to temperature increase or direct loading such as further wind deposited snow, external loading due to natural sources such as a Cornice falling from above, or external loading from an artificial source such as a skier, snowboarder, snowmobile or touring party.
Where as on soft direct pressure might only affect the small area to which it is applied, due to the solid bond with in a snow slab, any external loading applied to any one single point of the slab can be transmitted to a much wider area. If the increase in pressure is enough to break the weak bonds in the snow pack they will collapse. If the slab extends into steep terrain strongly affected by the pull of gravity then the slab will release and be pulled down the slope.
Weak layers in the snowpack are often caused by extremes in temperature.
Extreme cold temperatures running through the snowpack will re freeze loosely bonded snow crystals causing them to re-crystallise. The re-crystallisation of loose layers already buried in the snowpack causes the growth of flat facetted plate like crystals and granular hexagonal crystals called depth hoar.
Cold clear nights can cause re-crystallisation on the snow surface to occur and the formation of flat feathered shaped crystals called surface hoar.
Both these types of ‘non precipitated’ flat and angular hoar crystal form loosely bonded extremely fragile layers within the snowpack when buried.
These layers collapse and fracture very easily, this is often recognized by a whumping sound within the snowpack as it settles due to external pressure, and indicating perilous conditions. On slopes steep enough for a slide this whumping sound is usually followed by a slab release!
‘Non precipitated’ crystal growth within the snowpack is a very real hazard in the high mountains both during and after extremely cold temperatures. These weak layers can remain in the snowpack through out the winter causing serious problems later on in the season, especially on the cold North facing slopes unaffected by the action of the suns rays.
(Last year in the western Alps a large number of the fatal slab avalanches, released throughout the winter, were caused by the growth of ‘non precipitated’ crystals layers formed in a relatively shallow snowpack during a very cold period in early January!)
Other types of precipitated crystals that can form week layers are graupel or ice pellets such as hail, Once buried these little balls of ice can maintain their form for many weeks whilst the layers above literally sit on a layer made of roller balls.
Freeze thaw layers such as ‘sun crust’ and Ice also make layers on which further snow deposits cannot bond.
It is important to keep looking at the formation of the snowpack beneath your feet and build up a mental picture of what might be happening at different locations on the mountain by digging snow pits. Keep in mind the affect of temperature on the snow and how this changes with aspect and altitude!
Slab Avalanche risk assessment and avoidance, the signs!
Visual signs are our best indicator of what is happening or has been happening and previous or current signs of Avalanche activity is the greatest visual clue.
Sign of current activity is a sure sign of an extremely unstable snowpack!
Signs of recent activity show changing conditions, a settlement is in process and caution need be applied.
Signs of previous activity is also a good clear indicator of what the wind has been doing and shows clearly which slopes have been loaded.
If you see signs of avalanche activity try to think about how and why it took place.
Look for similar signs and symptoms in other areas of the mountain and see if you can recognize other areas that might be of similar risk.
Wind sign and affect!
Wind is the biggest creator of snow slab and so wind sign on the mountain is a great indicator of where the dangers might be found and hence avoided.
The greatest risks are posed during wind affected ‘snowfall’ or within the first 24 hours after, however, snow slab conditions can change hour by hour and day by day even weeks after the last fresh snowfall.
On a windy day, you can literally see the loose snow being blown along the surface of the snowpack and deposited in sheltered spots around the mountain.
On a non-windy day, it is important that you learn to recognise the signs that the wind has left and so know where the dangerous areas of Snow slab might be found.
Snow slab forms on slopes sheltered from the wind so it is very important to understand the wind, see how it is affecting the mountain and where it might be depositing its load.
Wind becomes compressed and accelerates when it hits the mountains. It becomes directed by the terrain and so can blow in many directions even opposing the global wind direction, this is known as the localized wind affect.
As the wind accelerates over mountain ridges it hits the still sheltered air hidden behind and deposits the snow load that it is carrying.
Over sharp ridges this change is dramatic and causes the air to swirl in a vortex on the lea side of the mountain building a friable overhanging cornice and loading the slope below.
Over rounded ridges the change is more subtle and the transported snow is deposited further down the sheltered slope building more depth to the slab and forming a dangerous convex bulge overlooking the wind loaded slope below.
The wind can deposit snow as it blows around the mountain too. Cross loading onto slopes and into gullies and couloirs can be a common problem at lower altitudes.
Slopes that faced into the wind will become ‘scoured’, sculpted and rippled into jagged waves called ‘Sastruggi’. Sastruggi is an important indicator of wind direction as the wind sharpened points always face into the wind.
Other wind sign might be wind plastered Rime that builds up on trees, trail markers or lift pylons, the rime deposit is found on the windward side and not on the sheltered side!
The Wind transports and deposits snow around the mountain, it is important to understand how this works and the effect it has on the snow pack.
Lea or sheltered slopes pose the greatest danger, they also however, can provide the greatest riding!
Analysing the terrain.
As Slab avalanche danger is for the most part hidden under our feet, one of the greatest tools in our risk management arsenal is Terrain analysis!
Simply put, Terrain analysis comes down to looking at the slope and asking ‘what is holding the snowpack in place’ whilst gravity tries to pull it down the hill?
Using you knowledge of the stability of the snowpack and your understanding of slope loading you can already begin to make some very important judgments about whether or not the slope in question is safe to ride or not.
Terrain assessment however, will help you make your final decision and hopefully, if you’ve judged it well, make your descent both safe and fun.
Terrain assessment is basically all about looking at what is holding the snow in place and looking at how Gravity is playing with it.
Begin by looking for the areas of most tension and least tension. Those areas that are being pulled by gravity, such as the steeper areas of the slope and those areas that are stabilised by gravity and are less steep or flat.
Slope shape will tell you a lot about the risks.
Convex Slope, slopes that bulge outwards and get steeper hold the greatest amount of tension. The mass of the snowpack is being pulled rather than supported by the steeper slope below.
Concave slopes, slopes that ease in angle in descent are somewhat more stable as the snowpack on the steeper slope above is supported by the lesser angled slope below.
It is also important to look at the transitions between the slope shape as these areas are often prone to the most tension or loading.
A sharp concave transition beneath a steep slope for example, might mean there is a lot of support loaded in one tightly packed area. Whilst in general this concave terrain shape should mean stability, in this case, direct pressure to the snowpack could break the support for the slope above!
A sharp convex bulging transition also puts a lot of tension into a small area and so depending on what is supporting the steeper slope below can increase the risk considerably.
A longer convex transition can create a substantial risk as the gentle transition will create wind loading over a bigger area and snow slab could reach further into the steeper slope below pulling tension in to the easy angled snowpack above.
Other terrain features might offer support to the snowpack. Trees can act as support, as can rocks and constrictions such as the verging walls of a gully.
Think about what is under the snow, what is holding the snowpack, is it sitting on grass or on rocky terrain? You might get an idea of this by looking at the terrain on the scoured side of the mountain or exposed parts of the ridge above the slope.
Look at the shape of the terrain, look at what is keeping everything from sliding down the hill and be aware of the tensions created by the transitions within the terrain.
Temperature, Aspect and Altitude.
Once you have assessed where the risks might be (wind sign) and where the greatest local risks are (terrain shape) you next want to look at the external factors affecting the snowpack such as aspect temperature and altitude.
As already noted extreme cold temperature can have a big influence on the snowpack, creating faceted snow crystals and forming week bonds and layers within the pack.
Temperature increases also greatly affect the snowpack and can be the sole cause of slab release on a fragile, unstable snowpack.
As the temperature changes it affects the movement of water vapour within the snowpack. With decreasing temperatures the vapour moves upwards through the pack, drawing it out of the layers causing crystallization and loosening the weakly bonded deep layers.
As the temperature rises, the transfer of vapour works its way downwards settling the pack but increasing the mass of the upper layers. This increased in mass might cause enough pressure increase to collapse or fracture weaker layers hidden deeper and could be the cause of a slab release.
Be aware of long cold spells, especially early in the season when the snowpack isn’t very deep.
Be wary of temperature increases, especially after fresh snowfall, wind and cold spells.
The direction in which a slope faces greatly affects the stability of the snowpack in relation to how much sun it receives.
A slope receiving more of the suns radiation will settle quicker than one that remains in the shade.
This settling process eventually leads to stabilization but in the initial phase of warming after snowfall or wind affect the risks will be high.
Long time exposure to the suns rays and warmer temperatures will eventually lead to instability due to wet loose snow avalanche risk.
Slopes that remain in the shade will settle much slower than those in the sun. Here the cold temperatures can crystallize the snow and weaken the bonds within the snowpack.
Only when the ambient air temperature begins to rise will the settlement process really begin and shaded slopes can offer fresh cold crystallized powder riding well into the spring time albeit it with a substantial element of risk attached!
With every 150m of height gain the air temperature drops by approximately 1 degree centigrade.
Altitude has a simple affect on the snowpack in that the higher the elevation the colder the air temperature affecting the metamorphisis and stabilization of the snow crystals.
In areas with a large vertical height difference between the top of the mountain and the bottom (such as in the western alps for example) this means that conditions between the summit and the valley can be of two very different extremes and opposite risks. In the spring the higher elevations might offer loose cold powder snow, the middle elevations a mixture of unstable settling and already stabilised conditions. Further down might offer good spring corn snow on a melt freeze stabilised snowpack whilst the lower slopes in the valley thaw towards dangerous levels of wet loose snow avalanche risk!
Temperature plays a big part in the metamorphosis of the snowpack and so the cooling affects of altitude also play their part.
Slab Avalanche risk conclusion.
Danger from Slab Avalanche is the number one cause of concern for all Backcountry and off piste/trail enthusiast.
Slab avalanche danger is very difficult to detect unless you know the signs and the symptoms and follow the clues given by the mountain. Current or recent signs of Slab avalanche activity are the most obvious sign of an unstable snowpack.
Slab avalanche risk is at its highest during or immediately following fresh snowfall influenced by the wind.
Slab avalanche risk is heightened by the activity of the wind and the risks can change and increase even days or weeks after a storm due to wind loading. Understand and follow the activity of the wind and the sign it leaves and use it in your risk assessment
Snow study is an integral part of Slab Avalanche risk assessment. Learn to recognise the signs and symptoms of slab build up and understand the fragile layers within the snowpack. Dig regular snow pits at different altitudes and aspects on the mountain and build up your understanding of what is happening under your feet.
Temperature plays an enormous role in the weakening, settlement and stabilization of the snowpack. Note cold spells and clear nights and the affect they might play on the snowpack over time. Be aware of temperature increases, especially after fresh snow or wind loading.
Altitude and aspect go hand in hand with temperature affect and so also play very important roles especially in areas where there is a large Vertical difference between the upper and lower slopes as found in the Western Alps.
The more information and understanding that you have at your disposal the greater your ability to make important safety decisions in the Backcountry.
Be aware of your surroundings and changing conditions. Listen and learn from the mountain, heed the warnings signs and make decisions based upon ‘what you see’, ‘what you know and discover’ and ‘what the terrain tells you’.
If in doubt, always turn on the side of caution, find an alternative or turn around and live to ride another day.
Make this your safety mantra ‘Look up (danger from above), look down (danger below), look all around (danger under your feet)’ and enjoy the ride!