Avalanche Basics

Chamonix veteran Chris Fecher unlocks some of the secrets of avalanche safety in this piece covering avalanche basics. He hopes to leave you with a basic understanding and a thirst for developing your own avalanche awareness and off-piste skills.

Avalanche basics
Avalanches are one of the most dangerous factors in off-piste skiing and backcountry travel. Chances are you’ll struggle to out-run one and digging your friend out with a ski is pretty much impossible. Taking part in avalanche awareness courses, reading books and talking to the ski patrol is a good place to start. Learning to recognise potential hazards and carrying out you own avalanche safety evaluation can, with careful planning, help you reduce the risk of being caught in a potentially lethal situation.

Studying the forecast and relating this to the specific hazards is one of the best places to start, the info is out there so collect as much as you can!

There are many factors that can trigger avalanches and there are a number of forces and elements that dictate whether a slope is safe to ski or not. The safety of a slope is determined by the stability of the snow pack, which in turn is affected by wind, terrain, aspect (the angle to the sun), moisture content, weather conditions the gradient and of course how well the different layers are bonded together. A good freerider takes many years to develop this snow sense and considers all these factors when deciding what and where to ski.

Official Avalanche training partners

Avalanche basics – The Facts:
If you are buried in an avalanche for up to 18 minutes there is a 91% chance that you will survive. After 35 Minutes this chance drops to 34%. These figures show that every second counts and it is of vital importance to find and rescue a victim as quickly as possible.

It usually takes rescue teams, dogs and helicopters longer than 18 minutes to arrive at an avalanche site so it makes sense to give yourself, and the people you’re skiing with, the best possible chance for survival by adopting the ski buddy system.

The basic equipment you must take and that’s including your buddy, when skiing off-piste is an avalanche transceiver, a shovel and a probe. But you must also have the skills to use them. Equipped with the correct equipment and a knowledge of how to use it you stand a much higher chance of surviving an avalanche or finding your skiing buddy if they get caught. Transceiver practice can be fun, why not turn it into a drinking game or challenge your friends to see how fast you all are at finding a buried transceiver in a 20m square area. Can you find it in under 2mins?

Avalanche basics – Types of avalanches:
An avalanche is triggered when the slope is close to or can no longer hold the weight of accumulated snow. The most common types of avalanches are slab and loose snow avalanches. Slab avalanches are the largest and most dangerous. They start from a fracture line and, as the name suggests, an entire slab of the snow pack breaks away. Initially in a single piece, before it liquefies and breaks up into smaller pieces.

Slab avalanches happen most frequently on 30º to 45º slopes. They occur when a layer of tightly packed new snow, or wind packed hard slab, is weakly bonded to the layers below. The top layer slides over the layer below initially as one slab, then it breaks up as it flows over the ground. Cracking or dull booming noises are clues that you may be on a hard slab.

Loose snow avalanches start from a set point and are progressive – they gather more and more loose snow as they move down the hill. They usually happened on 40º to 60º slopes where the snow is
loose, like sand.

Avalanches start when forces and triggers affect the stability of the layers of snow in the snow pack. Weak layers, like icy layers, or a weak bond between layers, like powder on top of hard packed snow, make the chances of avalanche more likely. As a seasonaire you should know the history of the snow in your area by keeping an eye on the avalanche bulletins and European avalanche safety scale. Digging a hole to look at the layers will give you an idea of the state of the snow pack but without knowing what to look for it’s only a small part of the puzzle.

Avalanche basics – Terrain factors:
The avalanche path has distinct sections: the starting zone (this could be high above you), the track and the run-out zone.

Start zones are typically steeper slopes, the top of gullies and bowls, ridges and slopes exposed to wind deposited snow.

It’s also important to be aware of how a potential avalanche would react with features on the slope when planning your line on a face. An avalanche can be directed by trees, gullies and ridges, try to note where a potential avalanche could start, travel and run-out.

Slopes in the lee of the wind (sheltered from the wind), cornices and the sheltered crest of slopes receive more snow so avalanches are most likely on these slopes.

Convexed slopes are a hazard because the slope is under tension, below rock faces, trees and rock out crops are also potential start zones.

Be aware of terrain traps when considering whether to ski a slope. If an avalanche occurs watch out for any natural features that could potentially trap and bury a person. Steps, bowls, crevasses and obstacles may cause injury and assist burial.

Generally safe areas when skiing in the backcountry:
Ridge tops; gently sloped windward side of ridges; level ground far away from steep slopes and dense forest.

Potentially unsafe areas with an unstable snow pack:
Steeps slopes; lee slopes; convexed slopes and cornices; lee side of ridges; slopes exposed to the sun for several hours and gullies.

(The answer to some of items listed below could mean ‘DON’T GO’)
Preparation before skiing: !Check the local avalanche forecast!
1. Check the general and local weather conditions.
2. Talk to ski patrollers and/or guides about where you are going.
3. Check transceiver battery life and wear the transceiver under your jacket (not in your backpack).
4. Switch mobile phones off or make sure it’s not near your transeiver – they interfere with the transceiver signal.
5. Never ride by yourself.
6. Choose your line carefully. (Consider the consequences of an avalanche)
7. Choose an escape route.
8. Travel at the top of start zone, bottom of run out and avoid mid slope
9. Move from one safe spot to another.
10. Ski down and across, reduce disturbance and consider a safe speed

Avalanche basics – European avalanche danger scale:
The perfect reference point to the current avalanche stability is the avalanche rating scale. The scale goes: 1-Low, 2-Moderate, 3-Considerable, 4-High, 5-VeryHigh. This is also represented by a number of different coloured avalanche flags. A solid Yellow flag 1-2, Yellow & black check 3-4 and Black 4-5. The highest percentage of people get caught at levels 3-4 on the scale. This considerable to high avalanche danger rating is more challenging to forecast as it depends on a wider and more detailed number of factors. Forecasting at the extremes are easier as the information is more obvious and more readily available. For example if you’ve seen naturally triggered avalanches running and or experienced slope settlement then that should be a huge indication of the current situation and a big ‘Don’t Go’. The other extreme may be that there has been no real weather change and moderate temps for days leading to a low hazard rating and a stable snow pack.

How snow forms:
The birth of our beloved white stuff ‘snow’ occurs when super cooled water vapour (with the help of tiny particles) crystallises into tiny snow crystals high in the atmosphere. On this journey snow crystals can be affected by wind and temperature and they don’t always end up looking like the perfect Stellar crystal we have grown to recognise. In fact, there are several different types of snow crystal.

Basic snow crystal form:
• Stellar Dendrite (dendrite). Most recognisable snow flake from, six-branched star
• Plate. Hexagonal flat plate
• Column. Six-sided, hollow column,
• Needle. Fine, long, crystals

Factors that alter the snow pack
When snow crystals finally hit the ground they begin to change, or transform. From the moment they hit the ground (depending on the temperature) snow crystals (or grains), begin to change size and form. Over time the snowflakes decompose into round-ish looking shapes. Since the snow pack is constantly changing, the bonds between the grains can either be gaining or losing adhesion. Internal changes in the strength of snow pack layers can also be caused by the mechanical action of packing the snow underfoot or during an avalanche this is usually related to a temperature change.

How the snow pack gets stronger
Water vapour moves from areas of high vapour concentration, like the ends of the snow crystal’s arms, to low, hollow areas of low vapour concentration, like the place where two round crystals touch each other. Over time, the snow crystals will become more rounded and individual crystals will become glued together, this is called sintering, necks develop between grains and this strengthens the snow pack. Characteristics of this snow type would be hard snow with small densely packed grains.

How the snow pack gets weaker
If there’s a big difference in temperature between the top and the bottom of the snow pack, free water molecules in the pore spaces between the crystals tend to move upwards towards the surface. When they reach colder air, these molecules cling onto the under-side of the snow crystals above them.

Over time, these crystals will develop a faceted, or square appearance and will lose their ability to interlink, or stick to each other. This is known as Kinetic Growth because the temperature gradient within the snow is usually higher than 1ºC per 10cm. The faceting process creates un-cohesive, sugary snow, and nearer the ground it is called depth hoar. Melt-freeze metamorphism also weakens the snow pack and is as its name suggests – the rapid melt and re-freeze of snow crystals in the snow pack. This creates a strong pack when it is frozen, and weak when not. This is characteristic of spring snow, when the snow pack becomes weak in the afternoon sun and gets stronger over night as it freezes.

The development of surface hoar crystals can be very dangerous in a snow pack. Not to be confused with depth hoar crystals, these crystals are produced when there is a deposition of water vapour from the air on to the cold surface of the snow. They are typically fan shaped, or feathery, in appearance and occur when the air temperature is higher than the snow surface temperature. If found deep in the snow pack, say from a period of warmer weather, hoar crystals can create a major instability that can only be improved by a lot of time, rain and avalanches. In the backcountry hoar crystals are a major factor in instability and typically little or no traffic allows them to continue to grow.

Wind
The way the wind affects the snow is one of the most important factors to consider. Wind transported snow is deposited in large amounts on more sheltered, lee, slopes. The wind blows snow crystals around and breaks them down into smaller grains forming denser layers (slabs). Warm winds heat the snow pack while cold winds cool. Loose snow is redistributed by the wind. Watch out for wind blown slabs, wind lips and cross loading.

Snow Profile:
We have discussed snow grains and the factors creating a strong or weak snow pack. But the density, weight and thickness of each layer of snow are as important as the grain size, type and shape and the bond between these layers is, perhaps, the most important factor.

The best way to look at the snow pack to help you build a better picture of the history is to do a ‘snow profile’. Find a safe slope that represents the aspect, or steepness and angle of the slope you are interested in. Dig down to base or the last layer you observed and look at the different layers. To firmly understand a snow pack, the information really needs to be built up over the season, so test profiles on different aspects and suspect slopes should be done throughout the season. When visiting an area it’s a good idea to speak to the ski patrol or local guides to see what’s been going on before heading out.

Make your test pit big enough so you can get in there and have a good look, smooth off the sidewall with a ski and carefully cut through the snow with a thin card (your mates credit card if you like) feeling for the layers as you go. Try to identify the soft and dense layers and the weak bonds between them. Using a crystal card and magnifying eyepiece to identify crystal form and size will help you determine the state of the crystals. If you can push you fist into the layer then that’s pretty loose and soft if it’s to dense and if would be easier to push a pencil into the layer then that’s getting pretty hard and dense.

Compression test:
One of the most useful tests is the compression test. Practice doing lots of these as they provide a quick way of determining the upper layers stability. Clear off the rear wall of your pit and isolate a test block 30cm wide 30cm long and 70cm – 100cm deep cutting each side and down the back. Place your shovel blade squarely on the top of the block.
Tap the shovel blade 10 light with your finger flexing at the wrist, failures at this point would be ‘Easy’. Next Tap 10 times flexing from the elbow, failures at this point would be ‘Medium’. Finally Taping 10 times from the shoulder would produce a ‘hard’ score. Canadian and New Zealand avalanche research suggests human triggered avalanches are more likely associated with ‘Easy’ fractures than with ‘Hard’. The more test’s done the more accurate your results will be. Examples of this process can be found on the Tinderbox Ski School Blog.

The key to successfully making avalanche predictions is practise, time on the hill and number crunching, From a more practical point of view, a skier needs to be aware of all the elements and how the snow is changing from day to day. Joining an avalanche awareness course is the best way to learn, mixing the theory with practice on the snow.

If you’re interested in learning more about avalanche safety contact Chris Fecher at Tinderbox Ski School Chamonix about our weekend Avalanche awareness course’s and Backcountry ski courses. You’ll get a real practical look at the finer points of avalanche safety and backcountry travel.

www.tinderboxskischool.com

Avalanche Basics

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