HOME to avalanche.org.nz
All photos © Gordie.Smith , unless otherwise stated
Avalanche 101 Online - Avalanche New Zealand's free Online Theory Course!
NZ avalanche specialist and educator Gordie Smith put this great course together. It brings together many of his photos, online videos, and text that walks you through the basics of traveling in avalanche terrain, covering (quickly) all the topics that you'd encounter in a fundamental avalanche class. It also includes an interactive exercise (Flash req'd) where you can try and find your way safely through avalanche terrain, avoiding the hotpsots where avalanches might be lurking. This resource gives students the chance to prepare, giving them a framework of theory to hang the field skills they will practice during any snow based course.
These core skills are the same for those who ski, snowboard, climb, snowmobile, hike and hunt.Once you have these basics under your belt, enroll on one of the many Avalanche Courses offered around the country. Avalanche safety is a practical skill that cannot be learnt by just reading a book, or completing this online course. Learn with expert guidance out in the field.Use the menu to the Left to quickly jump to a topic.If you would like to try the quiz to see how much you might already know, then CLICK HERE
Hi, welcome along. By understanding a few basic concepts, we hope you will start to understand why avalanches can occur. They are not random events. Through good planning, and looking for clues in the field, we hope these basic concepts will help you to make good decisions, navigate around areas of avalanche danger and avoid their grim consequences.
So ‘What is’ an avalanche? – An avalanche is a moving mass of snow on steep terrain, which has been triggered by adding weight/stress to a weak area of the snowpack.
Below are a couple of video examples of triggered slab avalanches
Okay, so alot is going on there. Let’s look at the different parts of the puzzle.
Weather creates avalanche conditions, primarily through a combination of Rain or Snowfall, Wind and Temperature.
Snow – it comes in different varieties, and can be described by many words. Powdery, chunky, buttery, chalky, funky, crusty, light/heavy, dry/wet, and even the oxymoron 'packed powder'?(according to the ski resort marketing depts. – haha)
The list is long and seldom scientific. Alot of these words imply an indication of strength which is good, as some snow types are relatively strong, and others are relatively weak. (THINK – making snowballs – sometimes it’s easy to press together snow to form a strong hard packed ball, other times it’s tricky because even if you squeeze the snow hard together it doesn’t want to bind and remains loose and weak.)
Snowpack layers - As snow settles on the ground it forms itself into layers of different strengths. These layers start to bind together forming what we call the ‘Snowpack’. Not all of the layers stick to adjacent layers with the same quality. Some will bind together well, and others relatively poorly. (To have a layer of weak snow underneath a stronger layer can be a potential slab avalanche.)Here is an example snow pack test that highlights where weak layers can be located.
These stronger and weaker layers are not evenly laid out across the terrain. Some places will have more layers than others, and the depths of the layers can vary from place to place. These variations are heavily influenced and modified by changes in Wind, Temperature, and humidity.
Exposure to Wind
In the mountains, especially close to ridge lines, it is rare to have a storm without wind, and as it blows, the wind can strip the snow away in places and pile it up in others. Different parts of the mountains will retain more or less snow depending on:
ASPECT- which direction a slope faces. Some aspects will be sheltered from the wind and collect more snow (sometimes called Lee areas), and other aspects will be exposed to the wind and have much of the snow stripped away.ALTITUDE - Generally the higher on the mountain the stronger the wind will be.
Lee Slopes –When the wind blows, snow is taken from upwind areas and gets redeposited in downwind, sheltered areas. We call the sheltered areas lee slopes (from the sailing term leeward). You can see this alot around terrain which changes abruptly like ridge lines. Signs of lee slope snow loading include smooth, pillow shaped snowdrifts and overhanging cornices.
In the picture below you can see that the wind has been from left to right. Left of the ridge it is back to bare rock and tussock, but on the right side of the ridge (the lee side), cornices have formed and there are large deposits of snow.
Lee slopes are not always at the top of a slope. In the picture above you can see where the wind has swept across the mountain face from left to right, overloading the bright white side of the vertical gullies. We call this ‘cross loading’.
Mountains act like large sails sticking up into the sky. They bend and deflect the speed and direction of the wind as it flows over the terrain. We call this ‘local wind effect’, and it is always present. Example: Even if you know it’s been blowing Northwest all night, and expect to see wind drifted snow piled up on Southeast aspects, there will be variation due to ‘local wind effect’.
Exposure to Sun
Over time the heat from the sun can help to stabilise the snowpack, but if we get too much too quickly it can do the opposite. If a slope gets little or no sun, it can also prolong unstable conditions.
SHADY SLOPES - slopes facing south (southerly aspects) will remain colder as they receive little or no sun. New snow will take longer to stabilise, and weaknesses in the snowpack can persist.
SUNNY SLOPES - slopes facing North (northerly aspects) receive the most heat from the sun. When this heating is intense or rapid it can quickly weaken the snow on these aspects. It is common in springtime to see naturally triggered loose snow avalanches in the middle of the day.
As we are surrounded by ocean in NZ, our temperature and humidity can yoyo up and down. It is not uncommon to receive a mixture of rain and snow, even at high elevations during the winter months, especially when warmer air is coming down from the North or Northwest. These changes in temperature and humidity can warm and melt the surface snows, and then have it refreeze into crusts. You can find crusts on the snow's surface or it may be buried by the next storm or wind event.
Once snow has settled to the ground the story does not end. Layers of the snowpack continue to be under the influence of the weather. Here are two examples you may have experienced:
- Winds can shift snow. What might have been fresh loose snow in the morning, can get stripped back to reveal an old hard layer below by the afternoon(Wind transportation)
- The sun can heat the snow surface during the day making it damp, only to have a cold overnight temperature refreeze it, turning it into a hard icy crust the next morning.(Melt/Freeze)
Below: ski tourers boot packing on a firm morning crust
The weather is always changing, so it’s reasonable to expect that the snowpack’s layers are also always changing because of the influence from the weather.These changes can occur quite fast and produce dramatic results that are easy to notice, for example: the wind pushing loose snow to fill in your freshly made tracks. Change can also happen slowly and be more difficult to spot because the clues are hidden down below the snow’s surface.
Snow has a certain amount of strength, but if you apply enough stress and overload this strength, it will fail or ‘fracture’. We call the source of the stress a ‘Trigger’.
Natural triggers are ones that come from a weather influence, for example:
- Extra weight – this could be from new snowfall, wind loaded snow, or rain.
- Rapid warming caused by warm air, sun, or rain.
Human triggers are ones caused by...you guessed it, Humans. Here are some common examples:
Below is a short video of a human triggered slab avalanche. This video has sound - please excuse his 'French'Did you notice anything about the slope before the skier triggered the slab? YES - it had several tracks already on it. We will talk more about 'Common Trigger Points' in a few pages under 'Terrain'.NOTE: Nearly all the avalanches that have killed people in the backcountry were triggered by the victim, or by someone else in their group. Knowing how to recognise and avoid terrain that has avalanche potential can help keep you safe.
- Climbing/Hiking/On foot
While there are many different types of avalanche, we can place them into two broad categories; Loose, and Slab. Each has different characteristics to tell them apart.
Loose Snow/ Point Release Avalanche
A loose snow avalanche is also often called a point release avalanche. As the names suggest, they involve loose, unconsolidated snow which initiates from a point, gradually fanning out as it moves downhill. This often makes for a ‘teardrop’ shape.
The triggered weak layer tends to be the top layer of the snowpack, the surface layer(s). (Loose avalanches start where the stress is applied.)
Below is an example video of typical small loose avalanches:
A slab avalanche involves a cohesive block of snow that fractures within the snowpack. A crack spreads out across the slope leaving a tell tale fracture line called a crown wall. The slab will break up into smaller blocks as it moves downhill.
The triggered weak area (layer) is below the surface layer, within the snowpack. Slab avalanches need a relatively strong layer of snow over a relatively weaker layer. When stress becomes too much for the slab to take, it’s ‘back will break’ starting a slab avalanche in motion. Slab avalanches can break above you, giving you little or no escape. That's why slab avalanches pose the greatest danger - watch these video examples...
(To read more on avalanche danger problems that are used in the Backcountry Advisories CLICK HERE)
Avalanches range from tiny loose snow sloughs (pronounced sluffs) up to monstrous walls of snow like a tsunami. Generally the bigger they are, the greater the destruction will be.
We rate avalanche size using this scale:
NOTE: Small slides can be enough to cause injury/death if you find yourself in an exposed place like a steep walled gully, ice couloir, or above a cliff. (Terrain traps – more on this later)
The danger from avalanche varies with terrain. Avalanches are more likely to happen in terrain with certain characteristics and features. Being able to recognise these features is a key skill to develop in order to travel safely in the backcountry.
The most important factor in identifying avalanche terrain is slope angle. Most slab avalanches are triggered on slopes between 30° and 45°, with 38° being the most common and prone angle. This is similar to an advanced Black Diamond run on a ski field. Slab avalanches are the main killers so pay special attention to these angles. Areas where the slope angle is less than 25° are generally safe, unless there is a threat from a large steep slope above. When slopes get steeper than 55°, they struggle to accumulate enough snow to form a slab. They are more likely to empty themselves regularly with small sloughing avalanches.
Curved roof showing how snow slides off very steep angles, is just waiting for a trigger on moderate angles, and will never slide off shallow angles. Thanks to Bruce Tremper for images
If you are familiar with ski field colour ratings for their runs, then this chart (below) may help you start to get a feel for slope angles.
Judging slope angles is tricky and often your perspective is skewed depending if you are looking up from below, or down from the summit of a slope. It can also be misleading depending if you are up close or faraway from the slope you are estimating. The simple solution is to not guess, but measure the slope angle.
Here is Bruce Tremper, an avalanche specialist based in Utah, to give you some great tips on how to measure slope angles.
Common Trigger Points
These are places where avalanches are more easily triggered. They are areas where the snowpack is weaker, already under stress and overloaded.
Convex rolls– these are rollovers where the slope angle increases. If you are standing at the top of a slope and cannot see the bottom as it rolls away from you, then this is a convexity. Notice in the photo below the area between the two cornice overhangs. you can see where the slab avalanche has broken away along this convex roll
If a convex roll is abrupt it often will form a wind 'lip' which is like a mini cornice. The video example below shows a convex roll which is also a 'Lee' slope' full of recent wind loading.See how easy it is for the ski patrol to trigger shallow wind slabs.
Unsupported slopes - Some slopes end in a cliff. Snow relies on the 'toe' or base of the slope for extra strength. If there is no gradual concave shape at the bottom of a slope, then the slope has little or no support to hold it up.
It is sometimes obvious to not travel on these areas, but you have to remember to give them a wide berth especially when traveling under these zones.
Near rocks or shallow areas of the snowpack - The Slab avalanches below have broken away from one rocky outcrop to another. These are weak points and basically holes in the snowpack, so any strength the snowpack may have is less at these points.
Video example below: the snowboarder rides close to the rocks in the middle of the slope where the snowpack is shallow. That is where their weight has triggered the buried weak layer. Others had already gone down this slope but did not ride over this extra weak spot (common trigger point)
Here is another video example. NOTE: when the Slab breaks away, you can see a series of rocks along the crown wall
Steep shaded slopes or gullies
In NZ slopes facing south (North in the Nth Hemisphere) will remain colder. new snow takes longer to stabilise, and weak layers can last longer
Close to ridgelines and lee to recent wind direction.
These are also sometimes called entry slopes. They are often overloaded with windblown snow and under tremendous stress due to the extra weight of the snow dirfts. Often they can trigger naturally under their own weight or will avalanche with only a light trigger.
What will happen if this slope slides? Where will I end up?
Terrain traps are places that increase the consequences of an avalanche. They increase the likelihood of either trauma or a deep burial. Even a small slide with a cliff, rocks or trees in the path can be deadly.
Rocks Cliffs Trees = Trauma
Other terrain traps increase the likelyhood you will be deeply buried even by a small slide as the avalanche debris will pile up more easily. These include:
Flat spots/road cuts
Photo - Andrew Hobman
Gullies or hollowsWe all come to the terrain in different ways depending on how we travel. Its a good idea to have some basic rules that you use each time you go out. This makes things more systematic, so you dont forget things when you're having a bad day. We encourage you to develop your own rituals and routines to approach terrain. Below is a short video from well known backcountry skier Greg Hill. He tells us his rules.
How do we know when there are unstable snow conditions?
In most avalanche accidents one or more of the following signs were present. Check the Backcountry Avalanche Advisories HERE when planning your trip, and see if the Forecasters have mentioned any of these red flags. Note where (aspect and elevation) these dangerous conditions have been observed. Stay alert when making your own observations in the field.
Some of these clues are obvious and will be hard to miss, others need you to be alert and actively seek them out. Making sense of what you see, hear, and feel takes time. Learn to scan the terrain looking for clues as you travel. Use small rolls/slopes where there are no terrain traps and where the chances of being caught are low to get a feel for the snowpack. If moving out onto larger more exposed slopes, scan for key details and avoid common trigger points. Communicate anything you observe to others in your group.
The strongest indicator of unstable snow is recent avalanche activity. It tells us there is a weakness in the snowpack that has been triggered. Take note of the where it occurred, as well as the type and size of the avalanche. You should treat slopes that have the same aspect and elevation as suspicious. Avoid them if you can, but if you need to use these slopes, stick to low angled sections (below 25°) that are not connected to larger slopes above.
Signs of Instability-
Collapsing snow (whumphing sounds), shooting cracks, and hollow drum like sounds are nature’s warning signs that the snowpack is unstable. When a sudden collapse occurs, you have triggered the weakness in the snowpack, but the slope may not have been steep enough to slide downhill. A whumph should make you nervous in the guts, perhaps even lower, maybe causing a pucker. Back off any steep terrain you might be on.
Heavy snowfall or Rain in the past 24hours-
Significant new snow or rain can overload and weaken the snowpack. Avalanches are often triggered naturally during and immediately after a storm, as the snowpack cannot adjust fast enough to the extra weight being added, and it fails. It is also common for people to trigger avalanches on the first fine day after a storm as the snowpack is still sensitive to light triggers, and needs more time to stabilise. Be patient before charging into steep terrain straight after a storm. Gather more information from local sources to see if there has been any local recent avalanche activity.
Wind can deposit snow ten times faster than snow falling from the sky. It deposits snow on lee slopes and can rapidly create dangerous ‘Slab’ avalanche conditions. Wind often accompanies a storm, but it can also build ‘slab’ conditions days/weeks after the last storm even when skies are clear. PICS Watch for tell tale signs like cornices forming on the lee side of ridges. These will often overhang a slope that appears ‘fat’ and smooth. Remember about cross loaded slopes too.
Rapid increases in the temperature of the snowpack can weaken the bonds between snow crystals causing unstable snow conditions. Sources of rapid warming can be direct sun on northerly aspects, warm air often coming from the north or northwest, and rain. Measure this with a thermometer PICS: Early signs can be pinwheeling/snowballing on the snow surface of steep ground (especially below rock outcrops), sticky or boggy feeling snow where equipment does not glide well. Late and very dangerous signs can be deep glide cracks that open up. These often appear where the terrain rolls over sharply and is unsupported.
Persistent Deep Weak layers-
These can be difficult to identify, and you will often not know they are there unless they are mentioned in your region’s Backcountry Avalanche Advisories. You should pay special attention to where they may be located (aspect and elevation). These areas need to be avoided due to the large amounts of snow that can be involved. Avalanches failing on a deep weakness can spread over a surprisingly wide area, and are seldom survivable.
4. Human Factors
Even with the best skills and years of experience under your belt, we all occasionally make poor decisions.
Studies into avalanche accidents have found that the victim often had the skills and knowledge to recognise the signs of unstable avalanche conditions but chose to go anyway. They ignored the facts and based their decisions on emotions.
How we feel on any given day can change. Some days we may be confident, cocky, bullish, self assured, be willing to take on more risk, and on other days we may be more reserved, sceptical and overly cautious. Who we travel with can also affect how we make decisions.
By understanding a little bit about these factors, you can assess whether they are affecting your decision making.
Familiarity: Are you feeling over confident because you’ve been there before? Always approach a slope as if you have never been there before.
Acceptance: Do you have a desire to be noticed and accepted or not wanting to speak out or communicate concerns for fear of conflict or embarrassment? Think of the respect gained from being the one that makes decisions that keeps everyone safe.
Commitment: Are you bound to a plan or the way you think you should act? Make the day’s goal to return safely and have good options that you would use.
Experts: Are you following someone because you think they know better? Question authority. Develop an atmosphere where everyone in the group has an opinion and voices it; use a team approach to decision making.
Tracks/Scarcity: Are you racing to get there first? Or get home? Remember that competition and danger go hand in hand.
Social Proof: Are you thinking, it must be ok because others are doing it? Ask yourself, would I do this alone, without a transceiver, or with no-one else watching?
Follow these links to find out more on how easy it is to miss, or turn a blind eye from obvious red flag clues that avalanche danger is eminent.
· Rogers Pass avy pros
To have the discipline to make judgments based on facts and not emotion takes a systematic approach. The use of checklists is common when there are many factors to consider, and the consequences of a poor decision are high. Airline pilots and surgeons have used checklists to help them eliminate errors and the possibility of overlooking valuable clues. Get yourself into a good systematic way of planning and preparing for your backcountry trips. Below are two videos by world renown backcountry skier Greg Hill. These short clips give us an idea of the rules/system that he tries do stick to when out beyond the ropes.
5.Safe Travel Techniques (Terrain tactics)
How you travel in the backcountry, and which terrain you choose, can greatly affect your safety in avalanche terrain. The following tips are standard minimum precautions that should be used at all times.
Spacing – travel one at a time or with enough spacing so that only one person is exposed to an avalanche path at any one time. This applies to going up and down.
The combined weight of people travelling together may be enough to overcome the snowpack strength and start an avalanche.
Watch what happens when all 3 skiers enter the slope. Their combined weight is enough to trigger the weak layer and cause a slab avalanche. Aside from the crew in the helicopter filming them, who is left to begin the rescue?Visual contact – always watch each other from safe spots. Should the worst happen and a slope does avalanche, give your buddies the best chance of rescue by watching them across suspect slopes
Islands of safety – only stop in safe zones, never in or below an avalanche path. Always spend the least amount of time in the direct ‘line of fire’ of a potential avalanche. Never stop or take a break in an avalanche path. (Bush clearing picture and or skidoo shot from somewhere).Safe travel routes – Use low angled terrain and ridges. Avoid exposure to terrain traps where possible. Never travel directly above or below a partner/group.
Communication – Get together at safe spots and discuss decisions with each other. Many accidents have occurred when the person out in front has assumed everybody knew the plan. Make sure everyone has a chance to contribute their observations, voice their concerns, and agree on the travel plan.
Here is a simulation game where you draw in your travel route trying to avoid avalanche terrain. Just imagine you are hiking/skinning up or down and try to avoid the common trigger points and places where avalanches might start. Navigate your way from the Green Hut to the Red Hut. Follow instructions on the screen.
If you are caught and buried in an avalanche you may only have minutes to live. If you are wearing a Transceiver you have a much better chance of being found quickly, but only if your buddies have the right gear and know what to do.Time is critical when someone gets trapped by an avalanche. The following tips will help make the rescue more efficient.If it is you that gets caught:
If someone in your group gets caught:These rescue cards are available in our shop HERE
- Make noise, yell and wave at the rest of your buddies - make sure they have their eyes on you.
- Attempt to get out of the flow by angling towards the edge of the slide.
- Roll onto your back with feet downhill, swim/fight to remain on the surface
- Insert avalung and/or deploy airbag
- Discard equipment like skis, poles - they will act like anchors pulling you down.
- If under the snow, as the debris slows down, attempt to create an air pocket in front of your face - bring arm across so mouth and nose are tucked into your elbow
- Remain calm and try to breath evenly