As a keen Snowboarder, this article is written with an emphasis on back country Skiing and Snowboarding, however, the information in this article is equally relevant if your focus is on Mountaineering, Climbing etc.
Avalanches are defined as a mass of snow, ice, rocks and debris, falling or sliding rapidly down a mountain. Avalanches are a dangerous phenomena which kill hundreds of people each year. With the recent surge in popularity for back country skiing and snowboarding, more and more people are heading off into to back country in pursuit of powder and/or riding or skiing the ultimate big mountain line.
The unfortunate reality is that many of them are heading out unequipped and unprepared for the dangers that lay in wait.
Avalanches come in three basic flavors:
- Slab Avalanches
- Sluff or Loose Snow Avalanches
- Wet Avalanches
All three can pose a serious risk but the slab avalanche has claimed the most victims.
Slab Avalanches occur when a weak layer in the snow-pack gives way, causing the layers above (the Slab), to slide on a firmer, base layer below.
The constantly changing conditions in the mountains, cause each layer of snow to be affected in different ways causing variations in the different layers which make up the snow-pack. Some of these layers are stronger and some are weaker. When a layer has been affected by the sun, rain or temperature and it has started to melt and then been frozen again it causes a harder crust layer. Bed layers such as these can provide an ideal platform upon which layers above (often referred to as the slab) can slide.
The three main Avalanche factors are:
- Precipitation (Snow or Rain)
- Aspect (the direction that the slope is facing, in context to the sun and wind)
Wind has a big impact on the snow-pack as it has the ability to move large amounts of show from one place to another. A slope which has a large amount of snow deposited on it by the wind is referred to as wind-loaded. A wind-loaded slope can be dangerous as the loading can add enough additional weight and tension to an already unstable snow-pack to either trigger an avalanche or provide a loaded trigger point on the slope.
Often, wind will collect snow from the windward side of a slope (wind facing) and deposit it on the leeward site of a ridge. This not only causes wind-loading on the leeward slope but also allows a large build up of snow to settle on the leeward lip of the ridge, this is referred to as a cornice.
When wind travels across the face of a slope it has the ability to collect snow and deposit it across the slope face. Because of the natural imperfections in the face of the slope, the snow is usually deposited in some places more than others causing variable snow depths across the face. This is called Cross-Loading. Cross loaded slopes are dangerous as they are subject to both wind-loading and variable snow depth.
Prevailing winds (winds traveling in a common direction for that region)
The direction that the slope is facing with respect to the path of the sun and wind.
In the Northern Hemisphere, South East, South and South West Aspects are subjected to the sun. The reverse is true in the Southern Hemisphere. In both Hemispheres, Eastern facing aspects will be affected by the sun in the morning and Western facing aspects, in the afternoon.
Shaded Aspects can cause unstable conditions to persist. Things on the surface of the snow, such as large crystals formed by dew on the snow surface, don’t disappear before new snow settles, because it is too cold.
These crystals form a weaker layer which can give-way under the weight and tension when it is loaded with other layers of snow on top, causing a slide.
Differences in elevation present different levels of avalanche risk. Below the treeline there is less snow and wind and avalanche danger is low. When approaching the treeline, there the trees start to spread out, you should start to be more alert as the danger increases. At the tree line, the danger increases again as these areas are subject to more wind and snow. As you head up into the more complex alpine areas, the dangers are increased yet again. There are many more things to consider up there. Along with loads of wind and snow, you must consider cornices, variable snow depth and the colder temperature.
Slope Angle and Shape
It is important to understand that Avalanches can happen at almost any angle, however, certain angles facilitate avalanche conditions better than others.
Slopes at angles of 45° and greater generally do a pretty good job of self regulating. At these angles, gravity does it’s job and pushes fine trickles of loose snow off the slope periodically. Slab layers don’t really get much of a chance to build up; they can but it’s not likely.
Slopes at angles between 25° and 45° are prime candidates for avalanche activity.
Slopes less than 25° are generally flat enough for gravity to hold the snow pack-down solidly, preventing large slab slippage.
Avalanche control is performed at most of the major ski resorts and some popular back-country areas by experienced avalanche professionals.
This usually consists of early morning bombing of trigger points in an attempt to induce any slides which are laying in wait and reduce the risk of avalanche activity while people are on the mountain. For the most part this is performed within resort boundaries so take caution when venturing outside the boundaries or when heading into the back-country.
Identifying potential avalanche areas can be tricky as they are an erratic phenomenon but the snow pack can provide a few clues if you know what to look out for.
Look for common signs of instability. “What do you see around you?” Take note of where avalanche activity has occurred in the past. Avalanches often continue to occur in the same places so don’t become complaisant if you see evidence of a fresh slide. You should be on high alert in these areas. There could be more of the same sitting dormant, waiting to pop at any time.
Assess the layers and the structure of the snow-pack below you. Layers are formed by varying conditions on the snow pack such as wind, dew, rain, frost and sun followed by more snow. These layers can create weaknesses in the snow pack. You can dig a deep hole in the snow to expose and assess the layers. There are a number of tests that you can do to identify weak layers. You can also push a ski pole into the snow pack and see whether initial resistance gives way to softer layers underneath. Understanding the conditions that the snow pack has been subjected to in the days leading up to the current day can be useful. Note the rate of change in weather conditions (Intensity). The higher the intensity, the more unstable the snow pack is likely to be and the higher the risk of it giving way under pressure.
Look for trigger points. These are places where avalanches can be triggered. Trigger points can be created by Weather Loading, Wind Loading, Temperatures, Precipitation and even Human or Animal Triggers.
Look out for cracks shooting out from in front of your skis when walking in them or when standing on your board. when walking across the snow on ski’s, how do they cut into the snow pack? they should be creating a sharp-ish edge. Also listen for if the snow-pack sounds hollow as you travel across it. These are signs the the snow pack is unstable.
Some people report hearing a “Whompf” sound. This can usually be felt as what some describe as a small earthquake.
Common Avalanche Triggers
New Snow Loading
This is when we get a fresh layer of snow which has fallen. As snow is frozen water, even as little as 15cm can be very heavy.
This is when wind blows a thick layer of snow onto a slope. Another form of wind loading is Cross Loading which is where the wind blows snow across the face of a slope and deposits it in uneven depths across the slope.
Causes the slab to get weaker as it starts to melt. Sun can also start to melt the top layer of snow, before the cold sets back in and re-freezes it. This causes an icy crust on the snow which is referred to as Sun Crust. Subsequent layers of snow falling onto a sun crusted layer don’t have much to grip onto and can easily cause a slab slide if the weight of the new layer gets heavy enough, or additional tension is applied by a human trying to ride it.
Rain has a double effect as it adds new load to the slope and causes the snow pack to weaken as it melts. Enough rain and it can weaken the slab to the point where it turns into a giant Slushie and slides down the mountain. Rain can be an instant avalanche trigger.
Now these guys are very dangerous. Cornices are formed when the wind carries snow across a ridge and deposits it in the leeward side (out of the wind) lip. Cornices can form the shape of a thick wave which is hollow inside. You must be careful when standing on the top of ridges as they can be really hard to identify when you are on top of them. They are also really dangerous from underneath also. Weight from the top can cause them to break off. When this happens, if you are not well back from the edge, they will more than likely suck you off too. When a cornice breaks off, it can be the size of a car or mini bus; I don’t need to point out the damage that that could do. But that’s not all, as broken cornices can be heavy and come down with brutal force, they will often trigger avalanches on their descent.
Rapid Changes in Temperature
Warming can weaken the snow pack enough to trigger a slide.
Skiers and Snowboarders
We can trigger an avalanche by applying more tension to a trigger point which is already under load and tension.
The same goes for Mountaineers, Climbers, Sledges & large Animals such as Mountain Goats and Polar Bears.
Identifying Trigger Points
These are rounded surfaces like the outside of a ball. On Convex Rolls the snow pack is under tension. It doesn’t take much additional pressure to trigger an avalanche on a Convex Roll. If you come across a Convex Roll on a slope, it is best to stop on the top and try a controlled trigger with your ski’s or snowboard to test whether it is going to pop. If it seams OK, head over it, one at a time, ensuring the the previous person is not in the direct path in case it does let go. It is probably safest to simply avoid it by going around it. In contrast, slopes which have a concave, which look like the inside surface of a ball, are generally well supported from the bottom and are not likely to be under tension.
Slopes with 20° + angle with rocks scantly popping out all over the place can be unstable. The rocks can warm up the ground around them weakening the snow pack and forming weaker layers of crystals. Rocks can also act as anchors which hold a weak snow pack in place until it comes under pressure. Never ski or board from rock to rock if you find yourself in these areas. Rocks can provide high ground safe zones but scattered about in avalanche start zones, rocky areas should be avoided.
Obvious danger; best to steer clear of them and be careful on ridges as you cant always see a cornice from above. They can be useful though as they are a great indication of the wind direction in that area, which will help to understand where wind loading is likely to occur.
Can provide enough additional weight onto areas such as convex rolls or steep angles, to trigger an avalanche. Wind loaded slopes on the leeward side can be very unstable.
Is the direction that the slope is facing in respect to the sun and wind. As the sun is closest to the equator most of the time, in the Northern Hemisphere, Southern facing Aspects get more sun SE, S, SW. In the Southern Hemisphere, this is reversed and NE, N, NW aspects get the sun. In both hemispheres, the East facing aspects get the most sun in the morning and West facing aspects get the most sun in the afternoon.