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.