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Factors that contribute
towards avalanches

It’s great skill to know the positives and negatives of new snow fall as you look to expand your off piste skill set. You might be chomping at the bit to hit those fresh slopes, but there could be a number of hazards just waiting to catch you out. So lets look at the factors beyond just the human element that can have an effect on the level of potential avalanche risk.

Weather — Precipitation

It's not just about the snow falling, but in what conditions it was falling and when it last fell.

How do you tell the amount of snow that has fallen? You can check your local avalanche report as well as stepping outdoors and seeing how much snow is on features like fences and cars etc, or when you hit the slopes seeing how much snow has fallen just off the piste.

Weather — Temperature

Temperature is one of the key factors that comes into play. In general on a ski tour we’ll be heading out early and aiming for most of the hard work to be done prior to lunch. The sun heating up the snow pack even on what would be a typically cold day still has an impact on safety.

The warmer the snow pack the more likely it is to slide as the water from the melting snow lubricates layers within the snowpack and soaks into the snow. That's why that last run of the day off piste excursion on that untracked south facing slope may not be such a great idea.

Also temperatures over long periods of time can create stable or unstable layers with the snow pack. Most commonly we look at the temperature gradient of the snow, i.e the air vs ground temperature. The stronger the gradient, i.e the bigger the difference (more than 1 degree per 10cm of snow pack) towards cold air temperature vs the ground that is zero degrees then there is more chance of troublesome faceted crystals forming with the snow pack.

These crystals do not play well with others and create unstable layers. This's why some great early season dumps of snow followed by very cold temperatures can bury hazards for an entire season.

Weather — Wind

Wind is a critical aspect in relation to avalanches. A large snowfall combined with high winds is a common factor in increasing the avalanche risk especially when slopes become wind loaded. Basically the wind can break down the awesome shape of snow crystals and make them more rounded and therefore less likely to bond together. The wind can then move those rounded grains around 10 times more rapidly than snow falling from the sky and deposit them in large volumes on certain slopes creating potential hazards. A slope with these types of hazards would be referred to as “wind loaded” You typically find these slope on the “leeward” side of a piste or ridge line. You can see this in the photo below.
That wind loaded slope could look really appealing but if you drop in on that cornice you just might trigger the rest of the slope to slide due to the sudden and massive movement of a large amount of snow.

You can also see the effect of wind on the “windward” side. You can see the desert like contours in the snow called Sastrugi. The wind will compact the snow and as in this case create a solid bonded layer that can sit on an un-bonded lower layer.

Terrain — Slope Shape

An essential part of route planning is making sure your hiking or riding routes that do not subject you to undue risk. These risks can come from things like terrain traps that include bowls, cliffs and deep river beds where if the slope was to pop then you would be dragged into or off. Other areas of awareness would be convex rolls or being in gullies, on cornices and steep slopes that can open you up to more hazards.

As part of our riding plan we look for islands of safety that isolate the group from any triggers from other members or provide a potential run to zone if something was to pop. You can see in the image below the risk zones in red and orange. Our typical route is through the middle and stopping where the two off piste routes split in the centre of the image. That gets us well away from the main hazard slopes.

A popular app showing off piste routes and avalanche hazard zones

Terrain — Slope Aspect

Its something to really be aware of in relation to what slopes you ride and when. Ideally you want settled and stable slopes with some fresh snow on them. That would typically be on a south facing slope, as the sun will melt and help settle the snow pack (The Rounding Process). But over time the sun will start to have a detrimental effect as it melts too much snow and starts to saturate the snow and lubricate the lower layers of snow pack.
Image for post

The inverse of that is that north facing slopes will hold the snow quality longer as they’re not so effected by the sun, but it will mean that the snow pack might not be as stable because it has not had as much warming action over the duration of the winter. You can see from the statistics in the image how the risks increase as you head on more north facing slopes.

Terrain — Slope Angle

The most obvious thing often discussed in relation to avalanches is slope angle. The general deal is the steeper the slope the more likely it is to slide. Around 97% of all avalanches happen on slopes over 30 degrees. This chart is a really good indicator that relates well to the avalanche readings provided by forecasters each day.

Although what you will need to do is get used to judging slope angles. That is really hard. There are various apps and tools that can help you. When judging a slope your looking at the steepest part that is at least 20m x 20m. Its well worth setting yourself a challenge and starting estimate some slope angles when your out in resort. You can then start to get a natural feel for the terrain.

To get a quick overview of some of the keys areas of focus you should check out this useful video.

Source — Ortovox Safety Academy Guide Book.

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