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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.