Conditions for a slab avalanche
Snow and weather
But what is the difference between a hard or soft slab, what is a weak layer and when is a slope steeper than 30 degrees? This section will discuss this in more detail.
Three ingredients for trouble
After years of study we have found out that there is a higher risk when:
- The angle is larger than 30 degrees AND
- There is a (hard or soft) slab AND
- There is a weak layer present.
Combine these three and it is a guarantee for a dangerous situation. But with only two of three ingredients the chance on a slab avalanche is very slim.
We are going to add another important element to these three ingredients. A slab avalanche is triggered by something. And in most fatal slab avalanches the skier or snowboarder is the trigger. The chance of a slab avalanche being triggered by an external element is much smaller. It's possible that new snow could cause a slab to release naturally. If cornice breaks, a rock falls down or a spontaneous loose snow avalanche starts, that might also trigger the slab avalanche.
You might be the trigger. And you probably are.
Weak layers beneath slabs can be extremely sensitive. But the deeper a weak layer in the existing snow cover, the more limited the impact of additional stress, and therefore the less the chance of causing an avalanche. That looks like the figure below. Notice that the deeper a weaker layer is buried, the less additional stress a person exert on it. But be aware that a group will add additional stress on a snowpack.
Watch out for extra stress
Now you know that you are likely the trigger. Try to apply as little force and stress to the snowpack as you can. This makes you are lighter. But when applying more force or stress to the snowpack you are heavier and this increases your riss. The next cases cause the extra stress:
- Hiking up (1-2 times your own bodyweight as extra stress)
- Short turns (4-5 times your own bodyweight)
- Falling (6-8 times your bodyweight)
In 90 percent of the cases you or your friends trigger the avalanche. The extra pressure from the weight of the skier or snowboarder can be enough to trigger an avalanche.
What makes a slab?
Even with the slightest amount of cohesion it's consider it a slab, whether it is hard or soft. What often feels like loose powder snow is actually a slab. Hard slabs are easy to identify. The biggest nuance is between very loose, but not cohesive snow, and very loose cohesive snow. A slab doesn't need to be so hard you can barely make a hole in it; It just needs to be relatively stronger than the snow underneath. Light, dry powder can behave as a slab as long as it has an even weaker layer beneath it. You can determine this by doing the shovel test. This is a common method among professionals. Without it, it can be difficult for a layman to recognise a slab.
The shovel test is a term that originated in avalanche studies. The goal of the shovel test is to determine whether or not you are dealing with cohesive snow. Carefully take a small scoop with your shovel out of the suspicious layer so that it lays easily on the blade. Hit the bottom of the blade softly. If the snow falls off like loose sand, the snow is not cohesive. If you have to hit a little bit harder and the snow falls in smaller and larger lumps or stays whole, than you are dealing with cohesive snow.
Not every cohesive layer of snow is necessarily dangerous. Only in combination with a weak layer and a slope steeper than 30 degree it dangerous.
After years of studying data collected world wide, it became clear that slab avalanches happen on slopes steeper than 30 degrees. Most avalanches originate on slopes between 34 and 45 degrees—exactly the types of runs that are most fun to ski. Only in really unstable conditions do we sometimes see slab avalanches on slopes less than 30 degrees. Chapter four (terrain) will discuss how to measure the steepness of a slope. To summarize:
- Three out of four avalanches occur on slopes between 34 and 45 degrees;
- 38 to 39 degrees is the bulls eye;
- 10 percent of the slabs occur between 30 and 34 degrees
- less than 3 percent of slabs occur on slopes less than 30 degrees.
How steep is 30 degrees?
What is the steepest run you ever did? Wait, before you answer, how steep do you think the steepest black slope is? While we are at it, look closely at the photo below. How steep do you think that is? Take a test with your friends. Ask them these questions and show them the photo below. Answers will probably range from 30 to 80 degrees.
The steepest black slopes are between 33 to 35 degrees. Le Tunnel in Alpe d’Huez, the Harakiri in Mayrhofen, the Mont Fort in Verbier and Le Mûr in Avoriaz are examples of this. But when standing in the powder it becomes clear that the steepness is not that bad. The fun only begins at 27 degrees.
For most of us powder snow becomes fun between the 30 and 39 degrees. You feel the resistance underneath your feet, you can pick up speed and the snow hits your face. Unfortunately this is also the angle where most deadly avalanches take place.
The weak layer and bed surface
Below is a diagram of what a cross section of a snowpack, with its different layers, could look like. But what is a weak layer and what is a bed surface? Weak layers can be located at several places in the snowpack and cannot be seen from the surface.
Most recurrent weak layers
Nearly any kind of snow can be a weak layer, but in general weak layers tend to be these:
- Faceted snow: very weak, angular and mostly larger grained snow that forms within the snowpack because of higher temperature gradients within that same snowpack. Experts might call it hoar or depth hoar.
- Surface hoar: the name for the feathery frost that forms on a snow surface during cold nights with clear skies. Once buried this hoar layer creates a particularly dangerous weak layer.
- Loose and badly bonded low density snow somewhere in the middle of the snowpack (think of graupel or hail)
- Snowed in icy layer
- Snowed in Sahara sand
What is a bed surface?
In general a bed surface is a layer of relatively hard snow (or ground) on which a slab may slide. In most cases, avalanches descend on a harder and slicker snow surface in the same way the books were sliding in the earlier example. Bed surfaces tend to be:
- Rain crusts
- Sun crusts
- A hard and older snow layer
- Wind packed snow
- Melt-freeze crusts
Besides these, a grass or smooth rock surface can act as a bed surface too.
How do you recognize a weak layer?
Weak layers in a snowpack are not visible from the surface, so how do you know if you are dealing with one? The answer is, you dig. This is a lot a work—and you’re there to ride powder, not to dig—and recognising and assessing weak layers takes some training and experience. Luckily, in the Alpine countries local avalanche experts do this for you and post their findings in the avalanche forecast.
Our enemy is the slab avalanche. This avalanche generally consists of dry snow, happens mostly on slopes on shadowed aspects (between west-northwest and east-southeast (WNW-N-ESE)), is an average of 70 meters wide and 50 centimeters thick where the first fracture took place and the angle is usually 38 degrees or more.
For a slab avalanche the next three elements must be present:
- The slope angle must be larger than 30 degrees
- A soft or hard slab
- A weak layer / bed surface
A combination of these three elements is a guarantee for danger. If only one or two of these elements is present the chance of a slab avalanche is very slim. And do not forget, it goes wrong because you are there. Almost all lethal avalanches start due to an external trigger. In most fatal slab avalanches, the skier or snowboarder is the trigger. You are the trigger!