Snowpack

Understanding the composition of the snowpack is critical to understanding the risk of avalanches occurring.

Information about the snowpack is crucial to accurately predicting what type of avalanches could be caused given the snow conditions.

Weak Layers

A weak layer is a point at which two larger slabs of snow have not bonded securely. If these are present in a snowpack, the chances for dangerous slab avalanches are increased. Weak layers can be triggered remotely, meaning that depending on how widespread the weak layer is in the snowpack, it could collapse on steeper slopes above you. Avalanches that occur due to a weak layer are triggered by excessive loading and/or weakening of the overlying layer of snow.

Red Flags

In situations where the weak layer is near the top of the snowpack, the collapse of a weak layer will result in cracking on the surface of the snow, as well as “wumph” sounds where air from the weak layer is pushed out as the snow settles above it. Weak layers may also lead to naturally triggered avalanches in the area you are skiing. Be alert to recent avalanche activity in the area you are skiing and use that information to asses the stability of the likelihood of weak layers in the snowpack you are considering skiing. But just because these signs may not be present, that doesn’t mean that the weak layer isn’t present deeper in the snowpack.

Triggering Weak Layers

Weak layers will always have a breaking point in terms of the load they can support. “Load” refers to the force that a skier places on the snow below their feet. The minimum amount of load is generated by a single skier walking, or skiing carefully down the mountain. By jumping off of cliffs or features, falling, or adding additional skiers on a single face, the load is increased. Therefore it is important to make decisions about how and where you tour and ski based on how much load you believe the snowpack can withstand.

Thickness and Depth

The thicker the weak layer is the stronger it is. From research done in Switzerland, 65% of all the avalanches had a weak layer of less than 2 cm. The depth of the weak layer is a key factor in its strength. The deeper the weak layer is, the heavier load it can support. At a depth of 80cm below the surface, the load from a skier is only ¼ of the load at 20cm.

Hardness

Loose snow with low hardness will cause a skier to sink deeper down into the snow. If the snowpack has a weak layer, the skier will come closer to this layer, placing more load on it and increasing the probability of triggering an avalanche. In loose snow the forces from a skier goes pretty much straight down. Harder snow causes the pressure on the snow to spread out, resulting in less load on the weak layer, decreasing the chance of an avalanche.

Snow Profiles

Digging a snow profile can help to identify the weak layer, evaluate the risk of propagating it by adding the load of a skier, and how likely it would be to spread across the snowpack. Snow profiles are complicated and should be conducted based on in-person instruction received in avalanche courses.

More information from SLF - Davos

Helpful Rules of Thumb

In general, snowpacks get more stable over time, but the speed at which bonds between layers are formed is dependent on temperature.

A thick snowpack with low variation in hardness is ideal for snow stability.

Loose snow avalanches can occur when the snowpack is loose in very steep terrain.

Slab avalanches occur when the snow is uniformly packed into unique layers that have not bonded strongly together.

A deep, persistent weak layer in the snowpack caused this massive avalanche in Tamokdalen, Norway. Photo: Aadne Olsrud

Other avalanche factors...

Weather Factors

Temperature, wind activity, and loading of new snow can all impact the stability of the snowpack on any given day.

Terrain Factors

Recognizing avalanche terrain is therefore crucial and could prevent the possibility and severity of an avalanche.

Human Factors

At the end of the day, you are the one making the final decision based on your knowledge, experience and the influence from other people in your party.

Slab Avalanches

A slab avalanche is an avalanche that loosens as a large, cohesive area of snow of snow and slides downwards.

Loose Snow Avalanches

A loose snow avalanche is an avalanche that releases from a single point and spreads downwards in a cone shape.

Find an Avalanche Course

This information is meant to be an introduction to avalanche safety and a knowledge base for backcountry travelers.

But reading it cannot replace real experience in reading conditions and making decisions in the field. We strongly recommend that all backcountry travelers take instructor-led avalanche courses to get hands-on experience in wild snowpacks.