Avalanche Safety Week: Anatomy of an Avalanche

Trent Ernst, Editor


Anyone who plays outdoors in the winters knows avalanches are bad.

But many people don’t know how avalanches form and, even worse, when not to be heading out high-marking or skiing unprotected slopes.

So here’s how avalanches form.

At the heart of an avalanche is snow. Sure, rocks and broken trees and poorly prepared sledders can get tossed into the mix, but if the snow no go, there’ll be no avalanche.

There’s an old saying that no two snowflakes are alike. The corollary—that there are no two snowfalls that are alike—is also true, and the quality of snow that lands on the ground is affected by temperature, humidity, wind and amount varies from snowfall to snowfall.

In addition, the fallen snow is then affected by a variety of other factors: sun, wind, humidity, temperature, rain, etc. acting on the surface of the snow, which changes the snow from its original form.

As fresh snow falls on top of the previous snowfall, it creates another layer, which has its own qualities.

In addition, as moist air comes in contact with the surface of the snow, it can cause a layer of frost to form over top the previous layer of snow.

How well or how poorly these layers hold together is one of the major factors in determining the avalanche conditions of an area.

Note that many of these factors (sun, wind, etc) are localized phenomenon in the mountains. The sun can hit one side, or even one area of a mountain, but be blocked by a peak across the valley, creating two different avalanche conditions on the same slope.

Just to make things even more complex, the sub-surface layers can change over time, due to internal snow pack conditions (such as temperature, pressure, etc.). This will also change the avalanche danger to be found in an area. Factors that drive change in the layers of the snow pack are always present; therefore the layers are constantly changing. This change can be rapid (e.g. freeze-thaw of the surface layers) or slow (e.g. long-term metamorphism of the sub-surface layers).

The other main factor affecting whether an avalanche will form or not is the terrain. We don’t get many avalanches here in Tumbler Ridge proper. That’s because the terrain is not conducive to avalanches forming. The main feature of avalanche terrain is the angle of slope. Typically, avalanches happen with a slope angle of between 25 and 45 degrees. Any higher than 45 degrees, and the snow has a hard time sticking. Generally, the snow will just sluff off, not allowing larger avalanches to form.

Below 25 degrees, on the other hand, the angle is not steep enough (generally) to cause the snow to slide.

Trees and large rocks can act as anchors, though if the snow is deep enough to cover these features, then all bets are off. And sparse trees can actually create weak points in the snow pack, causing things to trigger easier.

Avalanches are more frequent at higher elevations, which are often steeper and get more snow and wind than lower terrain. Also, there tends to be fewer anchors higher up.

There are eight different types of avalanches that commonly happen in the mountains around Tumbler Ridge.

The first is a loose, dry avalanche. These often happen when the soft powdery snow that has just fallen looses cohesion with the bottom layer and start to slide. These are generally smaller avalanches that happen on slopes greater than 35 degrees.

Loose, wet avalanches, like dry avalanches, typically only affect the surface layers, and are generally very small. However, as the snow is wet, they are denser than dry avalanches, making them harder to fight against if someone gets trapped in the avalanche. Warning signs include small, snowball sized chunks of snow rolling down the mountain.

When a thick, cohesive slab of wet snow loses bonding with the layer below it, you have a wet slab avalanche. These can form on slopes that appear firm and not prone to avalanches, and happen when the layer below is weak. Because they are wet, they tend to move slower than dry slab avalanches, but they can be highly destructive because of how heavy the snow is. These avalanches tend to form into channels, and if you see one, there’s a good chance that other terrain in the surrounding area is setting up to go. They tend to occur in the late spring, after the snow has had a chance to warm up.

The fourth type of avalanche is a storm snow avalanche, and occurs during or after a large storm as the new snow load gets too heavy to stick to the layer below. This is the most commonly known avalanche type in this area, but only last about two days after the storm. And therein lies the danger, as many people assume that if they wait 48 hours after a dump of snow, they’ll be safe. As you should be figuring out from this list, that is not the case.

Wind slab avalanches are caused, you guessed it, by the wind. These are caused by wind packing a layer of dense snow over top a weak layer. These layers are often called pillows, and often sound hollow when crossed. They tend to develop during blizzards or other windy periods. They tend to stabilize after a few days, but can last longer if it is cold.

Persistent slab avalanches and persistent deep slab avalanches are among the most dangerous and destructive avalanches. And form when snow builds up on a weaker layer.  In the earlier stages, persistent slab avalanches are usually small and easily triggered, but as the top layer becomes more cohesive, and as the lower layer does not bond, the amount of snow that can be triggered by a deep persistent slab avalanche is staggering, and quite often fatal. Even worse, they can become dormant for extended periods of time, then re-awaken with horrific results. Often this re-awakening is brought on by spring, when they can go without warning.

Finally, skiers and sledders need to watch out for cornice falls. A cornice is a mass of snow that is deposited by the wind on the edge of a ridge. As the wind blows over the ridge, the snow swirls back and attaches to the edge of the ridge, creating a ledge that extends out beyond the ground. They are not supported from below, and, if a sledder or skier moves out onto the cornice, it can give way, with often fatal results. Even worse, they can often trigger other forms of avalanches when they hit the slope below.

Different types of avalanches behave differently, but there are a couple clear signs that you might not want to be out on the snow right now. If you can hear a whumpfing sound and see the snow start cracking, you might not want to be out.

Another bad sign is when the snow sounds hollow when you travel on it. And if you start to break through a solid layer because the snow underneath is powdery? That, too is not a good thing, and can lead to trouble.

Want to know more? Avalanche Canada reps will be at the Core Lodge this weekend for the Northeast Northern Rockies first ever Avalanche Awareness Days.

The events happen on January 17, starting at 11am and running until 2 pm. Representatives from the TR Riders Snowmobile Club, Canada Avalanche experts and Mayor Don McPherson will all be in attendance. The event is to provide out-reach on Avalanche Awareness, through the use of tools like the Avalanche Terrain Evaluation Maps, AvCan Data sparse Area Avalanche conditions reporting and BLOG. AvCan experts can also help you out with your personal safety equipment questions.

Hot Dogs and refreshments will be supplied.