Glide Cracks Mean Snowpack Stability….


…. but they need to be treated with care, all the same

Over the last few years we’ve been emphasizing that over 9 out of 10 off-piste accidents occur due to victims triggering the avalanche themselves. This usually happens because they weren’t applying basic accident prevention risk reduction measures at the time of the accident. We talk about ‘recent avalanche activity’ as a clue of instability. By this, we are referring to recent cold dry slab avalanches. This is the type of avalanche that’s involved with almost all off-piste avalanche accidents.

Glide cracks mean snowpack stability

“Brown frown”. Photo by Chris Souillac

One thing I’ve often noticed when chatting to other off-piste skiers is a deep myth/misunderstanding about glide cracks, sometimes called ‘brown frowns’.

The truth is that full depth avalanches that release from glide cracks are never triggered by skiers. One reason for this is because, unlike slab avalanches, glide cracks contain no significant weak layer. Glide cracks are not considered “recent avalanche activity” in our risk evaluation because they are not a clue to instability in the snowpack.

On the contrary, glide cracks are actually an indication of  general snowpack stability. This may seem counter-intuitive, and many people have resisted this fact. Glide cracks are spontaneous events. They represent general snowpack stability, except in the very localised areas where they exist. Most of the time they don’t release at all.

Comparatively little research has been done into glide cracks and the relatively rare full-depth avalanches that may release from the fissures (relatively rare compared to slab avalanches).

If a glide crack does release into an avalanche, it looks like a slab avalanche, as both occur as linear fractures. However, glide crack avalanches are very different in nature because of the composition of the snowpack:

  • A huge amount of research has been gone into understanding slab avalanches because of the frequency of their involvement in accidents and all too often deadly consequences. Slab avalanches are much easier to trigger by people and artificially trigger/blast by the piste patrol, to control the ski area and secure the slopes.
  • A glide crack avalanche occurs when a full-depth snowpack slides directly on the ground. There is no weak layer in the snowpack. The snowpack has become ‘homogenous’, packed down and stabilised, often due to a high level of liquid water content (but certainly not always). Glide cracks are next to impossible to release artificially with explosives.

The releases of glide cracks are very unpredictable, and almost always occur naturally. Unsurprisingly, they’re a nightmare for ski resort and road safety authorities when they hang over a piste or road that needs to be opened!

Although they’re not much of a threat to the off-piste skier who’s just quickly traversing under them, you certainly wouldn’t want to hang around there. Graham Predeger, Chugach National Forest Avalanche Information Center, SE Alaska, recommends: ‘Glide cracks are best given a wide berth. Limit your exposure time underneath and if skiing or riding in terrain with glide cracks, try and map them out first as not to end up directly on top of or inside one’. See “Don’t Mess with the Brown Frown” by Aleph Johnston-Bloom (The Avalanche Review, April 2016).

Glide cracks mean snowpack stability

A number of glide cracks across a steep slope

 

While glide cracks used to mainly be a spring-time phenomenon, they now seem to be appearing earlier and earlier in the season. For example, in January 2016, Chugach National Forest witnessed an unusually active spate of glide cracks appearing on slopes facing virtually every direction. This followed a warm storm pattern that had deposited over 4 m of snow in a very short timescale (glad cracks often form after a lot of snow has come down and stabilized).

 

To sum up, the problem with glide cracks is that:

  • It is impossible to predict with any certainty if/when they will release into an avalanche. Cracks may release after just a few hours of forming, may take a few weeks, and most of the time do not release at all. Graham Predegar says “Glide cracks are impossible to predict. There is no discernable pattern to predict a failure as they tend to fail naturally and on their own schedule’.
  • They are virtually impossible to control/trigger artificially.
  • Although rare, they do occasionally release some pretty massive avalanches, as seen in the video below. In the first example, a chairlift pylon was taken out in St François Longchamp, France, when a glide crack released into an avalanche. The glide crack had been around since early January, but didn’t release until March.

 

Our advice is to:

  • Keep an eye on glide cracks, even if there are tracks underneath.
  • Avoid them if you have a choice.
  • But it’s OK to pass underneath them while applying basic risk reduction measures (like avoiding terrain traps below, keeping distances between you and stopping at islands of safety) and NOT hanging around under them. 2-3 mins of vigilance while passing under is OK. 15 mins or more of ski touring underneath is more risk than we would take ourselves!

 

However, just to re-iterate that the presence of glide cracks is not a sign of recent avalanche activity, at least in terms of the avalanche risk reduction framework that we encourage. They are a sign of general snowpack stability except perhaps in the very precise localised area below the crack.

I’m personally fascinated by glide cracks, as I am by snow and avalanches in general. We have a longer version of this article, largely thanks to these two articles, who provided much of the information above:

‘Retour d’Observations sur les Plaques de Reptation’ by Damien Margueritat in Neige et Avalanches, No 152, February 2016, and “Don’t Mess with the Brown Frown” by Aleph Johnston-Bloom in The Avalanche Review, April 2016.

Let us know if you’re interested and we’ll publish it. And please feel free to comment through our Facebook page.

Also, why not also come along and find out more at our ORTOVOX Off-Piste Awareness Tour starting on 24th October and coming to various places around the UK.

We can do private talks for you and your group too.

 

Email hat@henrysavalanchetalk.com for more details on any of the above.

 

Safety is Freedom!