People do not trigger glide cracks.
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 reduction measures at the time of the accident despite clear signs of danger. According to experts, one clear sign of danger that many victims did not take into account is ‘recent avalanche activity’ … as a clue of instability. The clue of ‘recent avalanche activity’ is recent cold, dry slab avalanches. This is the type of avalanche that’s involved with almost all avalanche accidents. Therefore this is where attention should be directed for effective accident reduction actions.
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’.
Despite speculation, conversation and rumours – even in avalanche forecasting bulletins – there is no confirmation by the scientific community of a person ever triggering an avalanche from a glide crack any where in the world. There are confirmations from numerous professionals who have found it almost impossible to trigger them even with explosives. One reason for this is because, unlike slab avalanches, glide cracks contain no significant weak layer except for the interface at the bottom of the snowpack and the ground surface. Thus, glide cracks are not considered “recent avalanche activity” for off-piste skiers and ski tourers in snow stability evaluation regarding the risk of cold, dry slab avalanches because they are not a clue of general instability in the snowpack that could lead to an accident.
On the contrary, glide cracks are actually an indication of general snowpack stability during winter conditions when most accidents happen. This may seem counter-intuitive, and many people have resisted this fact.
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’ can be triggered by people and artificially triggerd/blasted 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! See this article on a new novel technique that uses flooding glide cracks with dozens of tons of water in order to get them to release by the Val d’Isère Piste Patrol
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).
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
- Glide cracks do however, 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.
In sum, for the off-piste skier and ski tourer:
- 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!
Here is a quick summary that Alec Vanherwijnen from the SLF snow and avalanche science center in Davos provided for me after some long discussions on the subject. His conclusions here are based on a current study – they are approximations.
- Glide cracks: 50% of glide cracks result in an avalanche.
- In winter: glide cracks can be reactivated after a snowfall event
- In Spring: glide avalanches essentially behave like wet-snow avalanches.
Content in this article is also thanks to these two articles: ‘Retour d’Observations sur les Plaques de Reptation’ by Damien Margueritat in the ANENA.org publication, Neige et Avalanches, No 152, February 2016, and ‘Don’t Mess with the Brown Frown’ by Aleph Johnston-Bloom in the AAA publication, The Avalanche Review, April 2016.
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