The recipe for avalanche risk

Joining Chris Smith and Phil Sansom are Mark Diggins, co-ordinator of the Scottish Avalanche Information Service, and Jim McElwaine, a professor of geohazards at Durham University. They’re going to help unpack what happened to avalanche survivor Lawrence. Chris asked Mark, who's in charge of producing the avalanche hazard forecast for part of Scotland, why there might have been a high avalanche risk on that day...

Mark - There are five typical problems. Wind; the wind destroys grains and makes a compact small grain layer. There is something called a persistent weak layer, where you have a layer of grains or crystals underneath the surface, and you actually have no idea that they may exist, but what's happening is they may be developing, getting weaker and weaker. And then the next is a new snow, of course. If you get lots of new snow, logically that's going to load a slope and slide away, and then wet snow, which is obvious, a warming or rain, which will make the snow turn into a sort of slurry. And then gliding snow, which is spring effect where you get big cracks and slowly the snow is creeping down the mountainside. And then finally what we have, and other countries have, is a problem which is a cornice, which overhangs a slope and if it collapses, that can produce a big trigger. But the two key ones are the wind slab, and the persistent weak layer underneath the surface. Which it sounds from Lawrence's case that this may well have been one of the culprits.

Phil - Jim, does that jibe with your research into avalanches? Do you look at the same sort of factors?

Jim - Yeah, exactly. I mean it's precisely how Mark described it. I'm the sort of, more theoretical end of it, but it's all these factors. If the load from gravity in the snow pack exceeds the weakest layer, then the avalanche will occur.

Chris - And Mark, the factors that you described, those are things that you know, you can tell without having to actually potentially go onto the slope. You could potentially observe those from a distance. Can you actually, if you go and look at a patch of snow, tell: "Hey, that's a big avalanche risk?"

Mark - One can, if one is aware of what you need to do when you're going into into the mountains. And that's the real challenge for recreationists, people going skiing or climbing, is the boundary between having a fantastic time in sunshine, and having some terrible event occur is unseen. And so what you really need to know is what's gone on. So you need to know the history of the snowpack. You can't just turn up and expect to know the complete picture. You need to have some understanding of what's gone on before in the days before.