This is a great video that illustrates one of the trickiest avalanche problems in Colorado: the super hard slab, windslab.   Windslabs are such an interesting problem.  They come in a wide variety of forms and can present with very different characteristics.

Understanding Windslabs

Wind slabs are essentially a change of density in new snow deposition resulting from what we might call “mechanical rounding.”  The new snow, the stellar dendrites, are broken apart by the turbulence of the wind into smaller pieces and when those smaller pieces come to rest they do so with less pore space and more overall density in the layer.  

Essentially the stronger the wind, the smaller the grains, the harder the slab.  More wind equals harder slabs.  Less wind means softer slabs. 

This gives rise to a tremendous variance in slab presentation.  Some wind slabs are nearly indistinguishable from a ‘true storm slab.’ They present with a very slight increase in density: still riding like low density powder but reacting on steep slopes with a more predictable storm slab-like character.  

Wind slabs formed under high intensity winds can be incredibly hard.  Presenting on the surface as cold chalky snow that is hard to even get an edge into.  They can also form in really weird spots we wouldn’t typically associate with classic top loading or cross loading because the winds can swirl and contour through the terrain in unpredictable ways at such high intensities.  

These hard ‘Colorado windslabs’ have a very different character as far as ski quality goes, but also very different character as far as how we interact with them in steep terrain.  They are able to ‘bridge’ extremely effectively and resist triggering even when quite thin—10-12 inches of crown height.  This means they can be extremely unpredictable and lure us out onto them before releasing.  Even with relatively low snow volumes.

Understanding Triggers, Propagation and Bridging.

To understand ‘bridging’ we need to understand the mechanism of avalanche trigger.  Many people talk about the ‘stress bulb’ a skier or rider creates.  But what is this stress bulb really?  It is a deformation of slab created by the weight of the rider.  As the slab deforms and bends under the rider’s weight, it creates a failure of the weak layer under it.  If this deformation—essentially the start of the wave of propagation—is of the appropriate length relative to the weakness of the layer and the tensile strength of the slab, a self sustaining propagation ensues.  This is known as ‘critical crack length.’

This is the reason deep slab avalanches are more easily triggered by air blasts from slow speed explosives above the snow surface as opposed to fast speed explosive charges on the snow surface.  We want to push more gently across a large area of slab to get it to bend instead of punching a hole in it.  

It’s the same reason you can boot through a thin soft slab and not trigger it and then get it to propagate pushing with the larger area of your skis.  You need to get the slab to bend without breaking to start the propagation. 

The Hard Slab Windslab

The really hard wind slabs we often find in Colorado are able to bridge extremely well.  The means we can easily ride out onto slabs of 10-12in crown depth and not get them to trigger.  They are also extremely spacially variable, which means it’s hard to say where they will thin out and trigger.  

Imagine a classic top loading situation.  Right off the ridge the most loading is occurring, the slab depth is 12-24 inches.  Lower into the slope the slab depth diminish.  The slab is essentially a big lens shape.  When you jump on the convexity at the top of the slope to see if it will rip, you just bounce off hard, chalky snow.  Doesn’t seem like a wind slab is there.  Soon as you make 3 turns you get to the bottom of the lens and get the slab to propagate.  This quickly rips out all the terrain you were just jumping on.

These kinds of avalanches are even more devious because their surface character doesn’t really look much different from old wind blasted snow–old surfaces that were possibly old windslabs at one point, but are no longer reactive.  It’s the same snow surface we find all over the alpine.   The snow surface we ride all the time without issue. 

Haiyaha 

This ski run in Rocky is a place many people have been caught by similar avalanches over the years.   I myself have been caught off guard here as well.  During high winds the run can experience really inobvious loading from the top and from the side at the same time.  

This kind of avalanche is the culprit for many bad accidents in Colorado every year – especially in the spring.  When it’s corn season many people stop thinking about dry snow avalanche problems and often small windslabs in the alpine escape the notice of the public forecast.  In conjunction with firm frozen melt form snow surfaces and extreme terrain we get really high consequence avalanches even when they are really small.  

If you misread or don’t notice that 10in windslab at the top of a line it doesn’t take much to take a really dangerous sliding fall down a face when it gets you sliding on frozen corn. 

Managing these problems requires a good sense of the clues wind transport gives you, as well as keeping an eye to the snow available for transport and weather history.  

These are insidious and won’t always present obviously – they make you think you are just skiing down some old wind blasted chalk and not a well bridged slab before the magic carpet comes out behind you.

There are a couple other key points of discussion with this video: how do we understand the role the ‘hangfire’ played? As well as how does a season of this kind of snowpack deposition relate to our snow environment in Colorado? There are also some observations from the difference in snow character in the ski cut at the top of the line vs the snow character lower in the line. If you want to be involved with more high level snow and avalanche discussions check out the Avy3 Series: Pro Level Training for Rec Riders and Skiers.