Windslab on top of rock-hard old snow

'Windslab avalanches are what kill people in the UK' essential advice from Andy Say MIC

By Andy Say

Outdoor headlines

29 January 2009 15:10

The recent tragedy in Glen Coe has highlighted how vital it is that people are aware of avalanche risks before heading to the hills.

This piece, written by LFTOer Andy Say (who's also the Executive Secretary for Mountain Leader Training England) gives you a detailed understanding of avalanches and how to avoid them...

"Snow.

Snow is wonderful.  It varies from the lovely star-shaped flakes that little kids (and some big kids I could name) try to catch on their tongues to the wet slushy stuff that clogs up the drains in a thaw.

For people in the hills and the mountains that variety is a source of fascination, adds to the beauty around you and is also a potential danger. 

Understand snow and you are just a little bit less at risk in the winter hills.

Those star-shaped, crystalline flakes that come drifting down out of the sky are really good at hooking on to each other (it's all the sticky-out bits).  They form light, fluffy masses that are pretty well bonded – that’s why we can build snowmen; fresh snowflakes can easily hook on to other bits of fresh snowflakes, it's like mini velcro.

The only real problems immediately come from the snow that has landed on a surface that it cannot hook itself on to. 

We’ve all seen the slapstick moment when snow slides off a roof to cover someone knocking at a front door?  Snow can’t hook on to roof tiles. 

Snow can’t really hook onto things like granite slabs either. 

That’s why in many winters thousands of tons of snow come shooting off the Great Slab in Coire an Lochain. 

And if you’ve ever seen that lot go it’s not slapstick; it's just scary.  So think about what’s underneath all of that snow. 

Your average lumpy, bumpy, knobbly mountainside should be OK, rock slabs and smooth grassy slopes may not be – especially late in the season when thawing might wash away any bonds that are there.

As we all know that nice, freshly fallen snow doesn’t just lie there being pretty. 

It changes.  It melts, it freezes, it gets wet, the sun gets on it.  Typically it initially gets harder and it gets a crust on it. 

When that crust gets solid enough to support a person we are in winter walkers' paradise; no more wading through thigh deep drifts, no more stumbling over rough ground; we just zip along on that lovely smooth surface. ‘A lovely smooth surface’?  Uh-oh!  We’re back to the roof tiles again aren’t we?

So there’s old snow lying on the ground and there is a fresh dumping of snow on top.  Think about the bonds between those layers.  Pretty poor! 

So if you are going out into a winter wonderland that has been snowy for a while and there has been much fresh snow recently the traffic lights in your head should be at amber. 

Climbing up the lower part of convex slopes or the upper part of concave slopes might be a thing not to do – ridges are good.  Mind you, ridges are just good anyway so that’s no hardship! 

So we look at weather forecasts to see if it’s been dumping snow before we go off to the hills.  We also look at the wind speed and direction. 

When the wind picks up those lovely, fragile crystalline stars and knocks them about across the ground they break. 

They turn into much smaller icy crystals that tend not to hook onto each other with a lot of air trapped in between; they tend to form a fairly solid mass of closely packed particles.  It’s called ‘windslab’. 

It looks dense and chalky instead of sparkly and it squeaks when you walk across it.  And it hasn’t got our little hooks!  So it forms a dense layer that doesn’t bond well with what’s underneath. 

Windslab forms on lee slopes where the wind drops it; so if the wind has been from the SE for a while and there has been fresh snow and we’re approaching a NW facing slope then our mental traffic lights should be at red. 

Windslab avalanches are what kill people in the UK.  So don’t just look at the hill when you get there; look at the weather for the week before.  The weather man might just help you plan the safest route. 

So. We’re looking at a snowslope.  How can we tell how dangerous it is? 

It’s all about the layers of snow, the types of snow and the bonds between them, isn’t it?  So if in doubt: dig!  Dig a pit that exposes the layers.  You want one that goes ‘down’ (though on a slope you’re actually going to be digging ‘in’, aren’t you?) a fair way and has a fairly vertical uphill wall of a meter or more.  That wall is what we look at.  It’s like looking at the rings in a tree.

The layers of snow are a record of the past weather and with a bit of practice you can see when it snowed, how much it snowed and even what the weather was like between snowfalls. 

But that’s incidental.  What you are looking for essentially is layers that are very dissimilar and have poor bonding.  There’s a whole range of tricks and techniques for getting information out of a snow pit. 

Like using your compass base plate as a knife and running it down through the pit face. Where it goes easily is likely soft, aerated snow. 

Where it ‘catches’ momentarily is likely to be an icy crust on the top of a layer – so there may be poor bonds there. 

Snow analysis is a book or three in itself.  Simply looking at the face of the snow you’ve exposed will tell you a lot.  If you want to know more then you can cut slots back into the slope about 30/40 cm apart to effectively isolate a column of snow. 

If the top of that column slides off of its own accord then just run away.  Go to the pub.  With a bit of pushing and pulling on the column you can work out where the weakness are, how deep they are and how nasty they are.