Thursday, 28 November 2013

Winter Part 1 - wedding cake and avalanches anyone?

Around this time of year, when the first snows of autumn have covered the Cairngorms, Glencoe and even the Lake District (sort of), I start to switch my head into 'winter climbing, mountaineering and skiing mode' quite a change from 'summer rock climbing in the sun' which was pretty good this year despite the usual wet weather headlines. 

David Monteith enjoying early season conditions on Milky Way (III)
 Coire an Lochan, Cairngorms, November 2010. Photo: Alasdair Monteith

Such a change in activity requires more than just a cursory glance at weather forecasts and an idea of a venue (although the Mountain Weather Information Service is the place to go for reliable weather forecasts all year around for 8 mountain areas across mainland Britain).

Winter days in the mountains can bring some of the most rewarding and exhilarating experiences when stunning weather, good routes and excellent banter all come together. However, they can also bring fear, danger and worse - injury or death, if the cards are stacked against you and decisions don't go your way. 

From a geography point of view, winter presents a wealth of interesting physical processes to look at, explore and enjoy. The weather is often unpredictable and brings disruption as well as fun, and although weather forecasting is improving snow events like the December 2010 covering are hard to predict.

Ski touring Yorkshire style: Andy Saxby taking advantage of a fairly rare few feet of snow on the North York Moors, December 2010. Photo: Alasdair Monteith

But of all the unique geographical 'things' that winter brings, one of the most interesting - and deadly - is the behavior of snow as it accumulates on slopes and then detaches itself from slopes: avalanches

Snow -  more than just a few Ice crystals

Whether an avalanche occurs or not is down to a number of factors all of which affect the stability of the snowpack- the accumulated snow that's sits on the hillside over a period of time, perhaps the whole winter season or just a few days.

Changes in temperature, the type of snow (yes really, there are types of snow), wind speed, wind direction, and the presence of people on the slopes. These factors can all trigger avalanches. 

It's the weaknesses between layers in our snowpack and how they can be triggered that is worth exploring. I'm writing with Scotland in mind here, not the Alps or North America. I mention this as it's important to recognise that due to a combination of location, latitude, altitude and maritime air masses, we suffer a range of weather in winter including wet mild days and storm force winds. Compare this with the cold, stable high pressure days (thanks to continental air masses) that often dominate areas such as the European alps and you see how the snow pack can develop very differently.

The snow pack as the layers of a wedding cake

To help visualuise the changing characteristics of a snowpack, think of it like a strangely layered wedding cake ( not that I'm comparing a billowing, white, wall of death with marriage of course. Wedding cakes and snowpacks just so happen to both be white and have lots of layers!) Below is just one possible example of how weaknesses can develop in a snow pack. The authority on avalanche assessment and forecasting in Scotland is the SportScotland Avalanche Information Service. Check it out.

Wedding cakes and snow pack. More in common than you'd think.....
 Photo credits: Andy Huddart, wedding cake. Lucy Monteith, snow pack.

To begin with we have our first layer of snow, or layers of cake. In Scotland, significant snowfall tends to start around mid to late October and depending on the weather, such snowfall can hang around and form the base for our snow pack. This snow could have fallen on dry frosty ground or partly unfrozen wet soil and rock . A few days or a week later we might add another layer born on the wind, but this time our layer (in the cake) is more like a layer of thick meringue and quite a contrast to the underlying texture of the first layer (my baking skills are rubbish, the cakes always look odd, but I like meringue). This is our wind slab.

In Scotland strong winds are a common occurrence. Snow doesn't tend to stick around on the slope it was deposited on but is transported to the lee ward, or downwind slope, perhaps sheltered behind a ridge or corrie rim. This violent process of transporting snow tends to damage the crystals and deposit large amounts of snow that can be poorly bonded to the underlying layer. It can give off a hollow 'wumf' like sound when loaded and shooting cracks may move out across it when loaded.

Now, at this stage I run out of time so either keep the cake in the kitchen, where the temperature varies from being fairly mild to below freezing (it's a cold mid terrace Lancashire house; below freezing IS possible). Such a variation in temperature in the mountains tends to have a consolidating effect on a snowpack. Freeze thaw cycles result in the layers of our snow bonding together to form a fairly cohesive structure. 

However, I could of course put the whole half finished cake in the freezer, I am an amateur cake maker after all. In this consistently cold, dry environment we see the formation of crystals on the surface of the cake. With our snowpack a similar process may occur, as surface hoar on the top of the snowpack, or as a layer of faceted crystals buried within the snowpack (see the photo below for a glimpse of their delicate and intricate structure). These crystals can present a real danger in the form of a hidden weak layer, ready to release when loaded.

 Cold weather beauty: faceted crystals. Photo: David Monteith

Now I cover my fairly odd looking wedding cake in a final layer of icing. This gives the cake a smooth, white, consistent looking appearance. This is our surface layer which falls as soft, slightly damp snow which quickly freezes over that night forming what is a fairly dense, consistent surface layer. In reality it's a smooth blanket hiding a variety of layers with individual characteristics and instabilities.

To finish we want to visualise our cake (or snow pack, if you insist) on a slope of between 30-45 degrees, the range In which an avalanche is most likely to occur. So let's put that very expensive wedding cake on a table and start tipping...

Sometimes the slope may have all the right ingredients for an avalanche but just needs a trigger to get things moving. The 2nd part of the post will look at how humans can be that final factor and what happens if you're unlucky to be caught in an avalanche (with a personal account thrown in).

Thanks to David Monteith - MIC and IML for the 'peer review'. Check out his website here: 

Lucy Monteith for the cake based inspiration!

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