So far there have been two very distinct parts to the winter, says Mark Diggins, senior avalanche forecaster at the Scottish Avalanche Information Service (SAIS). He explains what the SAIS do, and offers some tips on staying safe for the remainder of the season.
Between December and the end of January several storms swept across the Atlantic from America. They brought really strong winds with sharp gusts. We had a similar pattern of storms last season but this year wind speeds reached 140 mph on the tops, and one day 100 mph speeds persisted all day, both of which are very unusual!
These gusts could really catch you unawares. The SAIS has been going for 25 years and we had our first accident when a forecaster was blown off the path on the Ben, trapping his ankle in rocks and fracturing his lower leg.
Then at the end of January a change of weather system brought in more arctic conditions. More northerlies, colder temperatures, and fresh challenges. If it stays really cold (say below -5 on the summits) for long periods of time it creates complex conditions. It can be dangerous because the snowpack essentially looks fine but a lot is going on under the surface.
How the snowpack develops is determined by the temperature gradient between the ground or old snow, which is around 0 degrees C, and the surface. The steeper that gradient, ie the colder it is, the more weaknesses may develop. Certain grains that don’t bond well will be preserved in the snowpack, and snow grains may grow and become sugar-like.
People often think that warmer weather is the most dangerous, as it softens the snow and avalanches occur. Actually, these natural avalanches often purge the slopes, and then the refreeze cycles consolidates the snowpack. So you get a few days of instability with the warmth, then everything generally stabilises.
In 90% of avalanches, if people are involved, then they are the cause. In the first 15 days of February we had 17 human-triggered avalanches. That was purely because an unstable snowpack period coincided with the February holidays. People tend to plan objectives, which they want to stick to, especially if they have invested a lot of time and commitment.
The most important thing I would say to mountaineers is: be flexible and read the avalanche forecast. The more you know about the history of the snowpack and the current hazards, and the more you’re prepared to adapt to the conditions you find, the less likely you are to put yourself at risk.
The SAIS get very accurate weather information directly from the Met Office, which we use for our forecasts. The data comes in the form of graphs and numbers generated by their super-computers down in Exeter. We know what the wind shift and precipitation will be every hour for the next 24 hours, and the level at which snow will fall (known as the wet bulb freezing level).
Our team of 16 forecasters follow the weather patterns and the evolution of the snow pack throughout the winter. We have a very good handle on what is going on, and report it all on daily blogs for each area. Teams cover six areas: Creag Meagaidh, Glencoe, Lochaber, Northern Cairngorms, Southern Cairngorms and Torridon.
We check on persistent weak layers to see how they are developing, and if they are spread around the mountains or in small isolated places. We also look at how new snow is bonding onto other layers. To do this we go to a safe place that represents the worst place we can think of in aspect and angle, and dig a snow profile.
Digging a snow profile only gives us about 15% of the information we need, though. The key element it provides is allowing us to take the temperature throughout the snow. It’s an important indicator of whether the snowpack is going to improve or maintain weaknesses.
The most important thing we do is to look around and actively test how the snow behaves - if it cracks underfoot for example. On a daily basis we spend four hours or more travelling around many different aspects and altitudes in each area.
Unusually, in this country the person who observes the snowpack also writes the forecast. We record what we’ve seen and the hazards on our blog, then we get the weather forecast and think about what is going to happen for the avalanche forecast. Last winter we had 680,000 reading the blog and 300,000 looking at the avalanche forecasts over the four-month operation.
There’s usually something you can do, whatever the weather. It’s about being cunning and aware of the hazards. If it’s really cold for example, then weaknesses will be preserved in the snowpack and it might be unstable above 35 degrees or on unstable terrain shapes, but good on 20-degree snow fields, or perhaps you could climb a ridge rather than a snowslope.
Here you'll find both avalanche forecasts and blog posts for six areas: Creag Meagaidh, Glencoe, Lochaber, Northern Cairngorms, Southern Cairngorms and Torridon.
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