/ Help me understand...
I'm looking to head to the Southern Uplands this weekend for a bit of XC skiing and gully scrambling, and I'm assuming that at the momement there is a large amount of powder scattered around the place. As I understand it, windslab is likely to have formed at the moment, and that it is the current avalanche risk because it is on top of a loose powdery base. Is that assumption correct, or is the risk because of just loose powder that wants to flow downhill?
Looking forward to Sunday when I'm heading out, the MWIS forecast is saying -4 today and gentle winds, then -3 on Friday with heavy snowfall. As far as I know, this will put fresh powder on existing windslab, which could cause a bigger problem. Is that right? (Assuming the winds are constant, which they seems to be).
Then on Saturday the forecast is for an increase in temperature to above freezing, and some rain. Would this stabilise the snowpack because of the rain? Or would it cause weaknesses because of the rising temperatures?
If it froze again after the rain (Sat night) would the conditions on Sunday potentially be really good?
I don't know the area well so can't offer specific slopes of interest - this is a more generic look so I'm aware no advice can be too exact! I think what I'm after is what effect the rise in temperature, and the rain, will have on powder on top of windslab.
Any suggestions or links would be appreciated.
If you don't know what to look for and don't undertand the basics, you shouldn't be heading into the hills.
My opinion of course but a winter skills course and a weekend learning about avalanches was my education before I started winter walking and climbing.
Please don't take offence!
I agree you should read some more and understand the basics. Please buy a copy of "A chance in a million?" which is about Scottish avalanches. It is almost mandatory reading I think. You need to also read the SAIS - Which although doesnt provide information for that area, will allow you to get general ideas about conditions giving current weather patterns. Temperature, elevation, and slope aspect all playing their own part.
Windslab forms where snow is blown onto lee slopes into a slab which has little cohesion to the underlying layer(s). The avalanche risk is because of the lack of cohesion.
Your assumption is generally correct about rising temperatures helping stabilise the snowpack, but that is assuming there is enough rain to penetrate the snow pack - and this depends on how much rain there is and how deep the rain needs to penetrate to bond the poor connection between layers.
Warm temperatures and rain themself can make avalanches more likely as well as upper layers gain more weight on the poor layer and the tension on the slope becomes greater. Ideally you want to wait until it gets cold again and freezes up.
Most avalanches occur 24/48 hours after a storm so that is new snow or possibly new winds transporting unbonded snow to different slopes again so Sunday could be fine but it might not be.
I am perfectly able to take the long route around when I suspect a slope, or am not sure about it. It's only when I'm totally sure of a slope/gully that I'll cross it. I fully understand your concern, but I'm more asking about the details/science behind it. Links/comments on that basis are what I'm looking for really. Do the books you recommended have that sort of information in, or are they less science and more empirical based?
I'm trying to look a litte ahead of that and predict the conditions for this Sunday, and then see if I'm correct on the day (in a good way!).
You mentioned windslab. I understand how it forms, but it's failure mechanism is what I'm after. Is it the slab that slides on top of the underlying powder?
is a good starting point, with more info to be found elsewhere on the site
Yes the slab slides on the weak layer but there are different type of weak layer that can exist such as Graupel (like hail), depth hoar (Facets), surface hoar (frost).
I think the question I should have asked was "Check out the MWIS forecast, and let me know what you think the avalanche conditions on Sunday will be like." Never mind! Some good info anyway, thanks.
The "Winter Skills" book (MIC/WML handbook) gives quite a lot of detail
That means studying the map in detail to fully understand the terrain and especially areas with slope angles in the critical 30-45 degree range. I am not intending to climb in the Southern Uplands so I can't help there.
That means studying the weather forecasts, (proper ones with synoptic charts) ideally for the preceding week, so that you have an understanding of what the weather has been like and how the weather is currently developing. You are already doing this and your general conclusions don't seem unreasonable - in terms of improving bonding some rain is generally helpful.
Finally, you need to know about the snowpack. This is much more uncertain. You can study the SAIS reports and most importantly the snow profiles, again for the preceding 1-2 weeks but they just provide a snap shot and unfortunately they are not available for the Southern Uplands.
There is likely to be more windslab than loose powder currently but judging from Glencoe http://www.sais.gov.uk/profile_flash.asp?id=11303 it is possible that the snowpack may be both complex and variable, which is about the worse situation. More fresh snow will again just complicate matters. Equally, given the lower altitude you may find a more straightforward situation with better bonding as at Aonach Mor http://www.sais.gov.uk/profile_flash.asp?id=11301
Without detailed knowledge of the existing snowpack, arm chair predications are not particularly helpful. You will just need to do what all experienced and cautious mountaineers do - once you are on the ground, keep stopping and regularly dig snow profiles. You can study weather forecast as much as you want, but there is no substitute for getting out your shovel and checking the snowpack properly every time you substantially change elevation or aspect.
> Please buy a copy of "A chance in a million?" which is about Scottish avalanches.
I think you're on commission for that book.
I'd be interested to find out how that put all the data together to generate the liklihood of avalanche. Some algorithm I assume? Or would it be based purely on their experience?
The SAIS observers are definitely not a computer programme. They head out in to the hills every day whatever the weather, observe the conditions, pour over several weather forecasts and look at terrain. Then use the combination of these observations and their massive experience to make predictions.
There is a lot of science involved obviously, but I think for we ordinary peeps, its easy to get bogged down by the science of snow profiles and reams of data. Better to look at the big picture and apply it to what we find on the ground- as someone posted early, the triangle of:
weather, terrain and snowpack.
You already seem to have a good idea of how weather might affect stability. The books/websites listed above will give you more information to help you understand how the weather affects the conditions. Snowpack involves getting out there an looking at it. Are there tell tale signs of instability? Again- the books will tell you how to look for these, and finally terrain- thats down to you, looking at maps, and understanding gradient, terrain traps and slope convexities.
For info, I'm just back from a week in the highlands, and the current conditions have formed a diverse array of snow types, some very unstable. We've seen it all, from bullet hard neve, to firm slab overlying a very weak layer, to soft powder. Some of the instabilities were well buried and hard to spot without digging. As a rule this week we treated any snow that wasn't the old re-frozen snowpack as suspect. The change to stormy conditions this weekend is bound to create a whole new bunch of hazards which we await with interest.
> The SAIS observers are definitely not a computer programme. They head out in to the hills every day whatever the weather, observe the conditions, pour over several weather forecasts and look at terrain. Then use the combination of these observations and their massive experience to make predictions.
Sorry, maybe I phrased that wrong. I know that they head out every day and dig pits etc etc. It's what they do with that data in order to achieve the prediction that I was asking about. I'm an engineer, these sort of things interest me and it's also how I understand things - from the fundamentals rather than just 'on the ground'.
In Glen Coe last weekend we experienced a lot of varying snow conditions too. It's how to assemble that data into a prediction that interests me. Rather than just knowing 'Area x is dodgy today', I want to know WHY it's dodgy, from a fundamentals point of view, i.e. how different snow structures intereact with each other. I know it's a hugely complicated problem, but was hoping that some links to papers etc might come to light on this thread. Still, some really good information has come out of it, just not the sort I was expecting!
Bruce Temperer's "Staying Alive in Avalanche Terrain" is the most recent comprehensive book I know of with decent technical information that's still accessible to us punters. Aimed at skiers and snowmobilers in western North America, but the snow science is just as valid in Scotland, even if our snowpack is generally much less deep than theirs. The front cover picture is properly scary, too (the skier managed not to get buried - very impressive)
simply do a course. theres so many subtle factors that need on-the-ground attendance with a certified instructor its insane to rely on theory. the theory has so many exceptions its often more dangerous.
plus, its one thing to predict avalanches, and other to do something about the risks.
Best to read the the sais avalanche forecast the night before. It's no good speculating now, I guess, 3 days away. And then, once you have read it, just be very aware (seems like you know a lot of details already). Don't be afraid, embarrassed or whatever it may be, to walk away. I have done it many of times when it felt dodgy and my avalanche pits looked bad. Don't be lured by other people, there will always be someone who ploughs on regardless. Make your own decisions. Think: "'Better a life wuss then a dead hero" Or: "You are a long time dead"
So, my guess is it will be at least considerable avalanche risk and you have to be careful according to the avalanche blog and your own assessment on the ground.
I should have said wait til the night before to make your mind up, but follow the sais ( and the weather) everyday for several days as it will give you a good understanding of what kind of snow and weather happened before your day out as avalanche conditions build up over time
I'd suggest going to one of GML's Avalanche talks they do over two nights. Incredibly useful information given out. I've never thought of the Southern Uplands as a major avalanche area but i can imagine some slopes being prone to sliding. Stick to the lower level rolling hills.
Not so, the daily reports are edited and put live as soon as the forecasters are off the hill, usually around late afternoon.
If you look closely at a forecast (this one for instance http://www.sais.gov.uk/page_glencoe.asp ) you can see that it has two tabs. Firstly "Observed Avalanche Hazard", which is what they saw on the ground, and secondly "Avalanche Hazard Forecast", which is always from 1800 on day of forecast to 1800 next day. This is where they marry the previous observation, the weather as forecast and their knowledge of snow-science to produce their best prediction. Obviously this is not a perfect science, mostly because weather forecasts are not cast in certainty either, but I think it would be misleading to call it "yesterday's information"
Observers head on to hill and observe current conditions
They make a forecast that afternoon for the following 24 hours based on their observations and forecasted weather influences. This runs for example from 1800 hours on the friday to 1800 hours on the saturday.
The time period is clearly published with the forecast.
First - ice.solo made this comment "plus, its one thing to predict avalanches, and other to do something about the risks." There's an incredibly important but quite subtle point behind this . There's a natural assumption that if we are aware of a hazard and it's potential consequences we'll avoid it. That seems quite logical but, disturbingly, it's not necessarily true. Studies of education-based safety programmes in relation to sexual health, drug use, driving habits and avalanches show similar findings. The key point is that being aware of the risks, does not, in itself, make you any safer. *Changing your behaviour* is what makes you safer. There's an interesting paper on this subject here http://www.sunrockice.com/docs/Sex%20&%20drugs%20IM%202004.pdf In short, understanding the human decision-making side of avalanche safety is critical.
Back to snow. An avalanche starts when the gravitational load at a point in the snowpack exceeds its strength. This can occur because 1. The load increased or 2. the strength decreased (or a bit of both.)
Things that increase the load on the snowpack include humans, more snow (note that snow deposition rates from wind transport can be WAY higher than from falling snow) & rain.
Things that reduce the strength of the snowpack are things that weaken the bonds between ice crystals in the snow. Examples are anything that causes rapid warming (e.g. rain, sun) or the slower process of facetting.
You mentioned failure mechanism. It might help to think about "right way up" & "wrong way up" snowpacks. A strong layer (e.g. windslab) over a weaker one is the wrong way up - it is more likely to slide. The sliding layer can either be a weak layer (i.e. poorly bonded crystals) or a good sliding surface (e.g. an ice layer.)
One other thing - warming then freezing. The *overall* tendency of melt-freeze cycles is to stabilise the snowpack. However the melting part of the cycle is weakening it, the freezing is strengthening it. A slope that's bullet-proof cramponing first thing on a spring morning and that would not avalanche with the entire UKC community dancing on it changes into a weak, slushy unbonded mess that could slide all the way to the ground in the afternoon sunshine. When it comes to melt-freeze cycles, timing your run is critical.
There's any amount of information in the books about terrain, the three (or five) "As", weather and it's effects and how the weather and terrain interact to make the snowpack how it is. But good on you for asking the questions and getting us all thinking.
Thanks for the link to that paper - I'd not seen it before and it makes some interesting points. I've seen McCammon's earlier work on heuristic traps and have tried to incorporate that into my decision making, particularly what he and Tremper have to say on "social proof" - I reckon this is a factor in most avalanche accidents involving groups, such as the one recounted here: http://www.outsideonline.com/outdoor-adventure/snow-sports/Tunnel-Vision-November-2012.html?page=1
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