South facing, no snow is lying on the trees so presumably weather has been or is warm and consequently snow is heavy, a clear candidate slope for a slide. Lucky to have just missed that tree.
Last edited by Obviously A snowHead isn't a real person on Wed 9-01-13 22:38; edited 1 time in total
Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
The slide also happened just has he rode on to a convex slope.
Megamum, the snow slab will experience more tension on the convex slope, and therefore more likely to break apart.
Then you can post your own questions or snow reports...
Then you can post your own questions or snow reports...
meh, rob@rar, I see - the snow has a further distance to travel on top of the slab than it does at the bottom. The same theory that can create crevasses in glaciers as they go over un eneven base.! Great! Thanks I can see there is a lot more to consider than seems immediately apparent!
After all it is free
After all it is free
hard to tell at the point it slipped, but he seemed to be slamming an edge in pretty hard at times. As soon as you see him sliding you see he's edge is across the slope, so maybe he chose to stop at just the wrong point and with enough force to trigger the slide.
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He looked lucky to get away with stopping in the finish.
I reckon he knows it's getting dodgy. As he crests the brow you can see some disturbance in the snow by the small tree, he slows dramatically starts to cut across a bit and is pointing and signaling the skier on his left.
And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
rogg, I'm not so sure, he has his hand up (pointing? signalling?) and has slowed down before the fracture happens. I think he's surprised that the slab has let go.
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
Scarpa, Slightly??!! I found that really difficult to watch.
You know it makes sense.
You know it makes sense.
If you stop the vid at 2.54 his board is buried, slamming an edge in (which he did a lot in the vid) perhaps an understatement. Yes the slope is convex, which probably didn't help, but I think that guy was just shifting a lot of snow, and it didn't like it..?
Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
Schuss in Boots wrote:
Scarpa, Slightly??!! I found that really difficult to watch.
That is because you are easily distrac... oooooo, look... a squirrel!!!
Poster: A snowHead
Poster: A snowHead
Megamum wrote:
meh, rob@rar, I see - the snow has a further distance to travel on top of the slab than it does at the bottom. The same theory that can create crevasses in glaciers as they go over un eneven base.! Great! Thanks I can see there is a lot more to consider than seems immediately apparent!
It's not to do with the distance, it's to do with the shape and how the snow is stretched over the hogs back shaped hill, when the slope is concave(the opposite shape to the diagram) the snow fights against a it's self and makes the slab stronger,
If you want to know more about avi's read Bruce Trempers "staying alive in avalanche terrain" and re read it and re read it till you know it word for word, it's a great book
Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
Scrapa,
To me Probably the video that really has drove the message home the most about how easy it is to get caught up in a slide. I think examples like this make the reality of the dangers clear for all to see without the need to actually witness a slide to appreciate the gravity of the situation.
Not nice to watch, but does reinforce many of the messages from other threads re skiing in group and being equipped.
Gazmob
Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
Interesting - I see it like this. Cruising down with his mate (not far enough apart), come to some trees, see some tracks, turn to mate and signals left to avoid them, and if you stop at 2:52 you see his mate near a small tree. Forward on a second, and the fracture appears to come from that direction. I reckon he and his mate initiate the fracture around a weak / stress point caused by the lone small tree. Had they been further apart???
All I really note in Scarpas video is the aftermath of an avalanche, expected or otherwise, being dealt with without any shovels, so I'm guessing that the group was not carrying much in the way of the correct gear? Ok no-one expects an avalanche but please carry the right gear. Has anyone tried digging through solid ava debris with a ski? Its far from easy. My comment is not meant to act as a catalyst for retibution by the way.
To me Probably the video that really has drove the message home the most about how easy it is to get caught up in a slide. I think examples like this make the reality of the dangers clear for all to see without the need to actually witness a slide to appreciate the gravity of the situation.
Not nice to watch, but does reinforce many of the messages from other threads re skiing in group and being equipped.
To be honest, being of a cautious nature, these sort of videos - and the Stevens Pass report the other week - are enough to stop me going off piste at all. I'm not too confident in powder anyway, but I want to be. But then the Peter Bruce book "Heavy Weather Sailing" also gave me the screaming abdabs - although it was immensely useful on what to do if you do get caught.
Then you can post your own questions or snow reports...
Then you can post your own questions or snow reports...
eng_ch, it's the tripadvisor mentality. People only ever post when the avalanche happened, just as they only post when they had a bad stay in a hotel... so you miss out on the millions of lines that didn't end in disaster, just as you miss out on the millions of hotel rooms that were occupied without incident.
(Yes, before I'm flamed, I'm not suggesting anything other than caution etc. etc. etc.)
After all it is free
After all it is free
James the Last, I see way more videos of cool lines being skied without incident than I do those that end in disaster.
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Megamum wrote:
meh, rob@rar, I see - the snow has a further distance to travel on top of the slab than it does at the bottom. The same theory that can create crevasses in glaciers as they go over un eneven base.! Great! Thanks I can see there is a lot more to consider than seems immediately apparent!
No not like a crevasse, this is the wrong way to think about it. A crevasse is a moving sea of ice, as the ice flows over the convex slope it gets stretched and breaks. Snow doesn't fall at the top of a hill and then flow down, it falls at an even rate all over the mountain (it may be moved around by wind or gravity but not to the same degree as a sea of ice). It is not getting stretched over the slope till it splits, at least not to any degree until the spring when it thaws.
Concave slopes
On a short concave slope the uphill snow is supported by the downhill snow, it has some "foundations" if you like. This makes it more difficult for a slab to slide, it may collapse with characteristic whoumping sound and cracks. On a longer slope with a convexity at the bottom this compression effect is much less of a factor and the convexity forms a terrain trap. To be avoided, especially if climbing.
Convex Slopes
A convex slope has a larger surface area than an even slope so assuming an even snowfall rate the snow will be thinner on the convexity. A thinner snowpack is a weaker snowback, just as a thinner bit of wood is a weaker bit of wood. Thinner snowpacks are also more prone to the formation of weak layers. A convex slope also gets steeper as you ski down it, remember you won't trigger a slab avalanche on a slope less than 25 degrees. As you ski across the convexity you may go 25->30->35 degrees than baam, the slide goes. Where the convexity leads into a long even slope there is no compressive support, if the weak layer collapses all you have is the top of the snowpack holding the slab in place and on a convexity this will be a relatively thinner layer of snow. A convexity will tend to accellerate wind which will remove snow from the convexity and deposit it downslope. Wind is an important factor in slab formation.
As for the video, you have a convexity leading into a gully that appears to have been wind (cross) loaded. Although generally south facing (assuming this is in the northern hemisphere) the gulley and convexity will present different slope aspects. Maybe a weak layer formed on the more shaded aspects (south facing slopes tend to stabilize with with diurnal temperature changes and form fewer weak layers in the snow pack - the exception is when it is very cold). Temperature can be a factor in slab release but it is not a major one: the most important factor is a weak layer, a cohesive slab of snow and a load (the skier).
Ski the Net with snowHeads
Ski the Net with snowHeads
James the Last, meh, oh absolutely. And knowing the precautions to take and what to do if it should all go wrong is the best defence of all. Maybe there's just a small minority of testosterone-fuelled adrenaline junkies out there with a penchant for plastering their exploits all over t'Interwebby while the rest of the world has a healthy dose of caution?
snowHeads are a friendly bunch.
snowHeads are a friendly bunch.
davidof,
Quote:
it falls at an even rate all over the mountain
actually that's not really accurate, part of the problem is that it doesn't and many factors - slope angle, large vegetation, boulder fields, plateaus, etc. can all cause different snow distribution patterns, leaving aside the wind issues and the fact that more snow tends to fall and accumulate at altitude. Part of the issue that it is more likely for snow to accumulate on the flatter top surface of a convex slope than on the steeper 'peak' so a convex slope has a tendency to 'top load' as well as be in tension rather than compression.
Looking at the 'lip' the guy is skiing in the second vid and the very even height differences between the skied field to the left and the untouched powder he is trying to track on the right, I think there is another explanation. I think the height difference is partly due to an earlier avalance. The field on the left is what resulted after a horizontal and vertical sheer. What remained on the right - the bit he is on - is the remaining field at the original height. After some time and a bit more snow the remaining snow mass weakened - partly through lack of the support that was there before - and got to slip point that the skier triggered himself higher up the slope. The way his legs are taken out without any warning suggests the whole mass started moving as one rather than an accumulating sheet slide that you see in the first vid. I know I am always nervous of defined vertical faces across the slope unless I can see a geological reason for it them be there.
Just my opinion, tho'.
And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
You take two adjacent slopes, one convex, one at an even angle but same overall angle. You add snowfall. The amount of snowfall will be the same in mm/hr over both slopes but the convex slope will present a larger surface area and will see less depth in total. It is simple geometry.
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
So if you're just off somewhere snowy come back and post a snow report of your own and we'll all love you very much
davidof, assuming all other factors are equal, I agree. Prob is that they generally aren't. It's especially noticeable on a generally even slope where there is a notch in the face at the top. Invariably, the slope in front of the notch tends to keep less snow than the slope on either side. It can also get funny with sheer faces, you would think they would pile up snow at the base and to an extent they do BUT wind shadow can cause a reverse effect that actually lifts snow from the base. (next time you get a strongly directional wind and some snow find a vertical face with no trees in front and look at the base of the face carefully and how the snow moves – it can be very odd). You then add in all the other variables and that's why we pay guides with local experience money rather than relying on just looking at the shape of the slope. Proof of the point – the slopes in the first two vids don’t look particularly convex to me (although the scary one does look very plateau-y)
You know it makes sense.
You know it makes sense.
Oooo....the problems caused by one seemingly innocent question - sorry folks!
FAQ
Poster: A snowHead
I can see there is a lot more to consider than seems immediately apparent!
I found that really difficult to watch.