Weak Layer Considerations

Page 152 of  Temper’s book places this number at 1.5 m(5 feet) with a caveat about triggering deeper  slabs from shallower areas. I believe that the problem with using any number is that they can become a“rule of thumb” and does not take into account the mean deviation that Gary points out above. My number will continue to be 1.5m+.5 m (error margin)=2m

I’ve also included an updated picture (bad light, no crown detail) of the Silver Star Mt. slides that I’ve been tracking this year. The path flanks were clearly visible this time and involve a large volume of snow. Also the slide tended to involve snow that was out of the path as it passed by and over the steeper rock bands. This indicates the presence of weak layers that fail as the anchors are removed by the avalanche.

Another implication of this slide path (or any slide path) is that there may be  now a shallower snow pack in the slide path, which may lead to future avalanche activity. Since the snow in path is thin, it is subject to temperature gradients that may disappear in the deeper, surrounding snow that did not slide.

Any way this is my understanding and any corrections or additional knowledge is welcome. One question I have is this; Does the energy input(frictional heat energy) generated by the avalanche destroy or modify the weak layer that was responsible for the slide?

In reply to your third point, in 1978 we had a helicopter out and were skiing north of Mt. Baker. Everything was fine until we skied a slope in the Damfino Creek drainage. The slope was a steep avalanche path and the snow structure on that one slope only was 18" of unconsolidated new snow overlying 3' of depth hoar. At that time I didn't fully grasp all the ramifications of depth hoar but did recognize it as such. I think the reason it didn't slide was that the new snow lacked any cohesion. But, obviously, skiing the slope made me very nervous. Had we been able to reasonably reverse course and walk back up, I would have done so. The main point is that the slope had obviously avalanched earlier in the winter; your point exactly.

To your fourth point, an avalanche is very likely to take out weak layers like facets or surface hoar, but the friction of an avalanche would likely replace those weak layers with a hard, smooth surface. This would be more obvious lower in the starting zone and in the track.

That is the number that is bandied about. It is thought that deeper slabs are triggered from areas where the weak layer is more shallowly buried. But I can say definitively that I've triggered a 4' slab which I described elsewhere and which I believe had as a weak layer a faceted suncrust. I've also remotely triggered a pair (nearly simultaneously) of 3 to 3-1/2' slabs on surface hoar. In both cases if I was in a location where the weak layer was buried less deeply when I triggered it, I didn't know it at the time.

Page 230 of The Avalanche Handbook ( 3rd Edition ): "However, when buried persistent layers are deeper than about 1 m, instances of skier triggering become rare, while snowmobiles may still trigger avalanches..."

I think it is a probability thing where the number of slides that can be triggered is something like three standard deviations from the norm at 3'. But there are undoubtedly outliers that are rarer that can be triggered at greater depths.

Where did you get this figure? Is this speculation, opinion, or is it sourced? This isn't a request for justification; just clarification.

Page 152 of  Temper’s book places this number at 1.5 m(5 feet) with a caveat about triggering deeper  slabs from shallower areas. I believe that the problem with using any number is that they can become a“rule of thumb” and does not take into account the mean deviation that Gary points out above. My number will continue to be 1.5m+.5 m (error margin)=2m

Using a 2 metre margin of safety is a rational individual choice, although it's probably too conservative for some people. I have a question: since 2 metres covers the entire depth of the snowpack in a lot of places, do you just assume that you can trigger an avalanche if there any weaknesses present?

Any way this is my understanding and any corrections or additional knowledge is welcome. One question I have is this; Does the energy input(frictional heat energy) generated by the avalanche destroy or modify the weak layer that was responsible for the slide?

It's just about impossible to say whether or not an avalanche will eradicate the weakness in which it formed. Sometimes it happens; sometimes it doesn't happen. Here's what I wrote in the April 2010 issue of The Avalanche Review ( for an article on this subject ):

***

The size of the forecast region is the most important factor, with precise answers only available for very small areas. However, even for small areas, the chaotic interaction between terrain and weather makes it difficult to predict the effects of widespread avalanching on future snowpack instability. The following scenario, which is just one possibility out of many, hints at the overall complexity of this forecasting problem.

Instability will persist when a bed surface composed of faceted crystals is immediately reloaded during a storm. On the other hand, future snowpack instability on that slope will be very different if the faceted crystals exposed by avalanching are subjected to multiple melt/freeze cycles prior to the next storm. Melt/freeze activity is often limited by aspect, so it is possible for the faceting process to continue on cold aspects, while faceted crystals on warm aspects undergo rounding as a result of melt/freeze metamorphism. In this highly general scenario, the weather builds new patterns of snowpack instability that are difficult to uncover without careful observations.

Therefore, for most recreational skiers, knowledge of a recent avalanche cycle is a very general and imprecise piece of information. General information often has a dangerous and unwarranted influence on individual or group beliefs about the presence of instability and its parameters. Without abundant information, expert knowledge, and significant experience (Randy and Nick provide great examples of this), a recent avalanche cycle should not exert undue influence on recreational travel choices and decision-making at any operational scale.

More than anything, incremental changes to the snowpack caused by synoptic scale weather events will alter the characteristics of the danger but won’t eliminate it.

* The chaotic relationship between terrain and weather is a primary source of uncertainty.
* Incremental changes to the snowpack are a primary source of uncertainty.
* Avalanches remove weak snow from some, but not all, slopes.
* Avalanches may or may not remove all the weak snow from a specific slope.
* Use multiple sources of information to determine the likelihood of avalanche formation.
* An avalanche cycle over a large area certainly does not mean a specific slope is safe.
* Proactively managing uncertainty is essential to safe decisions.

Wind reaches its maximum velocity at the ridge top, which gives it the most scour power. Unless I missed something, I'd expect to find sastrugi in those locations, not consolidated powder.

Your hypothesis about the relationship between energy and the apparent stability of consolidated powder over sugar snow is probably correct, but the line can be exceptionally fine.

Cookiemonster. Thank you so much for your reply to my question. Reading your stuff is like eating candy. In answer to your question. I always assume that I can trigger an avalanche in powder on a steep avalanche prone slope.

If the pack is only two meters deep and a weak layer is present, especially so.

Also, thanks for the kind words. Most of what I write is just a combination of research from other people!

I think regarding weak snow near cols that the aspect is what usually matters. Of course east aspects and most northerly aspects usually would be lee, so that leaves the other aspects. While southerly and southweasterly slopes near the tops of gullies and couloirs are often blown down to a shallow snowpack,  in my experience more often than not there is considerable strength even in faceted snow there because of solar influence and in the Cascades by wind driven rain that once refrozen adds strength. Westerly and northwesterly aspects get more interesting because the sun has less affect while the wind influence remains. Particularly in the Rockies and Purcells northwest slopes when they are loaded can be very problematic as they are often underlain by depth hoar. They would of course be loaded by easterly (less likely) or southeasterly (more likely) winds. The gullies and thin snowpack areas can also easily be cross-loaded by more southerly winds. Those aspects scare me in regions where depth hoar is often a factor or there can be a unique situation elsewhere.

I would also add that moraines because of the porous nature of the rock and because of the way they (moraines) stick out into the wind are classic locations for depth hoar in almost all mountain climates. The large moraine on the southerly portion of Spire Gully for example, has avalanched to the ground several Springs when the snowpack is not particularly deep.

Freeskiguy, a gut feel is that although these west and south facing gullies (as in the first of your last four photos) would slide with new snow instabilities they are probably unlikely to go as depth hoar slides until springtime when the faceted snowpack becomes very weak and isothermal. The gut feel is just based on the width of the starting zones and the amount of lateral anchorage in the gullies.