Solar Aspects - to avoid or not?

_Got a question…guys were talkin and I’ve been doing a little more reading. The discussion was when or what time of day should we decide to ride south facing or north facing slopes. Shouldn’t the warm weather generally settle and stabilize the snow? But then I was thinking or would that just start to load up on the facet layer below causing it to fail.
I know there is alot of variables but what is the usual general rule??....

_ Bob P., March 2009

It always depends on the blueprint in the snow.

Solar heating of a dry snowpack can trigger a loose, dry avalanche. Of course, solar heating of a snowpack with warm upper snow layers can trigger a loose, wet avalanche. These point release avalanches can happen with increasing frequency as the temperatures rise. Although this activity is hard to miss as steep slopes release noisy sluffs, these slides usually only involve the surface of the snowpack and aren’t typically responsible for high consequence avalanches.

Let’s look deeper into the snowpack. Strong layers….weak layers….? Are slab avalanches a possibility? If so, what are the layers of concern? Are they layers that will heal with warmth (windslab, storm snow layers), or are they layers that will fail due to a softening and warming of the surface snow (buried ice layer, midpack PWL)?


Image credit: Laura Bakermans

What are the internal snow temperatures? If the internal temps are -5C or lower, then the radiation might be a moderating force. If the upper layers of the snowpack are nearing zero degrees, then the solar aspects may be more unstable due to the daytime heating.

Are things warming enough that liquid water is moving throughout the snowpack? If so, where is it pooling? On a mid-pack dense layer? On the rocks below the snowpack? A safer bet in these conditions is to retreat to the shady or cooler slopes and beware of overhead solar aspects on your trail ride home. Naturally triggered avalanches are a VERY SERIOUS CONCERN in these ‘spring-like’ conditions. Are the trees raining? Dropping ‘snowbombs’?

Consider how much heat the black rocks are soaking up on a warm sunny day. This heat is radiating out to the surrounding snow, potentially weakening the snowpack above you…even if the temps are relatively moderate in the valley below.

But what if it doesn’t refreeze over night? What if the overall air temp is super warm? You might find that if the temps are warm enough, even the shady slopes might let loose.

How quickly and how intense did the temperatures change? If the change was fast and intense, the upper snowpack layers may be seriously weak, and yet the situation may not allow the heat to penetrate to the unconsolidated snow that is insulated beneath the surface layers. The unconsolidated snow could be fresh snow that hasn’t had a chance to set up or even small facets. Both of these weak layers could be healed by the process of rounding once the warming penetrates deep enough into the snowpack to moderate the temperatures and allow for settlement.

Generally, north east slopes are the site of statistically more fatalities, however much of this is due to the fact that our prevailing winds are from the south west, thereby making the north east the lee slopes. Windloaded slopes are linked to the vast majority of all fatalities.

I realize that I didn’t give you a definitive answer. Unfortunately there really are no hard and fast rules. Avy safety is all about responding to the current weather, snowpack and avalanche conditions. Read the Public Bulletins, browse the Snowmobile Forums for recent avy conditions in the riding areas that you are headed to and stay alert and gather observations throughout your riding day.
Make it a habit to use your best travel habits no matter the avy conditions. For example: travel one at a time, park, play or gather in safe areas (not exposed to overhead hazards), maintain visual with anyone exposed to danger, always plan an escape route, choose survivable slopes (i.e. no terrain traps). And the list goes on.

For a discussion about the foundation of our snowpack CLICK HERE to read ‘The Spooky Snowpack Dec 2008’
It is this poor foundation that is creating the potential for massive avalanches this spring…entire snowpacks releasing to the ground. It is entirely likely to see new avalanche paths created this season from avalanches ripping out mature timber by running on slopes that have never slid before, or by slides running further down slope than normal. Please maintain a LARGE safety margin during these conditions.

Photo credit: Randy Toronchuk
This photo is from the Dore area near McBride. It was taken ~March 20/09. Here are some of Randy’s comments.
We were in Dore when the slides came down, none were visible when we went in. The pictures are of the biggest slide at about KM20? Thought we were trapped…about 2 hours to get a trail around the slide. This slide was at least 350 yards wide, up to 40’ deep, 15’ crevasses like a lava flow. The amazing part was the run out. The terrain at the base of the mountain is almost flat and the length of snow at this depth was about 1km long. The entire mountain must have let go at the top. This slide encompassed the previous runout area in all directions and took out every tree in its way. This was wet heavy rained on snow, if some one would have been on the trail I don’t think they would have had time to get out of the way. I do not believe an air bag would help in this type of a slide, a scary sight. There was another slide at about km12. The third slide was at KM6 on the sharp bend. It was stacked about 40 to 50’ high across the road. We had to do about an 80’ drop down the bank onto the river ice, go around the slide and make a track up through the trees.

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For a discussion about why experienced riders might be getting caught CLICK HERE

To everyone reading this: how recently have you completed your AST1 (16 hr hands-on avy training)? If your answer is never, or more than 2 years ago, we recommend that you check out our upcoming courses by CLICKING HERE.
There are always new skills to learn and practice.

  1. Lori, the discussion article is great. Here are a couple of thoughts/suggestions:
    1) It might be worth distinguishing between loose solar activity (sluffing) and solar-induced slab avalanches. The former really aren’t that dangerous (unless they initiate the latter) while the latter can be very destructive.

    2) (Maybe too technical for the article, but I’ll include it because it’s interesting.) Solar-induced slab avalanches result from two mechanisms:

    a. “Glide”, which is where there is free water present in the snowpack. As you say, as this percolates down to the ground or an ice layer, the lubrication along the plane where free water is concentrated reduces the basal friction enough to allow the avalanche to slide. Often these avalanches are preceded by glide cracks which open up on south-facing slopes. This kind of avalanche could release at any time, but are most likely to release when free water is abundant and strain rates are highest, i.e. during intense heating by solar activity. Areas with smooth ground cover (especially smooth rock) are particularly susceptible to this kind of avalanche and should be avoided every year. Years with well developed lower crusts will likely see greater amounts of this kind of activity, like last year, and this year in some places. Rein things back on solar aspects if there are lower crusts and it’s a hot sunny day.

    b. Moist snow over a dry weak layer. Moist snow is able to deform (creep) at quite a high rate, due the increased moisture content. Where this overlies a weak layer that is dry, and not able to deform at the same rate, a very high strain rate gradient is set up. If this strain rate becomes too high, the avalanche releases. In order for this release mechanism to be widespread, a particularly weak weak-layer that persists all season is required (e.g. basal facets). Again, the most likely time and place is a south facing slope facing full sun. This season is unusual in the extent and the weakness of the lower facets. I suspect that this mechanism will be a big factor this year. However, the timing will be very difficult to determine and will vary from place to place. Best advice: rein things back on solar aspects during periods of intense warming. Don’t regroup below big south-facing slopes.
    Cheers, James


    — James F    Apr 2, 11:36 PM    #

  2. It’s really interesting how a really good question can get a whole bunch of avy types going in different directions!
    My initial thoughts for answering Bob P’s question are:
    1.Short term pain (weakening snow) and long term gain (it settles = harder = stronger). There are often two stories to warming.
    2.Crusts and corn: It’s good to go if there’s a thick crust protecting any weak layers below. Corn snow. Watch overnight temps and how hard it freezes. Get out of there when it warms and crusts break down. This is the “Spring Regime”, but we aren’t there yet.
    3.I like the third image in your article Lori – the one with the perc channels (percolation) and snowballs. That to me says solid safe snowpack that’s good to go as long as I’m not sinking to my shins cause it’s in the schmooey melt phase.
    4.I like that you talk about temperature and how close it is to zero (melting, and we all know how strong water is compared to snow).
    5.Is it the first time new snow is warming & melting (touchy) or has it been there and done that (settling)?(i.e. The short term pain?)
    6.Are there buried crusts? How’s the bond with the new snow? Is there a tiny layer of facets (like 1 or 2 mm thick) on it? Is there surface hoar on it? (I was just spanked really hard by SH on a crust!)
    7.I’m often spooked by S slopes at this time of year: all sorts of crusts (#6 above) but we’re not yet in a spring regime (#2 above). It’s interesting that many accidents do indeed occur on N’ly aspects and lee slopes (where skiers at least spend most of their time), but they typically are only a single or two people), Contrast that with S’ly slopes where some of the BIG commercial wrecks with numerous people. S’ly accidents are fewer but bigger.
    8.Cornices and overhead hazard.
    9.Don’t be fooled by good traction and hard surface layers, but if you poke you pole through (again, skier talk) the top 15 or 20 cm then it’s a pile of schmooo below. (That’s the Monitor Creek incident that you have photos of in your colum “But they were experienced…” the top 30 cm of the snowpack gave good traction and was warm, but the mid pack and bottom was wintery – it was a January or Feb thaw that released the sledders into terrain that you don’t usually get the traction for until April.)

    James et al. I’m struck by your second mechanism or increased strain rates. You say:
    In order for this release mechanism to be widespread, a particularly weak weak-layer that persists all season is required (e.g. basal facets).
    I agree, but at this point I’m even more interested in mid-pack crusts where strain rates concentrate. Rationale: It’s still late winter or early spring. I expect layers to peel like an onion and see mid-pack problems first. Well developed FC on cr in the mid-pack won’t have healed or melted out. I think strong buried crusts in the mid-pack are my primary fear when it comes to large avalanches at this point. Any thoughts?


    — Ilya S.    Apr 2, 11:37 PM    #

  3. Thanks Ilya and James for adding in your comments!


    — Lori Z    Nov 25, 12:14 AM    #
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