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Apr 13, 2007: Slabs, Loose Snow, and Convexities
Slabs of snow are continuous bodies, and are subject to deformation and the development of internal stresses for that reason. Loose snow without any cohesion does not form a continuous body and its behavior is that of a collection of unconnected grains. Comments in at least one public advisory show a lack of understanding of the fundamental differences.
New snow may be loose to some extent, although the shape of new snowflakes almost always means there will be some interlocking of many adjacent grains. Wet snow can also be loose, although the water involved can play some role in connecting adjacent grains. The epitome of loose snow is cold faceted snow. At its extreme this can be large depth hoar grains, but the grains are usually smaller when it's not the basal layer along the ground.
Truly loose snow generally slides in an entirely different manner than slabs. It tends to start at a point and fan out; in most cases it does not run all that far relative to the path and stops in the track somewhere. We see this with new snow and wet snow, although both can also avalanche as a slab in some cases. To the extent that you ever see faceted snow sliding down a hill it will be a loose slide. These cold grains are totally disconnected from each other. Try making a small model of a slab avalanche using sugar and you'll find it is difficult to impossible.
When people fail to understand this difference we end up with statements like this (which was used on 1/27 as well):
Good evening, this is the Eastern Sierra Avalanche Center with your weather and snowpack conditions update for January 25, 2007. The avalanche danger rating continues to be LOW on all aspects ... also watch out for steep convex rolls in mid elevation trees where you could trigger a sugar snow sluff.
What we have here is the combining of two different unrelated phenomena.
Convex rolls are often trigger points for slab avalanches. The slab, a solid body, builds up additional tension in the surface layer(s) on these spots due to creep. As the slope steepens the pull of gravity along the slope increases, so below the convexity the slab is being pulled harder than above. This causes both a stretching and a bending of the material. (This is explained very clearly and concisely in the Snowpack 1 module of the Level 1 Course in our Avalanche Institute.)
Loose snow such as facets will not be subject to this effect because you can't stretch or bend such snow. Again, try to do it using sugar. The steeper the slope angle is the easier it will be to cause the faceted snow to slide downhill. The shape of the slope won't be important, only the actual angle.
So this is a situation where someone took a common statement about convexities and misapplied it. Unfortunately this is a common problem, even more so in education than in forecasting. It seems everybody wants to be a scientist, but most don't really understand the basic science behind what they've read or been taught. Avalanche educators frequently just teach others what they've been taught themselves, but since they don't understand the underlying physics they often apply things to the wrong situations. Or offer poor or incorrect explanations for things. They also have trouble answering questions from astute students that require a deeper understanding of the material.
During the same winter, a statement about "sugar snow sluffs" was included in another bulletin for a different part of the country. However, that one was properly written and mentioned only that it was a possibility on steep slopes and did not refer to convexities.
