4) Why can weak layers be triggered long after for

4) What are the effects of weak layers that allow them to be triggered long after formation?

From Cookie:

* Their persistence is the most important factor for avalanche formation long after burial. For persistence, see above.
* Another important property of ice grains in a weak layer is the ability of the ice grains to rearrange themselves into smaller spaces through crushing.
* Poor bonding to layers above and below caused by lower number of bonds per unit volume.

Sources:

Proceedings of ISSW 2010 www.avtrainingadmin.org/pubs/2010_ISSW_Proceedings.pdf

* Some insights into fracture propagation in weak snowpack layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716
* Fracture energy of weak snowpack layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Crushing and densifying of weak layers is a slow process that leads more often to collapse rather than settlement because the grains are very large with low bond densities. Pressure that is transmitted to the weak layer by overburden pressure of new snow or by the more dynamic pressure applied by a passing skier can collapse the layer and initiate it's downslope movement since it is resting on a slope (if avalanche is a consideration).

Between storms overlying new snow layers gain some strength and in the metamorphic process of rounding begin to develop a structure that dissapates energy multi-directionally. Hence, the structure becomes more difficult to trigger with passing time. However, when additional snowfall loads the still weak structure or when a passing skier is able to transmit stress down to the weak layer, an avalanche may result.

Crushing and densifying of weak layers is a slow process that leads more often to collapse rather than settlement because the grains are very large with low bond densities.

I'm not sure that it leads more often to collapse rather than settlement.

* The speed at which densification proceeds is determined by meteorological and metamorphic phenomena at various spatial and temporal scales.
* The complex nature of these phenomena is one reason why it's so hard to create precise forecasts of instability for persistent weaknesses.
* The Avalanche Handbook lists "incremental changes to the snowpack" as one the primary sources of uncertainty.

Pressure that is transmitted to the weak layer by overburden pressure of new snow or by the more dynamic pressure applied by a passing skier can collapse the layer and initiate it's downslope movement since it is resting on a slope (if avalanche is a consideration).

According to the anti-crack model proposed by Heierli, Herwijnen et al., there are two basic requirements for avalanche formation:

1.) delamination of the slab from the snowpack system, and
2.) after delamination the crack faces come into contact with each other and must overcome static friction ( otherwise you have a failed avalanche ).
3.) gravity, not weak layer collapse, initiates the downslope movement of snow.

Source: www.issw2008.com/papers/P__8212.pdf

Between storms overlying new snow layers gain some strength and in the metamorphic process of rounding begin to develop a structure that dissapates energy multi-directionally.

If there are early strength gains, they come from loss of branches and pore space reduction from overburden pressure, not rounding.

* As I'm sure you're aware, rounding only occurs when a critical temperature gradient does not exist.
* In the presence of a critical temperature gradient, crystals will still lose branches, but they'll begin faceting instead of rounding.
* Loss of branches occurs because the vapour and temperature conditions on the ground are different from conditions in the clouds.
* Pore space reductions occur because overburden pressure rearranges the ice grains.
* Counter-intuitively, early strength gains in aggregates of tiny ice grains are from inter-crystalline bonds that form rapidly.
* This is precisely what makes wind slab so dangerous... wind slab basically stabilises TOO rapidly.
* Weaker snow below may not have time to adjust.

There are a multiplicity of factors that determine the relationship between new snow and any persistent forms beneath it. I'm pretty sure that avalanches form quite readily in weak layers below slabs composed of rounded polycrystals... assuming the energy requirement is met.

In re: multi-directional energy dissipation. Do you mean with respect to anisotropy and isotropy?

In re: multi-directional energy dissipation. Do you mean with respect to anisotropy and isotropy?

I'm referring to a model ANENA produced and displayed at the Jackson ISSW. While their three dimensional model was for well-developed rounding, I am sure that in the rounding metamorphic process there would be a somewhat steady progression towards a structure that develops 60 degree angles multi-directionally while at the same time densifying and developing a high bond density.

The model clearly showed that downward pressure onto the structure would be dissapated multi-directionally, which must be one of the reasons that it is harder to trigger a weak layer/weak bond between layers beneath such a structure (Rounded grains).

Let me add a little bit on warming to this topic:

* Warming with melt-water percolation to the weak layer has obvious implications for short term and long term stability.

*Warming of a structure that consists of a slab over weak layers causes re-arrangement of the slab structure within a short time after warming occurs (a matter of minutes to an hour or so). I think this was critical in the still-sub-freezing surface layers in making it possible for me to trigger my largest avalanche. Avalanches can be more easily triggered in the initial phases of warming.

* For deeply buried weak layers significant warming (as in the springtime) often correlates with instability some 4 to 5 days after warming begins. From a study by Jamieson regarding deeply buried weak layers in the Canadian snowpacks of 1997.