2) What snowpack structures are most dangerous?

2) What snowpack structures incorporating different types of weak layers are most dangerous?


*For virtually all weak layers the most dangerous structure is one of a sandwhich where the top and bottom layers are distinctly harder than the bounded weak layer.

The key is that the top layer(s) have a slab like characteristic, although how slablike it must be depends on the degree of weakness. That is, if a weak layer is very weak any layer overlying it will act as a slab. The classic example here is surface hoar which can form a weak layer even when sandwhiched between relatively soft and low density new snow (or partially settled snow). A faceted surface snow layer, when buried, can act the same way. 

The tendency for the slab to slide can of course be mitigated if there is sufficiently strong snow composing or"capping"  the slab. In this situation two things can occur: 1) Energy can not be transmitted down to the weak layer (see the stress bulb diagram www.turns-all-year.com/skiing_snowboardi...ex.php?topic=22760.0) or (2) the slab can be too strong to be broken at it's boundaries.

*For surface hoar, faceted snow and depth hoar grains, the worst structure is when the weak layer sits on top of a firm and smooth substrate. For an ice layer, of course, the hard substrate is the ice layer itself.

When one looks at the stress bulb diagram www.turns-all-year.com/skiing_snowboardi...ex.php?topic=22760.0 one would see a flattening of the base of the bulb at the depth of the hard substrate. In otherwords, the stress is concentrated at the base of the weak layer, with the potential to destroy the weak layer's structure at that depth.

The hard structure then becomes an efficient planar low friction surface, meaning that avalanches can release and slide at comparatively low slope angles. Any hardening, even a very slight warming crust or wind -hardened surface can have this effect. Th

*For depth hoar, the ground can become this hard sliding surface.

*The more difference between the underlying hard and overlying weak layer, the weaker and generally more persistent the weak structure. 

   

Cool list!

These can be generalised to:

1. Grain size
2. Hardness differences
3. Shear quality