1) What are weak layers and why are they dangerous

I should have used the word advanced as opposed to final as the meaning is more concise. Depth hoar is the most advanced form of the faceting process (have to be careful with wording here as Surface hoar is also faceted).

From my experience from literally hundreds of pits near Blewett Pass in classes faceted grains usually become Depth Hoar forms exhibiting cups, solid plates (often hexagonal), and solid prisms once the size of the crystal grows to about 1.5mm or so. The largest Depth Hoar crystals I've seen in Washington were near 5mm in size and near the ground. I saw larger grains in Sun Valley to about 7mm but I understand such crystals can grow to 20mm or more in the Rockies.

I would think that the term Depth hoar refers to the fact that it is more often found at the base of the snowpack because of heat from the ground, shallow early season snowpacks, and a "longer growing season". But depth hoar can occasionally be found adjacent to crusts; usually beneath them.

Scary, to the ground slab. Not my idea of fun snow conditions.

The latest version of the SWAG book doesn't specifically say depth hoar is always on the ground. But in practice I have always heard it referred to as weak snow at the base of the snowpack. I'll talk to the writers of the SWAG book to clear up any discrepancy.

Surface Hoar is also in a different class than facets, and forms from a very different process than faceted grains within the snowpack. I would say that surface hoar is not a faceted grain. I also haven't heard it talked about as "shockloading" the snowpack. It fails and propagates as any other weak layer would, creating a concentrated layer with very weak shear strength as it fails.

Regardless of differences in opinion, I think it's worth reading the snow classifications put forth in the SWAG book:

www.avalanche.org/research/guidelines/pdf/AppendixF1.pdf

They have made a number of significant changes in the latest version, and it provides a lot of information on the formation and identification of the grains we are discussing.

Surface Hoar is also in a different class than facets, and forms from a very different process than faceted grains within the snowpack.  I would say that surface hoar is not a faceted grain.  I also haven't heard it talked about as "shockloading" the snowpack.  It fails and propagates as any other weak layer would, creating a concentrated layer with very weak shear strength as it fails.

Yes, Surface hoar forms from a distinctly different process than what we call facets, or from more elaborate facets called Depth hoar. But physically, it is faceted; that is to say it exhibits straight edges, square corners, and develops monomolecular steps in it's growth. I didn't mean that Surface hoar forms from the faceting process.

Surface hoar's collapse, where all of a sudden all the vertically inclined crystals fall flat, is a dramatic event in the snowpack. Personally, I've seen rapid (nearly instantaneous) propagation for over 400 linear feet (delineated by shaking trees) when that event took place. Fortunately, that was on low angled terrain. From limited observations, when a faceted layer has failed the process has been a much slower one, in my experience sort of progressive; although the net effect can be the same - an avalanche on a slope.