1) What are weak layers and why are they dangerous

While there are many weaknesses in new snow those without substantial enlargement of ice crystal sizes tend to be transitory because they are like the rest of the snowpack and can bond.

Classic weak layers, often called Persistent Weaknesses or PWKL's are of four primary types:

*Surface Hoar
*Crusts with faceting (almost all crusts have faceting associated with them but vary in degree.
*Faceted new snow or low density snow
*Depth Hoar, the extreme final form of the faceting process
*Bonds to ice layers which almost always have some faceted crystals on the ice surface. The ice layer itself is usually not weak but the bond to it is often poor.

Surface Hoar is an essentially two dimensional crystal that grows into the atmosphere under clear skies on an exposed snow surface. It is weak because of it's shape. The structure is that of sometimes quite large isolated crystals that are seldom bonded between one another. Platelike, when they collapse because of disturbance they shockload the overlying snowpack layers and then act as an efficient gliding surface in an avalanche.

Crusts usually have faceting associated with them because at the time of formation they are warmer than surrounding snowpack layers (in a wintertime snowpack) and that sets up a strong temperature gradient across and near the layer. In the refreezing process the latent heat of freezing provides heat to the layer maintaining the gradient. Even after formation and burial a gradient often exists near this layer because the layer is denser than surrounding snowpack layers and hence conducts heat at a different rate. In addition crust layers resist settlement and cap the upward migration of water vapor in the snowpack which furthers the faceting process.

The faceted grains have larger crystal sizes than the rest of the snowpack and have low bond densities making avalanche release both above and at times below the crust surface a possiility.

Faceted snow evolves when new snow is subjected to a strong temperature gradient while it is near the snow surface especially when skies are clear. The resulting larger grains with lower bond densitiy form a weak structure. When buried more deeply, faceted snow can begin to round and densify, strengthening it, but it remains somewhat weaker than the surrounding snowpack layers that may have undergone rounding throughout the process.

Depth Hoar is the final form of the faceting process and can sometimes be found beneath crusts and in shallow snowpacks nearer the ground. The grain sizes can range from a couple of millimeters to a centimeter or more, usually with poor bonding between grains. It can obviously collapse when loaded which can result in an avalanche.

Bonds to ice layers are generally poor unless the snow overlying the ice layer is saturated with saturation rates near the ice layer higher and gradually diminishing with distance from the layer. The latter structure is gradated in strength and ice grain size. In the absence of this gradation, adjacent relatively large ice grains and relatively smaller snow grains are unable to bond. Colbeck (Army Institute of Cold Labs) said in a paper that anytime that adjacent grains are greater than 1.66 times one another are at best able to form transient bonds but not permanent ones. Stress onto the snowpack is then concentrated at this poorly bonded surface and can result in an avalanche. In addition there are often at least a few faceted crystals on the ice surface, further minimizing any bonding, and, of course, the ice surface is often quite slick offering a good sliding surface and little mechanical friction (because of it's smoothness).   

If you're interested in simplicity, all of these can be generalised to the following:

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

Beware!
Out yesterday.
Dug some pits. North facing slope Crystal BC.....depth hoar like I've never seen before.!!!

If you're interested in simplicity, all of these can be generalised to the following:

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

I think the reason for understanding the different types of weak layers is that they are not uniform in distribution. It is necessary to understand the "avalanche dragon" on a particular tour in order that any tests performed are effective and that good terrain choices are made and group travel considerations are employed at the proper times.

Depth Hoar is the final form of the faceting process and can sometimes be found beneath crusts and in shallow snowpacks nearer the ground. The grain sizes can range from a couple of millimeters to a centimeter or more, usually with poor bonding between grains. It can obviously collapse when loaded which can result in an avalanche.

This is incorrect.  Depth Hoar is not the final form of the faceting process, nor is it found beneath crusts etc.  It is found on the ground, and forms due to large temperature gradients at the ground/snow interface, and is further exasperated by warm temperatures near the base of the snowpack.  It requires a smooth landscape to run on, and may lay dormant before presenting itself in a spring climax cycle.  There was a well propagating depth hoar event at Crystal Mountain in mid-November, due to a small early snowfall Nov 3rd this year.

Depth Hoar is the result of a mature faceting grain, the bigger the Depth Hoar, the older the grain is thus it does represent the final stage of the faceting process. There no additional form(unless warming occurs) faceting takes once it achieves the striated cup form.

Depth hoar can occur under crusts and near the surface during long, cold, dry periods, which have similar conditions to the temperature transfer that occurs in the basal portion of cold shallow snowpacks.

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.