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).
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