Angular snow with poor bonding created from large temperature gradients within the snowpack.
Faceted snow causes the lion's share of avalanche fatalities in North America with surface hoar as a close second. And no wonder. It seems like made-to-order plot device out of a very scary movie. It grows like a parasite within the snow--often out of sight--until it's too late. It becomes inexorably more and more dangerous during the seemingly most benign conditions--clear skies, cold temperatures--and it lays in waiting, sometimes for weeks, until it's brought suddenly to life by a fresh load of snow or rapid warming. Then, when its victim bumbles into the wrong place, it pulls the rug out from under them, rockets them down the mountain at a terrifying speed, ripping them limb from limb as they bounce off trees and rocks and finally entombs them under tons of icy, hard snow.
How faceted snow is formed:
Faceted snow forms from large temperature gradients within the snowpack. Big word alert!--temperature gradient. A temperature gradient is simply how fast temperature changes over a certain distance within the snowpack. Why? Because it's a fact that warm air holds more water vapor than cold air. This means that temperature gradients also create what we call "vapor pressure gradients"--more water vapor in one place than another. And what happens when you concentrate something--especially a gas? It wants to diffuse--move from areas of high concentration to areas of low concentration. When water vapor RAPIDLY diffuses it changes rounded crystals into faceted ones--changes strong snow into weak snow. In other words, temperature gradients create potential weak layers that can kill us. That's why we pay so much attention to them.
Here's another way to explain it. Imagine an old woman with strong perfume walking into a cocktail party. As the perfume diffuses through the room, the people standing nearby would smell the perfume the strongest and the people standing against the opposite wall would be able to smell it the least. Next, pretend that wherever the perfume RAPIDLY diffuses through the room, it changes people to frogs. Soon there would be nothing but frogs around the old woman where the perfume is diffusing rapidly and the rest of the room would stay the same since the perfume around them is diffusing more slowly. Finally, imagine 20 old women with strong perfume spread equally through the crowd. Now, there's no more strong diffusion because the perfume has the same concentration everywhere in the room. Since there's no more diffusion, all the frogs magically turn back into people again.
A stupid example, I admit, but maybe you get the idea. The point is that it's a completely reversible process. Strong gradient turns rounds to facets. Weak gradient turns facets back to rounds. The process in reverse, however, occurs much slowly because it takes so much energy to create a faceted crystal that when we take the energy source away (the strong temperature gradient) it take a lot of time for the crystal to return to its equilibrium state (rounds). In other words, it might take a week or two of a strong temperature gradient to form large faceted crystals but after you take the temperature gradient away, it can take weeks or months for them to stabilize, depending on the ambient temperature of the snow and how much compressive load is on top. In cold climates without much load on top of the faceted snow, it may never gain much strength--even without a temperature gradient. The take-home point here is that: small temperature gradients make the snow stronger; large temperature gradients make the snow weaker. Got that?
So, large temperature gradient—how large is large? For snow of an average snowpack temperature, say around -5 degrees C, the critical temperature gradient is about one degree centigrade per 10 centimeters (1 deg C. / 10 cm.). In cold snow, say colder than -10 deg. C, you need a higher temperature gradient to cause faceting and in warm snow you need slightly less.
For example, let's stick two thermometers into the snowpit wall, one 10 centimeters above the other (about 4 inches). Say we measure a difference of only 1/2 deg. C. in 10 cm., it means that equilibrium snow is growing (snow is getting stronger). If we measure a temperature difference of 2 deg. C. in 10 cm., it means that faceted snow is growing (snow is getting weaker). All you have to do is to find a faceted layer in the snowpack, measure the gradient and you know whether the layer is gaining strength of loosing strength. Cool, huh? This is actually a powerful forecasting tool.