Angular
snow with poor bonding created from large temperature gradients
within the snowpack.
Faceted snow:
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.
