...Surface Clues to Snow Instability...
Previous Avalanche Activity:
It
is amazing how often this major clue is overlooked. The annals of avalanche
accidents are packed with incidents where touring parties marched into trouble--right
past obvious recent avalanches. If the snow is sufficiently unstable that
it is releasing of it's own accord, then your weight will definitely be
a potential trigger. I have seen skiers do something interesting and rather aggressive: they ski the flanks of a slab avalanche on the assumption that if this snow was unstable then it would have released with the previous avalanche; if it does slide, the fresh bed surface provides a safe escape route.
This is high stakes poker, but it does show that the skier is trying to think like the avalanche, I guess.
Whoomphing and Collapsing:
Often, the weight of snow and people above a weak layer such as depth hoar or light density snow will cause the weakness to collapse, dropping the snowpack like a book onto a desk. This collapse sounds like a belly flop onto a feather mattress or distant cannon fire. Sometimes, a person will actually be able to see or feel the snowpack drop several inches. Needless to say, if this happens to you on a steep slope check your shorts and expect a quick ride to the bottom.
Shooting Cracks:
If the snow is storing a lot of tension, it may fracture as you walk on
it. These "shooting cracks" that spread out from your toes are not a problem
in a gently sloping meadow, but indicate that steeper slopes are suspect.
The deeper and longer the cracks, the greater the instability.
Hollow Sounds:
The wind can move huge quantities of snow, piling it into pillows and creating slabs. These deposits can be fluffy and soft or so hard you can barely kick your toes into them. Don't be fooled by hard slabs! Although the slab itself is very strong, it may be sitting on top of a weak foundation or a poor bond to lower snow layers. Sometimes a poorly supported hard slab will make a hollow sound like a drum when you walk on it.Snow in the Trees:
Avalanche hazard is greatest during the 24 hours following a storm. The
snowpack will tend to adjust to the weight of new snow with time. If the
trees are still holding snow after a storm, one might surmise that the snow
on the ground hasn't had a chance to settle, either.
Cornices:
Cornices form on the lee side of ridges and indicate that winds have been moving great quantities of snow. You should expect much deeper snow--and consequently, avalanche hazard--below a cornice than on the scoured side of the ridge.Recent Wind-Loading:
Fat-looking pillows of freshly-deposited snow are often obvious and very
sensitive to human triggers. Hazard Ratings Explained ~ Avalanche Tutorial ~ Site Map ~ Forest Service
Send comments, suggestions and field observations to: lsafc@lasal.net
