Snow Cornice
From HydroWiki
General information:
A cornice is a snowdrift feature that forms along a break in slope, typically along ridgelines in exposed mountain areas. These can range from less than one meter to tens of meters in width and depth and from a few meters to more than a kilometer in length. Cornices can present serious hazards to mountain travelers including climbers, skiers, hikers, and snowmobilers.
Formation:
Wind blowing up a mountain slope can transport a mass of snow through several means, including: suspension, surface creep (rolling and sliding), as well as saltation (bouncing or skipping). As the wind passes over a rapid change in slope, such as a ridgeline, the air diverges and the velocity is reduced. This reduction in velocity reduces the transport capacity of the wind and so a portion of the drifting snow is deposited, thus forming the cornice. The cornice grows in the lee direction of the prevailing winds and overhangs the underlying slope. The size and shape of these features is dependent on the local geography and vegetative cover of the windward slopes, as well as the snow depth, density, and cohesiveness, combined with the wind speed, direction, and duration.
Hazards:
Cornices are inherently unstable and can be hazardous to those traveling on them as well as below them. A broad cornice can hide the true location of the underlying ridgeline and the unaware traveler can unknowingly be walking on an unsupported cornice hanging over a steep slope. The two main hazards of this situation are breaking through the cornice and falling unto the underlying slope and, more commonly, triggering a cornice failure and falling into the ensuing avalanche. Anyone on a slope beneath a cornice is at risk of avalanches, even if the slope itself appears to be stable. A cornice that is being actively wind-loaded may exceed its strength threshold and collapse unexpectedly. A change in wind direction or rise in temperature may also destabilize a cornice leading to a sudden collapse.
Safety:
When traveling in avalanche terrain, always stay downslope and upwind of ridgelines to avoid walking on top of a cornice. Never traverse or climb a slope that has a visible cornice above it. Always practice avalanche safety measures, bring appropriate equipment, and take an avalanche safety course.
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