A mass of snow deposited by the wind, often overhanging, and usually near a sharp terrain break such as a ridge. Cornices can break off unexpectedly and should be approached with caution.
Cornice Fall Avalanches:
Cornices are the fatal attraction of the mountains, their beauty matched only by their danger. Cornices are elegant, cantilevered snow structures formed by wind drifting snow onto the downwind side of an obstacle such as a ridgeline. Similar to icefall avalanches, the weight of a falling cornice often triggers an avalanche on the slope below, or the cornice breaks into hundreds of pieces and forms its own avalanche—or both. Be aware that cornice fragments often "fan out" as they travel downhill, traveling more than 30 degrees off of the fall line. Cornices tend to become unstable during storms, especially with wind, or during times of rapid warming or prolonged melting. Each time the wind blows, it extends the cornice outward, thus, the fresh, tender and easily-triggered part of the cornice usually rests precariously near the edge while the hard, more stable section usually forms the root.
Similar to icefall avalanches, cornice fall avalanches don’t kill very many people. And similar to slab avalanches, the ones who get into trouble almost always trigger the avalanche, in this case, by traveling too close to the edge of the cornice. Cornices have a very nasty habit of breaking farther back than you expect. I have personally had three very close calls with cornices and I can attest that you need to treat them with an extra-large dose of respect. NEVER walk up to the edge of a drop off without first checking it out from a safe place. Many people get killed this way. It's kind of like standing on the roof of a tall, rickety building and walking out to the edge for a better view. Sometimes the edge is made of concrete but sometimes the edge is made of plywood cantilevered out over nothing but air. It feels solid until, zoom, down you go. Check it out first.
But cornices aren't all bad. You can use cornices to your advantage by intentionally triggering a cornice to test the stability of the slope below or to intentionally create an avalanche to provide an escape route off of a ridge.
Squeamish folks or lay-people might think cornice tests are dangerous but they have been standard techniques among ski patrollers, helicopter ski guides and especially climbers for decades. Cornices are the "bombs of the backcountry." First, make sure no one is below you--very important. Next, simply find a cornice that weighs significantly more than a person and knock it down the slope. A cornice the size of a refrigerator or a small car bouncing down a slope provides an excellent stability test. The smaller the cornice, the less effective the test. You can kick the cornice, shovel it or best of all, cut it with a snow saw which mounts on the end of a ski pole. With larger cornices you can use a parachute cord with knots tied in it every foot or so, which acts like teeth on a saw. Throw the cord over the cornice or push it over the edge with an avalanche probe. You can saw off a fairly large cornice in under 5 minutes. It's best to work with small, fresh cornices and not the large, old and hard ones. You can also trundle heavy rocks down the slope, which work just as well as cornices, but they’re often harder to find. This is also a great way to create a safe descent route during very unstable conditions. In other words, make an avalanche and use the slide path to descend.
It doesn't take much imagination to see that knocking cornices down avalanche paths can be very dangerous. ALWAYS use a belay rope on slopes with bad consequences and practice your cornice techniques on safe slopes until you get the techniques worked out. Cornices have a nasty habit of breaking farther back than you think they should. Be careful.