Backcountry Snow
Cornices: the feature that breaks further back than you think
A cornice is a chunk of snow held out over nothing by cohesion alone. People walk on them, ski under them, and are surprised by both outcomes.
Cornices form where wind carries snow across a ridge and deposits it on the lee side, building an overhanging lip that extends out beyond the underlying rock or ground. They are structurally impressive and they are held together by nothing but the cohesion of wind-packed snow.
Two hazards, not one
Being on top when it fails. A cornice collapse takes the person standing on it down whatever is below, which by definition is a steep slope or a cliff.
Being underneath when it fails. A falling cornice is a large mass of dense snow arriving at speed, and it is also a very effective avalanche trigger — the impact loads the slope below abruptly and can release a slab that would not otherwise have gone.
The second is more common as a cause of accidents, because far more people travel under cornices than on them.
The fracture line is behind the lip
This is the crucial fact and the one people get wrong. A cornice does not break at the visible edge. It breaks along a line well back from the lip, sometimes many metres back, on ground that from above looks like solid ridge.
Because the cornice is continuous with the snow on the ridge crest, there is no visible boundary from above. You cannot tell by looking where the supported ground ends and the overhang begins.
Practically: on any corniced ridge, stay well back from the edge. Much further than feels necessary. If you need to look over, do it from a spot where you can see the profile from the side, or from a position where you are belayed.
Where a group must travel a corniced ridge, staying on the windward side, well back, is the standard approach — and it is worth marking the safe line for the return, because visibility can drop and a track close to the edge in a whiteout is a serious hazard.
When they fall
Cornices fail most readily during and shortly after rapid warming — strong sun on the lee face, a rise in air temperature, or rain. Spring afternoons are the classic time.
They also fail during and after heavy loading, when new snow and wind build them beyond what the existing structure supports.
And they fail with no obvious trigger at all, which is the part that matters for route planning. Being under a cornice is a risk you accept for as long as you are there, and the only reliable mitigation is time — minimise it.
Route planning around them
Look at the ridge line on the map and in the field, and identify where cornices are likely: the lee side of prevailing winds, above bowls and gullies, along ridge crests.
Then plan not to spend time beneath them. If a route must pass under a corniced ridge, cross quickly, one at a time, early in the day when it is cold, and do not regroup underneath.
Do not stop for lunch on a bench below a corniced face, which is exactly the kind of sheltered, sunny, pleasant spot that groups naturally choose.
Getting through one
Sometimes the descent requires breaching a cornice to get from a ridge onto a face. This is a genuine technical problem.
Options include finding a gap or a low point where the cornice is thin or absent, cutting a channel through with a shovel from a belayed position, or dropping in from a spot where the profile can be assessed from the side.
Belaying someone over a cornice is standard in guiding, and it is not something to improvise for the first time on a committing descent. If the only way down involves a cornice you are not equipped to deal with, that is a route-planning failure to be resolved by going back the way you came.
Deliberate cornice testing
Guides and forecasters sometimes drop cornice blocks deliberately onto a slope to test its stability. It is an effective test — a cornice chunk is a large, realistic load — and it is genuinely dangerous to perform.
It requires being on the ridge in a secure, belayed position, absolute certainty that nobody is below, and a clear understanding of what happens to you if the ridge you are standing on goes with it. This is professional practice, not a recreational technique.
The rule that covers most of it
Treat the corniced side of any ridge as ground that does not exist, and treat everything beneath a cornice as terrain with an unpredictable object suspended above it.
Both are conservative. Both are cheap. And cornice accidents, when they happen, tend to be severe, because the failure is sudden and the terrain beneath is by definition steep.
