In mountaineering a cornice is an overhanging ledge of snow on the edge of a ridge, cliff, or crest of a mountain. Cornices form by wind blowing snow over the leeward edge in successive layers. They can be very dangerous from traveling above and below them. A person or skier putting weight on top of a cornice can cause it to break off and collapse and causing a major fall, with no ground beneath, just snow or ice.
In mountaineering the best practice is to avoid going onto the cornice by staying far enough back from the edge. This is hard to do psychologically since most climbers who reach the top like to look over the edge to get a better view. The other reason is it’s hard to judge or know where the true break point of the cornice is. One test is to stick an ice axe or probe through the snow to see if it goes through or you can see lighter shade of ice (from light coming through). This is still risky since many times it means you’re already on the cornice. A safer method is to view the cornice from a different (safe) angle and not venture beyond the break point. Realize this can change year to year. An experienced climber (Joe Bohlig) on Mt. St. Helens who had climbed it 68 times and knew the cornice hazard died in 2010 when he stepped out too far and the cornice gave, causing
him to fall 1500 feet.
Even standing on rock or dirt at the top near a cornice break can expose you to risk and cause you to fall when the snow break line extends over the rock.
Another danger of cornices when traveling below them is the risk of getting caught in an avalanche set off by a falling cornice. Cornices are particularly vulnerable to collapse during warmer weather or times of solar warming.
Here's some pictures to help you identify cornice hazards:
This one is my friend Joel who we roped up with double anchors to test the Cornice. Don't try this unless you know what your doing.
Cornice On Mount Hood
Cornice on Mt. St. Helens.
This last photo is of a fatal cornice break on Feb 2, 2011 near Snoqualmie Pass WA, where the victim took off her skis and pack after climbing to the top. Foot prints on left side of photo show her walking towards the section where it broke, plunging her 1500 feet.