Cornices are large, overhanging masses of snow that extend off ridges. They grow under windy conditions, when snow is transported across ridgetops. They are at their weakest while forming during storms, windy periods, or in warm, sunny periods. They are particularly dangerous because they extend into a void, with no ground support beneath them. If they break, they can drop a bus-sized load onto the slope below, which can trigger very large avalanches.
It can sometimes be difficult to tell if a ridge is lined with cornices. To avoid cornice-triggered avalanches, stay well back from the edge of ridges where they might exist. You should also avoid travel on slopes with cornices overhead since you don’t want to be underneath a cornice-triggered avalanche if one releases naturally.