The Physics of Avalanches
Cornices

Formation
Triggering

Slabs

Bibliography

Cornices form when wind picks up snow and deposits it on the other side of a ridge. This is because of air currents that cornices are able to form. As wind flows over the top of a ridge and it creates eddies that deposit snow. When this snow is of proper density and has the right crystaline structure it adheres with itself to form a cornice. The snow interlaces and forms a large hanging snow depository. You can thing of a mountain like the tail edge of a wing on a plane. Only this wing is tilted up. The wind hits the wing and flows over the top creating a vortex on the back side. This is what would be considered drag on an airplane. But for our purposes it is what causes the snow to be trapped and pushed back on to the mountain to form a cornice. This physical concept comes from the Bernoulli principle. When the angle of attack of an airfoil becomes too great a large amount of turbulent airflow or vortexes are formed.

http://airplanegroundschools.com/Weather/Theory/Figure%2010-16%20Turbulence%20encountered%20over%20and%20around%20mountainous%20regions.JPG