5.1.5 Friction

Friction inhibits the movement of the snowpack at its interface with the ground and is always directed parallel to the slope. However, its orientation is directly opposite that of the slope-parallel gravitational component. Frictional resistance increases as surface roughness increases.

Orientation of the friction force, F, on the snowpack.

Friction is the primary factor that lets snowpack build up on sloped surfaces, rather than just sliding downslope. To illustrate this, imagine a plane of glass with a coffee mug on it. You don't have to tilt the glass much to get the mug to slide off. The glass is very smooth and produces little frictional resistance. If you change the glass to a sandstone surface, you'll have to tilt it much higher to get the same effect. This is due to the rough surface of the sandstone, which inhibits objects resting on it from moving due to its high frictional force.