As I have it, snow appears in a scene for one of two reasons: there is some volume accumulated on the ground, or a layer of snow covers other features like rocks.
When it accumulates it becomes a feature of its own. It usually forms waves and dunes. I found it can be recreated as a 3D volume layer using some form of multi-fractal ridged noise. To this volume I apply a snow texture.
When it appears on other features like rocks and boulders, I model it as a set of additional texture layers for the rock or boulder material. This means that every material needs to know how it will look when covered by snow. I will post about the material system in the future. As you will see, adding snow takes little effort.
Here are some screenshots showing the latest results I've got:
The three screenshots were taken out of the same mountains.
I found not so hard to re-create areas that were completely covered by snow. The tricky part was the transition zones, where snow gradually disappears and the ground starts to show again. The last screenshot shows a transition area. I still need to work on them.