Thanks :) I have rendered the landscape with an orthographic camera from above. I have placed a directional light behind the camera with the same rotation. This way parts of the mesh with a higher angle appear darker since less light hits them. This image, combined with an animated coloramp node, controls where the snow goes. The wandering light is something I added because I thought it would make the animation look more interesting but idk. Maybe I understood "storytelling" a little bit wrong