dl-online - Winter scenery is at its most spectacular when trees, shrubs, fences, anything and everything become covered in a white layer of ice. If the sun is shining, almost any view is calendar quality. There are two very different processes that cause this: hoarfrost and rime. Hoarfrost is when ice crystals grow from high humidity in the air. The ice is actually a deposition of moisture directly from humidity in the air. This leads to the most delicate formations because the ice growth is entirely crystalline.