It doesn't have to be a cloud covering a snow covered ridge.
I was flying Puma across Norway in January. My destination was the other side of a ridge that I could see clearly in the distance on a gin clear day. Cruising along and there was a sudden squawk from the radalt and the side of a rock wizzed by my right ear. It was then, because it was now in shadow, that I saw this ridge I had just flown over without seeing it because it was invisible against an identical background.