We saw these really impressive cloud formations over the Cairngorms when we were driving back from Inverness. I think they are likely to be a huge version of cap cloud. There was a lot of snow on the tops of the mountains making it far more likely that the water vapour condense out into cloud at that level. Then winds blow the top flat. I have seen it before in the Lake District where the cloud evaporates away as soon as it leaves the cooling effect of the top, so the formation never seems to move. Hence cap cloud.
