When I started walking just before dawn, this band of low cloud was to the east. As I walked eastward, I found myself under the cloud. Then as I walked back, the cloud was in the same place, to the east of me again, and I was in the sunshine, as above. The cloud seemed to be gently advancing towards me, east to west, moving sluggishly all the time. I guess it was one of those dynamic equilibrium things — the cloud was constantly dissipating as it blew westwards down the lee slopes of the Pennines.
Also after seeing the internet, I realised I can’t distinguish a Balwen from a Zwartbles. Whenever I tell Janet what I’ve seen on a walk I shall now have to say ‘Balwen/Zwartbles’.