I posted before about this topic in August and was unclear about what had caused the darkened lines on the clouds. Yesterday it was obvious. The shadow even moved from the right of the Sun to the left of the Sun as the clouds blew. But looking at the top picture, the vapour trail must have been much higher than the alto-cumulus layer than it seemed. There was an equal angle subtended between Sun and vapour trail and vapour trail and shadow. It was about 5 degrees.
