I was intrigued when one plane came over leaving a contrail which looked like the white line dashes down the middle of a UK road. Camera wouldn't focus and it began to break up. But then another came past very soon afterwards and did exactly the same thing - it's the one on the left in the picture. I can only imagine that there are bands of slightly different humidity up there. It was a windy day, the sort when stationary waves form in the air over the mountains. The usual sign is lenticular cloud. Maybe it was such a day but with air humidity too low for cloud formation. Then the aeroplane exhausts might seed condensation in the more humid parts.
