Interesting-I also saw this on a day that was absolutely crsytal clear and presumeably very very very cold up at FL300 +
In fact my wife pointed it out because by the time it passed us-near Blackbushe it paralleled a 'normal' contrail and looked so odd.
It definately had the flat sine wave shape and from directly beneath it had a number of puffy circular 'blobs' along its length as tho every so often it expelled extra exhaust. Can jet engines have uneven combustion like that I wouldnt really have thought so without it being very evident?
I was interested to read the post about the B52 trails because that is an unusual aircraft having so many engines and perhaps that explains it but do B52s mix in with the westbound commercial herd or flock heading for the N A tlantic at mid day?
Unfortunately I didnt see this trail until the plane had passed well away to the west because with binocularls it would have ben easy to identify as it was so clear today
PB