Dave:

Yes, it's obstruction ridge or wave. The difference is whether it's
on the windward or leeward side of the cloud. The obvious sympton of
wave is laminar flow, which I've flown on the leeward side of large
cloud formations.

Gary: Nice video. Too bad you don't have any MK IV yaw strings --
they look so much more professional than the scotch tape and yarn
variety. Maybe Santa will bring you one!

Fred