Visible shock wave
Here is a reduced GIF of a photograph taken through the window of a 747
at cruising altitude over the atlantic. Running vertically down the
center of the
photo is a curvy gray stripe. This stripe is the shadow of some kind of
pressure phenomena occurring just above the wing. The stripe of shadow
slowly moved around over many minutes. Occasionally it would wiggle,
presumably as the wing cut through turbulent air. Besides this one
strip of shadow, there were
OTHERS (100K) in various places on the wing.
They were only visible in direct sunlight.
I've heard this phenomena described as shockwaves that attach themselves
to the places on wing surfaces where the airflow goes transonic. But they
might also be the place where the airflow detaches from the wing surface,
i.e. a transition from normal laminar flow to a turbulent "stalled" area
on the wing.
Here's some email discussion about it.
And a website about shocks attached to wings
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