Fly long enough and you'll see a river covered with fog and
you can see the fog flowing down stream with ripples just
like it was the water. My guess in the picture you posted,
there was a front with wet and dry air, moderate to strong
winds and terrain effects.
Lenticular clouds, roll clouds, wall clouds, all are
effected by or caused by the terrain.

I've seen that, but this wasn't that. There was a river down there, but
there was no front that I recall, and the winds were light (eight knots
from the south - a tailwind). This cloud was maybe a thousand feet
above the valley, and curved smoothly around.

It looked to me as if it were the path of the wind coming down the
valley in the manner of a stream.

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
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