"Jose" wrote in message
...
[...]
It looked to me as if it were the path of the wind coming down the valley
in the manner of a stream.

It might have been. Take my comments with a grain of salt, since I'm no
expert on low-level wind structure or cloud formations. However, I'll note
what I do know.

Oh, and also keep in mind that I'm having a very hard time seeing with
certainty the actual 3D structure of the clouds in the picture. In
particular, I can't tell if the curve in the cloud is a change in elevation,
or is a lateral curve. I'm guessing the former, but can't rule out the
latter from the picture alone. It's also hard to see how the cloud is
positioned relative to the terrain underneath.

On to guessing...

While we talk about "the jet stream", there are in fact several
high-altitude jet streams, and countless smaller ones at low levels. The
cloud in your picture looks *to me* as though it could be some isolated jet
stream pulling moisture through the air. Over the hilly areas, orographic
lifting causes extra condensation, making for a fuller cloud formation,
while over the valley things thin out.

Alternatively, perhaps there's a bit of an inversion going on (not uncommon
in connection with hazy, low-stratus conditions as seen in the photo), in
which the winds aloft are picking up a bit of the extra moisture evident in
the larger part of the cloud, and carrying it as a thin "smear" downwind
from that area.

In both cases note that the winds you experienced in the airplane may or may
not be the same as in the area where the cloud is.

Ah well...as you can see, I don't really know. I have to admit, this is one
of the things I enjoy about flying, and especially of having an instrument
rating. It's not uncommon to run across strange cloud formations that are
beautiful and enigmatic at the same time. You may not get a confirmed
answer here, but at least you were able to share one such formation.
Thanks!

Pete