What is this strange cloud?

Just a sugggestion - it might have neen the shadow of a contrail (an aeroplane vapour trail) above the low cloud in the photograph. High altitude winds could have blown it across the field of view in a few minutes.

But then it might be something else.

Hum, I see that you are a sailor ... and so am I ... :-)

I think that what you see are stratocumulus, the result of an old convection (low pressure) that has stabilized. But perhaps that in an earlier form, there was shear winds in the region and that some of the clouds moved across each other, thus forming a thicker layer on the boundary. The darker zone you see is the result of a thicker layer of clouds right above it.

I have sailed many sailboats (among which a 25 ft gaff-rigged Cornish crabber) for nearly half a century and now, I fly an ultralight aicraft. It could be fun then to climb over the clouds and see what actually makes the darker stripe. I don't think it is a contrail, as Za thinks. A contrail is not thick enough to produce such a darker stripe.

Anyway, thanks for sharing; nature is the greatest show on earth, isn't it? I remember reading about Alain Moitessier, the Frenchman who sailed alone one and half time around the earth. After months at sea, he wrote that, that evening, he sat half an hour looking at the sunset. It must have made an impression on him.

Here is how I see now the weather:

http://home.online.no/~michel/Lifjell/

Nice, isn't it?

List of my flying and sailing experiences:

http://home.online.no/~michel/

maximum possibly no longer something alien, for logistical reasons, so I would desire to settle for it at face fee and spot it as merely an unusual, per probability guy-made as has been suggested, cloud formation. It actual sparks the mind's eye nonetheless.