Shadows in the sky

 In the picture above, there is a thin dark band stretching down and right from the edge of the cloud. It is a shadow of the cloud as the Sun was away over to the left at the correct angle. Consider the photograph below. The shadow appears on the solid surface. There is no evident shadow visible in the air between us and the concrete. I have seen higher clouds project shadows onto lower levels so it suggests that tenuous water vapour in the air above the Solway might be substantive enough to scatter light from the Sun back to us, making it obvious where there is no light getting through to be scattered. It's like seeing a shadow projected onto smoke.