This is related to the previous three images I posted of star trails. On this particular night as I started to shoot low clouds came in and pretty much ruined star shots, but I saw something I have never seen before -- the shadows of trees on clouds!

While shooting this was barely visible, but when viewed as a sequence of photos sped up it was pretty clear what was going on. The image here is actually a composite of three individual images to better show the effect (the movement of the clouds shows more of the tree shadows).