A grey, leaden sky is completely covered by clouds. The photo is looking up, with various bare trees visible on the bottom and lower right. Running in an almost straight line from upper right to lower left is a line in the clouds, looking greatly like a wave coming in to the shore. The clouds to the left of the line are slightly darker than those on the right. Perhaps the warm air coming in over the cold below? This is the setup for our currently-falling frozen rain, anyway. Found it strange.