My only criticism would be where the light clouds meet the light grey clouds. Something looks a little off to me, and I can't quite place it. I'm thinking if the grey clouds are in front of the white clouds they should be rounded on top, instead it looks like they are "cropped" tops. Maybe I'm just going crazy, but I'm sure you won't mind constructive suggestions.
Anyway, that's just me nitpicking, because it really looks brilliant.