Clouds movements in the skybox are definitely dynamic on Alps. I did a quick comparison as the car passes the pedestrian (?) bridge on each lap, and the cirrus clouds have shifted.
Edit: Hot damn, Prague in the darker haze looks beautiful.
As I said in the other thread, I think the skyboxes are videos. High resolution 360º videos. Every other element of the game pass through a contour mask and is superimposed over the video. That's why if you enable MSAA, a black line of artifacts appear on top of the mountains. They can "play" the sky video or "stop" it in pre-defined turns so it gives the impression of dynamic clouds.
The problem with this solution is that you are always static in the same point under the sky. I mean, you will never move forward with respect to the clouds that you have above. When you know it and you look at the sky, it produces a strange sensation of being driving without moving. The sky is a big screen playing a video recorded with a static cam.