To get the cloud material, creat a new master material, map the diffuse map, transmission map and edge transmission map in the corresponding tabs. Now set the luminance tab at 0.5, white color and set the transmission tab at 1, white color.

Diffuse Map

Transmission Map

Edge Transmission Map

Now map your material on a very flat hemisphere.

You then duplicate the hemisphere twice so that you have three of them which you slightly move away from each other in the "y" direction.

To create the sun, you can use a parallel light with a light flare for which you check "enable background" and "enable offscreen". If you want, you can activate the shadow.

Now you just have to add the ground, a bit of fog and a sky backdrop.

If you want to animate the entire sky, just move the three aFraktal shaders in the same proportions.