Or you can fake it like I do. A simple quad with a texture of some clouds where you can see through to the sky colour (background colour of scene) behind and then animate the texture across the quad by modifying the texture coords of the quad. You can put e.g. two layers of these on top to get two cloud layers going (even with different movement directions if that is your fancy). Finally you can then have a stars quad that fades in when night arrives.
The only problem with this is how to get clouds, stars and fancy sunset/sunrise gradients going in the distant horizon. Perhaps you can use a normal skymap for this but would have to be a custom made one where you can do the same animation trick on the textures if you want them to animate as well. I havent solved these problems yet as I am still using the fog filter that “fogs” out everything in the horizon anyway. 