Have a look at the following animation - it's a technical test for volumetric clouds in Cinema 4D.
The setup for this scene is pretty easy. You basically need the following three things:
1. Create a simple 3D volume for the clouds (just using a magnet on a cube is fine, a very rough shape is good enough)
2. Fill the volume with particles (using thinking particles)
3. Add a pyrocluster material to your particles.
That's all there's to it! Of course, the final look heavily relies on how well you manage to tweak the settings. Your 3D volume should have believable cloud-shapes, your particlecount should be correct, and most of all, your pyroclustermaterial requires some tweaking to make it "cloudy" (and remember: lighting and texturing are not two separate things, they go hand in hand). Still, it's not a massive task, I managed to create the entire scene in one evening (okay, and a fair portion of the night - I always lose track of time when tweaking/rendering).
Rendertimes are in the region of 2 minutes per frame.
Keep me posted if you try to create a cloudy scene yourself!