Again, about volumetric clouds (or "spectral" clouds, Vue has two categories of volumetric clouds). Just a warning: some of these clouds are really slooooow to render. I'm talking of hours, not days, anyway. It really depends a lot on the noise function that you use to create them.

Those clouds in that image are the so called "metaclouds", that you can manipulate individually. There are other types of clouds, which are more random and can cover all the sky, editable together with the whole atmosphere but with less control over specific locations where you want them to be.