The 3D preview (optionally) shows the entire scene, roughly as it will look when rendered (you can turn off various aspects such as atmosphere and lighting, to speed things up when not needed). Clouds are not based on 3D objects, they are a volumetric shading of input noise (or other data) functions. So there is no wireframe to preview. The best thing to do if you want to get a preview of the clouds is enable the Atmosphere component of the 3D preview using the buttons on the top of the preview window (or switch to the Atmosphere layout using the top layout buttons, where atmosphere preview is enabled by default). Atmosphere may take some time to preview if you have a complex setup, of course.
- Oshyan