If you're animating your airplane through the clouds, a bazillion volumetric clouds is going to look best.  If not, then faking clouds with a shader -- probably best a photo, maybe from NASA.gov or something like that, edited to have an alpha channel or just basic transparency where the clouds aren't -- will do the job just fine.

I don't have the answer for the terrain thing; I'll be interested to know what Brycers will suggest.

 - Dex