for fly through clouds you can use PaintFx ....
press 8 to go to Paintfx mode hold space and
choose Canvas->paint scene
Then from paintfx tool "options" make sure that
"Paint at depth " is checked on.
Use the Visor to choose a cloud brush, adjusts the brush size
switch to perspective viewport and you "paint in
depth ..." . You can fly through and have "real"
clouds around you... Warning... Paint fx is bad for the cpu load...
use with wizdom and only when necessary... otherwise i suggerst imageplane backgrounds too....

Konstantinos