Just playing with clouds, and am still not sure if I like this, but I suppose you can all decide for me ;) ! I am having difficulties removing the grain from the darker portions of the clouds, but this is most probably due to my lack of understanding of the Terrragen render engine - there seem to be many different areas that can be used to tweak quality. All crits welcome. Cheers.
Did you boost up the quality of both the atmosphere and the clouds? usually where the grainy comes from.
If you increase the atmosphere quality samples to about 40 then it should fix most of the grain and increase the cloud samples to get rid of more. Cloud samples are directly proportionate to the depth of the cloud though so the thicker your cloud the higher the number of samples required. You may also wish to increase the detail option in full render, if that's what you're using. 0.7s a pretty good number.