There is another thread:
http://forums.planetside.co.uk/index.php?topic=1619.0 (http://forums.planetside.co.uk/index.php?topic=1619.0)
Which shows several examples of banded patches of cloud that were produced using Art Matic Voyager. Along with the question "Curious if anyone has attempted clouds such as these". I already posted a test render of my version of Mackerel Sky over there.
After two and a bit days on my slightly underpowered laptop here is a bigger, better quality version.
I have also included a clip file for the clouds, a couple of merged density fractals, one for the small clouds and another for the patches. There are also a few "Fudge Factor" functions that control the relative density. The seed values have been changed so you will get similar but different results.
I was making this up as I went along so there is probably a more efficient way to organise things. I have developed a function based distribution that more closely reproduces the banding associated with Mackerel Sky. This is fine when applied as a displacement but as soon as I try it as a cloud density TG just grindes to a halt.
Ooh, it's coming.