A few more notes on what I used. I'm not at all sure what my final advice would be, but if you're following along you may want to play.

I used the fBm node - it seems to be able to do certain features that none of the others can do.

I modulate its "Bottom" value using the inverse of the blob. Watch out for Bottom=1. When set to 1, this makes it go totally white instead of the desired totally black.

In matmatic parlance:

blob = ImageMap(...).asNumber()

cloudBottom = .999 * (1 - blob)

pattern = FBM(..., Bottom = cloudBottom)

Note I am not at my Poser PC and may have spelled Bottom incorrectly (might be bottom - lower case). 

I used the FBM as the transparency map (actually an opacity map).

Note that I get the positional modulation by plugging coordinates into the x,y, and z scale parameters of the FBM. This disconnects its internal use of model coordinates.

By using the P node, I get world coordinates to drive the FBM.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)