My initial masking wasn't really correct, it just looked like it was doing the right thing. I went back to the drawing board to nut out a real subtractive image mask. The first problem was to get around the norml cloud distribution outside of the image mask:
- Combine local image mask with a global white shader
- Subtract combined mask from cloud density fractal
In theory this should have been enough, however in practise I found that there was enough noise in the area outside of the local image mask to put scattered clouds over the rest of the terrain. For the global white shader I tried both a white image mask and a white shader, both with the same result. To get around this I repeated the image mask subtraction.
Here is a clip file of the cloud mask. Edit the image mask with your own image.
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I then built up multiple cloud layers with this clip file, tidying it up by using just the one global white shader.
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The cloud settings still need work but it's an improvement.
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Looking more closely at the image I was basing my masks on, it was obvious that some manual editing was necessary to overcome some dense shadows on some of the taller, billowing cumulus clouds. It's possibly easier to use lower resolution mask images and simulate the rest with the cloud settings (which is a good thing as it saves your RAM for surface image masks)
