Here is a render where I was using the 3rd sphere for the towering part of the cumulus. It gives you more freedom over the shape . What I tried also, was to make the towering part relatively tight, while the base is soft and feathery. To make the shape of the cumulus base more interesting I added turbulence (This can easily be achieved by connecting a Turbulence node to the Origin parameter of the Gaussian Tooth node).

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