Indeed, I don't think they'd necessarily need functions though. Just tweaking of the scales a bit. a low smallest scale would be in order, something around 0.04 with a lead-in scale somewhere between 35k and 50k. In either Perlin Mix 2 or billows. Maybe using the redirect trick that....um.. someone recently posted about. And some noise variation, in particular the bouyancy and clumping of variation. Those last two are probably the most important settings actually.
