The render time of my second image was way too long, and I'll have to go back and optimise it before trying again. In the meantime, I have done my first big size Terragen render, deciding to do something a bit less time consuming. Just shy of 24 hours later I had a 4000x8000 picture that is quite simple, but I think I like it.
I have also made a few other scenes that I intend to render soon. My computer will probably be working continuously day and night now.
I would increase the edge sharpness of the clouds. Not drastically, but just enough to give the cloud tops more definition.
This is a good scene. You can always render while you sleep and/or are at work. Pause the render and the preview update if you want to do something else.
I might try that, although I think I am quite happy with the visual quality of the clouds as they are now.