Thank you all for the links and book indication. And for all the suggestion.

I actually use to agitate to get contrasty results. But now I see it is better think on contrast when printing.
But thes scene was challenging.

I always think about Jerry Ueslmann works, to have a library of clouds and add it to the sky when needed. It's is super cool. But It seems to really difficult work to do.
I may try one day rsrs.