You understood me wrong.I am not looking for the brushes or a way to build them, just for a way to use them in a realistic manner.There was a tutorial on how to use these brushes.How to combine the colors and transparencies.And there were other things in those tutorials.I know for sure one of them was about cloud brushes (not about how to create them, but how to use them creatively once you have them).That tutorial was using brushes of real clouds.That is what I call a "stamp" brush.A brush made from a photo.

And that tutorial was explaining how to use such brushes so the "stamped" clouds would look like real...