This video is part of my Scenery - Making it Real clinic. In it I show my technique for:
Using an airbrush to paint clouds and distant hills. I like to keep my clouds amorphous, that is with out distinct shapes. This makes them perspective neutral so the look OK even when viewed form off to the side.
Painting distant hills topping them with a tree line or clear cut.
I also discuss the order in which I paint backdrops and do foreground scenery.