I like the 3 point one the best .
Have you picked up the tutorial on idrawgirls yet?
Anyway, I'm not sure what's above the horizon line in the first one (Top left), I'm guessing dark sky/clouds?
In anycase, just remember that the further objects are away, the more whiter they will be, whereas the closer they are, they will be darker. It's good that you're working in B/W first.
What I normally do is take a 100% soft brush in photoshop and make a new overlay layer (Or normal, up to you), and softly dab the horizon and anything in the distance with the airbrush. Conversely, I use a a combination of hard and soft brushes for the objects that are closer to the viewer. Overlay layer as well.
Nice work keep at it
(No need to visit my SB atm, nothing new. Trying to solve a problem I'm having in Zbrush with damaged meshes before I do any more sculpting)