I like the amount of fog you have here. It adds drama to the painting. I too agree that it depends on where you are, if you're in the San Francisco Bay Area, you get used to seeing alot of fog like this and even more on a daily basis.
Very nice painting, I think if this matte shot was up on the screen for the typical 4 to 6 seconds, you'd have 90% of the audience fooled into thinking it is all real.
My comments are just thoughts, I wouldn't say these changes were totally necassary, but here's what I would experiment with:
-add more blue to the distant mountains
-add a bit more haze to the left side horizon until some of the land completely merges with the sky (for dramatic effect)
-make sure the distant blues you use here don't get too purple, many photos like this (shot at this time of day) have more cyan-blue, not purple-blue
Keep up the good work!
Chris Stoski | Pirates III Matte Painting Supervisor
Industrial Light and Magic