"Golden Field" oil, 8x10
We painters are cursed with binocular vision. Having two eyes allows us to easily see the spatial relationships of objects. When I paint a line of trees marching off to the horizon, I can easily tell which trees are close and which ones are farther off. (It has to do with parallax, which is the apparent displacement of an object as seen from two different points - your eyes.) But if I try to paint with one eye closed, it's not so easy to judge distance. Don't believe me? Try driving a car with one eye closed.
So here's the problem. Unless you're into painting stereopticon images, a painting represents a single-eye viewpoint. If you paint the scene exactly as you see it with two eyes, you won't be painting a strong enough illusion for the one-eyed viewer.
There are five ways to create the illusion. By overlapping objects, diminishing size, lessening contrast, softening edges, and cooling and muting color, we can make a line of trees recede convincingly. By pushing these effects - making the distant trees even cooler and softer than we see them - the scene will look convincing. You can't simply paint the distance as you see it.
The scene depicted above didn't have anywhere near as much "atmosphere" as the painting shows, but I pushed the illusion to make it work.