To paint outdoors is to be influenced by nature and nature, as it turns out, is full of physics. One of those great little tricks of physics is atmosphere. We all know that atmosphere is just many, many feet of little particulates in the air that interfere with the light and shadows from distant objects but somehow that becomes very hard to translate into 2 dimensional notes of solid color. I think it's just the way we think about it. Personally, I believe if, at first, we train ourselves to paint what we see and not what we know then we can forget the physics of it and put down all the notes correctly until they turn into a painting.
Here are two examples of implied distance through atmospheric perspective. It's just paint, not atmosphere. Hah hah!... You see? I tricked you! Actually one is pastel and the other is oil. One of the things that suggests that something is farther away from something else is the comparative change in color/value. Like, the shadow of the tree in the distance is cooler, grayer, lighter than the shadow of similar trees in the foreground. Pourquoi? All of those little particles between you and the thing in the distance are interfering with the shadows. The light stuff in the distance reacts a little differently. Shadows= absence of light but the light on a distant tree is a collection of photons being bounced at you in a certain spectral range. Light makes it through, absence of light, not so much.
Think of it as a whole bunch of clear shower curtains between you and that thing over there. The color in the dark shapes is interfered with by the layers of light bouncing off the shower curtains but those cute little photons still make it through, though grayed down. I sampled colors from the above two paintings to get the gradations in these two examples. If all of this gets too confusing to think about, go back to painting the notes of color that you see. Just compare what is going on in the foreground to what is going on in the background. Oh and then there's that glazing trick with liquin and white and a tint that I've mentioned in previous posts (see Carmel redux), it really mimmicks atmosphere but requires a dried painting to do.
For my money there's no one better than Steve Bach for this. He's a master of moody, atmospheric landscapes. I put up a link to his site at right but it's http://www.stephenbach.com/ just in case.