It is said that Rembrandt coined the phrase, "without atmosphere, a painting is nothing." As far as art goes, no truer words have ever been spoken, and no other art-related quote has stuck with me more. Even in a portrait painting you can have a hint of it in the few inches from the tip of the nose to the ears.
But landscapes sing when atmosphere is included. Nothing makes your center of interest pop more than muted grays of distant mountains, trees or just thick air of hot Texas summers.