Variation of Optical density

How does the optical density of air vary with height. I know about variation of Optical density within smaller heights like that observed in phenomenon of Mirage and looming but when we are concerned about light coming from stars, is there a specific pattern for change in optical density.
Like when we talk about Mirage we say light goes from denser medium in upper air to rarer medium of lower air. So is there something like this when we talk about light coming from stars and sun.