1. The refraction of light passing through the earth's atmosphere, including both astronomical refraction and terrestrial refraction.
2. An apparent upward displacement of celestial objects relative to the horizon as light from them is bent toward the vertical by the decreasing density with altitude of the earth's atmosphere.

It is greatest for objects on the horizon and negligible at elevations higher than about 45 degrees.

3. The angular difference between the apparent zenith distance of a celestial body and its true zenith distance, produced by refraction effects as the light from the body penetrates the atmosphere.

Any refraction caused by the atmosphere's normal decrease in density with height.

Near surfaces on the earth, those within a few meters or so, are usually dominated by temperature gradients.