modified refractive index

In radio engineering, the refractive index of air on a hypothetical flat earth, given by

where z is the height above the surface, R is the earth’s radius, and n is the actual refractive index of air.
For a spherical earth with an atmosphere having a uniform refractive index, rays are curved relative to the earth. This is approximately equivalent to a flat earth with an atmosphere in which the refractive index varies with height as z/R.
See modified refractivity.