The first assumption of the prediction of propagation of a radio wave is that it is moving through air with temperature that declines at a standard rate with height in the troposphere. This has the effect of slightly bending (refracting) the path toward the Earth, and accounts for an effective range that is slightly greater than the geometric distance to the horizon. Any variation to this stratification of temperatures will modify the path followed by the wave. Changes to the path can be separated into super and under refraction: