Atmospheric refraction is the refraction of light rays passing through Earth's atmosphere, due to variations in the density and temperature which produce corresponding variations in its refractive index. Atmospheric refraction gives rise to a shift in the apparent direction of a celestial object; the effect increases the observed altitude of an object and is greatest (just over half a degree) for objects on the horizon.

 

Unusual density variations close to the surface may produce mirages, shimmer, and other deceptive effects.