Atmospheric refraction

From Glossary of Meteorology

atmospheric refraction

Refraction by an atmosphere (usually Earth's) as a consequence of refractive index gradients resulting from molecular number density gradients arising from pressure, temperature, and possibly water vapor gradients.

Near surfaces on the earth (within a few meters or so), atmospheric refraction of visible and near-visible light usually is dominated by temperature gradients. Although atmospheric refraction used without qualification usually means refraction of electromagnetic waves, it could mean refraction of acoustic waves.
See mirage.