In geophysics, a layer of the troposphere in which refractive properties are such as to trap a large proportion of certain high frequency radiations: An atmospheric duct is a stratum of the troposphere within which the refractive index varies so as to confine, within the limits of the stratum, the propagation of an abnormally large proportion of any radiation of sufficiently high frequency, as in a mirage.

Atmospheric ducting is a mode of propagation of electromagnetic radiation, usually in the lower layers of the Earth’s atmosphere, where the waves are bent by atmospheric refraction.