In
telecommunication, an
atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical
refractive index gradients are such that
radio signals (and light rays) are guided or ducted, tend to follow the curvature of the Earth, and experience less
attenuation in the ducts than they would if the ducts were not present. The duct acts as an atmospheric
dielectric waveguide and limits the spread of the
wavefront to only the horizontal dimension.