Atmospheric duct

The duct acts as an atmospheric dielectric waveguide and limits the spread of the wavefront to only the horizontal dimension.

It also causes long-distance propagation of radio signals in bands that would normally be limited to line of sight.

This is one reason that early long-distance radio communication used long wavelengths.

In some weather conditions, such as inversion layers, density changes so rapidly that waves are guided around the curvature of the earth at constant altitude.

Phenomena of atmospheric optics related to atmospheric ducting include the green flash, Fata Morgana, superior mirage, mock mirage of astronomical objects and the Novaya Zemlya effect.