The propagation of sound or radio waves such that they are detectable over abnormally large distances, beyond those reached by the usual surface waves. Such conditions arise when there is exceptionally rapid decrease in the atmosphere's refractive index with height (known as super-refraction), causing the waves to be refracted back towards the Earth's surface well beyond the normal limit of reception. Sounds become audible beyond a zone of silence, and the range of radio waves (including radar) is extended. Under extreme conditions, when there is a strong temperature inversion or major decrease in atmospheric density in a relatively thin layer fairly close to the surface, a radio duct may be formed, channelling ( ducting) radio and television transmissions to great distances. This often gives rise to what broadcasters call co-channel interference. Such conditions frequently occur in an anticyclone when there is subsidence from higher levels.
Subjects: Meteorology and Climatology.