radio duct[′rād·ē·ō ‚dəkt]
An atmospheric layer, typically shallow and almost horizontal, in which radio waves propagate in an anomalous fashion; ducts occur when, due to sharp inversions of temperature or humidity, the vertical gradient of the radio index of refraction exceeds a critical value.
The layer within which the state of superrefraction exists. The meteorological requirements for superrefraction conditions are the presence of a temperature inversion and the rapid decrease of height. Ducting seldom occurs below 1000 MHz. For the anomalous propagation of radio waves, it is necessary that the duct exist over the whole range of propagation (i.e., both at the transmitter and the receiver). See duct propagation.