A layer of troposphere bounded above and below by layers that have different refractive index, which confines and propagates an abnormally high proportion of VHF (very high frequency) and UHF (ultrahigh frequency) radiation. This results in freak long-distance communications and radar pickup ranges.
This is normally caused by an inversion layer. The thickness of such a layer is seldom more than 100 m (300 ft). See also anomalous propagation.