DUCT EFFECT

The duct effect is a form of tropospheric propagation that takes place at approximately the same frequencies as bending and scattering. Also called ducting, this form of propagation is most common very close to the surface, sometimes at altitudes of less than 300 m.
A duct forms when a layer of cool air becomes sandwiched between two layers of warmer air. This is common along and near weather fronts in the temperate latitudes. It also takes place frequently above water surfaces during the daylight hours and over land surfaces at night. Radio waves can be trapped inside the region of cooler air, in much the same way that light waves are trapped inside an optical fiber. Ducting often allows over-thehorizon communication of exceptional quality over distances of hundreds of kilometers at VHF and UHF.