Duct Effect:

The duct effect is a type of tropospheric propagation which occurs at approximately similar frequencies as bending and scattering. Also termed as ducting, this form of propagation is most general very close to the surface, at times at altitudes of less than 300 m.

The duct forms whenever a layer of cool air becomes sandwiched among two layers of warmer air. This is general all along and near weather fronts in the temperate latitudes. It also occurs often above water surfaces during the daylight hours and over land surfaces at night. The Radio waves can be trapped within the area of cooler air, in much similar way that light waves are trapped within an optical fiber. Ducting frequently permits over-the-horizon communication of outstanding quality over distances of hundreds of kilometers at VHF and UHF.

Figure: The troposphere can bend and disperse radio waves at several frequencies.

PROBLEM:

Assume that you are using a handheld radio transceiver to talk with somebody across the town. You stand on a hill and can view the house where the other person is situated, and the two of you are well in the quoted communications range for the radios. Yet the signal is tremendously weak. You move over only some meters, and the signal acquires strong. What may cause this?

SOLUTION:

The reflected wave and the direct wave from the other radio's antenna take place to appear out of phase at your antenna, therefore they almost cancel each other out. Moving only some meters remedies this, and the signal becomes stronger.