Atmospheric ducting
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Atmospheric ducting, in communications and radar, is a type of anomalous radio wave propagation, similar to a mirage in light propagation. It causes long distance propagation of radio bands that would normally be limited to line of sight.
Normally radio "ground waves" propagate along the surface as creeping waves. That is, they are only diffracted around the curvature of the earth. This is one reason that early long distance radio communication used long wavelengths. The best known exception is that HF (3–30 MHz.) waves are reflected by the ionosphere.
However, the Earth's atmosphere becomes thinner with altitude, which tends to bend waves downward. In some weather conditions, such as inversion layers, it becomes thinner so rapidly that waves are guided around the curvature of the earth at constant altitude. This causes mirages and atmospheric ducting.
