ANOMALOUS PROPAGATION

In Radio: Anomalous propagation consists of different forms of propagation that are not normal. Example: Tropospheric ducting or Sporadic E-SkipDoppler Weather Radar: Anomalous propagation refers to false radar echoes usually observed when calm, stable atmospheric conditions direct the radar beam toward the ground. Also caused by other phenomenon such as thunderstorms, ground debris, bird flocks, ocean reflections, and others.
See more at Wikipedia.org...

Abnormal propagation caused by fluctuations in the properties (such as density and refractive index) of the propagation medium. (188 ) Note: AP may result in the reception of signals well beyond the distances usually expected.