Anomalous propagation includes different forms of electromagnetic wave propagation that are not encountered in a standard atmosphere. While technically the term includes propagation with larger losses than in standard atmosphere, in practical applications it is most often meant to refer to cases when signal propagates beyond normal radio horizon.
Anomalous propagation can be subdivided as follows:
References: W.L.Patterson, C.P.Hattan, G.E.Lindem, R.A.Paulus, H.V.Hitney, K.D.Anderson, A.E.Barrios. Technical Document 2648. Engineer's Refractive Effects Prediction System (EREPS) Version 3.0. May 1994. San Diego, CA.
Doppler Weather Radar and previous forms of weather radar where applicable:
Anomalous Propagation (AP) refers to false radar echoes usually observed when calm, stable atmospheric conditions, often associated with a temperature inversion, direct the radar beam toward the ground. AP is also caused by other phenomenon such as remnants and after-effects of thunderstorms, ground debris, bird flocks, ocean reflections, and others. The term Anomalous Propagation is often used to mean echoes which are separate from ground clutter, a set of apparent echoes in a circular area near the site which is largely consistent over time. AP, in contrast, can take many forms and can be mistaken for precipitation at first glance. In some rare cases, the redirection of the radar beam towards the ground can be consistent enough over an area to make large-scale features such as rivers and forests distingusihable in the echoes.
AP in the sense of weather radar is colloquially known as "garbish" and ground clutter as "rubbage". The software for many Doppler radar implementations has a mode which can be used to learn and then subsequently cancel ground clutter (rubbage) and other known and predictable phenonena such as apparent radar echoes resulting from the sun setting (see http://www.spc.noaa.gov/coolimg/radssets.htm)
The cyclic expansion and reduction of the ground clutter halo seen at many sites in the evening hours and overnight especially during the warm season at many sites primarily with Doppler radar systems could be considered to be one, the other, or both as it does reflect a change in atmospheric conditions, but it is not precipitation.
Radar echoes from dust and sand storms, insect swarms, volcanic eruption plumes, and other non-precipitation meteorological phenomena as well as manmade conditions such as release of chaff, and the falling debris cloud from the Columbia space shuttle breakup are not considered AP as they are indicating something actually there and either relevant to the forecaster and/or readily explicable and theoretically able to be reproduced.