Editing Anomalous propagation (section)

== Radar ==

The position of the radar echoes depend heavily on the standard decrease of temperature hypothesis. However, the real atmosphere can vary greatly from the norm. Anomalous propagation refers to false radar echoes usually observed when calm, stable atmospheric conditions, often associated with super refraction in a [[temperature inversion]], direct the radar beam toward the ground. The processing program will then wrongly place the return echoes at the height and distance it would have been in normal conditions.<ref name="EC">{{Cite web
 |url=http://www.msc-smc.ec.gc.ca/cd/factsheets/weather_radar/index_e.cfm 
 |title=Commons errors in interpreting radar 
 |publisher=[[Environment Canada]] 
 |accessdate=2007-06-23 
 |url-status=dead 
 |archiveurl=https://web.archive.org/web/20060630014639/http://www.msc-smc.ec.gc.ca/cd/factsheets/weather_radar/index_e.cfm 
 |archivedate=2006-06-30 
}}</ref>

This type of false return is relatively easy to spot on a time loop if it is due to night cooling or marine inversion as one sees very strong echoes developing over an area, spreading in size laterally, not moving but varying greatly in intensity with time. After [[sunrise]], the inversion disappears gradually and the area diminishes correspondingly. Inversion of temperature exists too ahead of [[warm front]]s, and around [[thunderstorm]]s' cold pool. Since precipitation exists in those circumstances, the abnormal propagation echoes are then mixed with real rain and/or targets of interest, which make them more difficult to separate.

Anomalous propagation is different from ground [[Clutter (radar)|clutter]], ocean reflections (sea clutter), biological returns from birds and insects, debris, [[Chaff (radar countermeasure)|chaff]], [[sand storm]]s, [[volcanic eruption]] plumes, and other non-precipitation meteorological phenomena. Ground and sea clutters are permanent reflection from fixed areas on the surface with stable reflective characteristics. Biological scatterers gives weak echoes over a large surface. These can vary in size with time but not much in intensity. Debris and chaff are transient and move in height with time. They are all indicating something actually there and either relevant to the radar operator and/or readily explicable and theoretically able to be reproduced.

[[Doppler radar]]s and [[Pulse-Doppler radar]]s are extracting the velocities of the targets. Since anomalous propagation comes from stable targets, it is possible to subtract the reflectivity data having a null speed and clean the radar images. Ground, sea clutter and the energy spike from the sun setting can be distinguished the same way but not other [[Artifact (error)|artifacts]].<ref name="EC"/><ref name=NOAA>{{Cite web
|url=http://www.spc.noaa.gov/coolimg/radssets.htm
|title=Radar-detected Sunsets from Minnesota to Tennessee
|publisher=[[National Weather Service]]
|accessdate=2007-06-23| archiveurl= https://web.archive.org/web/20070706093302/http://www.spc.noaa.gov/coolimg/radssets.htm| archivedate= 6 July 2007 | url-status= live}}</ref> This method is used in most modern radars, including air traffic control and [[weather radar]]s.