Editing Anomalous propagation (section)

== Causes ==

=== Air temperature profile ===
[[File:Superrefraction fr.jpg|thumb|Super refraction in radar operation]]
The first assumption of the prediction of propagation of a radio wave is that it is moving through air with temperature that declines at a standard rate with height in the [[troposphere]]. This has the effect of slightly bending (refracting) the path toward the Earth, and accounts for an effective range that is slightly greater than the geometric distance to the horizon. Any variation to this stratification of temperatures will modify the path followed by the wave.<ref name=WMO/>  Changes to the path can be separated into super and under [[refraction]]:<ref name=Patterson>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</ref>

==== Super refraction ====
{{redirect|Super refraction|the music album|Super Refraction}}

It is very common to have [[temperature inversion]]s forming near the ground, for instance air cooling at night while remaining warm aloft. This happens equally aloft when a warm and dry airmass overrides a cooler one, like in the [[Subsidence (atmosphere)|subsidence]] aloft cause by a high pressure intensifying. The index of refraction of air increases in both cases and the EM wave bends toward the ground instead of continuing upward.

On surface-base inversion, the beam will eventually hit the ground and a part of it can be reflected back toward the emitter. In upper air inversion, the bending will be limited to the layer involved but the bending will extend the path of the beam, possibly beyond the usual transmission horizon.

==== Atmospheric duct ====
{{Main|Atmospheric duct}}
When the inversion is very strong and shallow, the EM wave is trapped within the inversion layer. The beam will bounce many times inside the layer as within a [[waveguide]]. In surface-based ducting, the beam will hit the ground many times, causing return echoes at regular distances toward the emitter. In elevated ducts, the transmission can be extended to very large distances.

==== Sub refraction ====

[[File:Subrefraction.jpg|thumb|Under refraction in radar operation]]
On the other hand, if the air is unstable and cools faster than the standard atmosphere with height, the wave is higher than expected and can miss the intended receiver.


=== Other causes ===

Other ways anomalous propagation is recorded is by [[troposcatter]]s causing irregularities in the [[troposphere]], [[scattering]] due to [[meteor]]s, refraction in the [[ionized]] regions and layers of the ionosphere, and [[Reflection (physics)|reflection]] from the ionosphere.<ref name=Patterson/>

Finally, [[multipath propagation]] near the [[Earth]]'s surface has multiple causes, including atmospheric ducting, ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects such as mountains and buildings.