Editing Atmospheric duct

{{Short description|Horizontal layer that propagates electromagnetic radiation}}
{{other uses|Duct (disambiguation)}}

[[File:Superior mirage of an island.jpg|[[Fata Morgana (mirage)|Fata Morgana]] of [[Farallon Islands]] with clearly seen duct|300px|thumb]]

In [[telecommunications]], an '''atmospheric duct''' is a horizontal layer in the lower atmosphere in which the vertical [[refractive index]] gradients are such that [[radio]] signals (and light rays) are guided or ducted, tend to follow the [[figure of the Earth|curvature of the Earth]], and experience less [[attenuation]] in the ducts than they would if the ducts were not present. The duct acts as an atmospheric [[dielectric]] [[waveguide]] and limits the spread of the [[wavefront]] to only the horizontal dimension.<ref name = "isbn0-19-509161-2"/>

'''Atmospheric ducting''' is a mode of [[Radio propagation|propagation]] of [[electromagnetic radiation]], usually in the lower layers of [[Earth’s atmosphere]], where the waves are bent by [[atmospheric refraction]].<ref name=NTRP1-02>{{cite book
 |title     = Navy Supplement to the DOD Dictionary of Military and Associated Terms
 |date      = August 2006
 |publisher = [[United States Navy|Department Of The Navy]]
 |pages     = 2–25
 |url       = https://www.nwdc.navy.mil/Documents/NTRP_1-02.pdf
 |id        = NTRP 1-02
}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</ref> In [[over-the-horizon radar]], ducting causes part of the radiated and target-reflection energy of a radar system to be guided over distances far greater than the normal radar range. It also causes long-distance propagation of [[radio]] signals in bands that would normally be limited to line of sight.

Normally radio "[[ground wave]]s" propagate along the surface as [[creeping wave]]s.  That is, they are only [[diffraction|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&nbsp;MHz.) waves are reflected by the [[ionosphere]].

The reduced refractive index due to lower densities at the higher altitudes in the [[Earth's atmosphere]] bends the signals back toward the Earth. Signals in a higher refractive index layer, ''i.e.,'' duct, tend to remain in that layer because of the [[Reflection (physics)|reflection]] and [[refraction]] encountered at the boundary with a lower refractive index material. In some [[weather]] conditions, such as [[Inversion (meteorology)|inversion layer]]s, density changes so rapidly that waves are guided around the curvature of the earth at constant altitude.

Phenomena of atmospheric optics related to atmospheric ducting include the [[green flash]], [[Fata Morgana (mirage)|Fata Morgana]], superior [[mirage]], mock [[mirage of astronomical objects]] and the [[Novaya Zemlya effect]].<ref name="isbn0-19-509161-2">{{cite book |author=Thomas, Michael Frederic |title=Optical propagation in linear media: atmospheric gases and particles, solid state components, and water |publisher=[[Oxford University Press]] |location=Oxford [Oxfordshire] |year=2006 |pages= 327–328|isbn=0-19-509161-2 }}</ref>

==See also==
*[[Anomalous propagation]]
*[[Earth-Ionosphere waveguide]]
*[[Predel-E]] (Russian OTH radar system based on atmospheric ducting)
*[[Sky wave]]
*[[Thermal fade]]
*[[Temperature inversion]]
*[[Tropospheric ducting]]

==References==
{{reflist}}

[[Category:Electromagnetic radiation]]
[[Category:Atmosphere]]
[[Category:Radio frequency propagation]]