Within Radar
When the Atmosphere Makes Radar See Objects
Atmospheric refraction can bend radar beams toward the ground, turning ordinary terrain or clutter into convincing UFO-like blips.
On this page
- How radar beams bend in unusual weather
- Why ducting can create many false blips
- How investigators test an anomalous propagation explanation
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Introduction
Among the many radar-related explanations for UFO reports, anomalous propagation is one of the most important because it can create convincing radar targets without any unusual object being present. In these cases, the radar system is functioning normally, but the atmosphere is not behaving in the way radar engineers expect. Layers of temperature and humidity can bend radio waves downward, allowing radar beams to strike terrain, coastlines, buildings, ships, or other surface features far beyond their normal range. The radar then receives echoes from those features and displays them as airborne targets, clusters of unexplained blips, or even apparent moving tracks. Aviation authorities, meteorological agencies, military analysts, and radar researchers all recognise anomalous propagation—often abbreviated AP—as a major source of false radar returns. The phenomenon is so common that the U.S. Federal Aviation Administration (FAA) explicitly warns that atmospheric ducting can place “many extraneous blips” on radar displays. [Federal Aviation Administration]faa.govFederal Aviation Administration4-5-2Air Traffic Control Radar Beacon System (ATCRBS)Jan 8, 2015 — (b) The bending of radar pulses, often called anomalous propagation or duct…
In the study of identified flying objects and UFO reports, anomalous propagation matters because radar evidence is frequently treated as stronger than eyewitness testimony. Yet a radar display does not directly show physical objects. It shows the interpreted result of radio waves travelling through an atmosphere whose refractive properties are constantly changing. Under the right conditions, the atmosphere itself can cause radar to “see” targets that do not exist in the sky at all. [amt.copernicus.org]amt.copernicus.orgIf a radar beam is subjected to atmospheric subrefraction, targets…Read more…
When the Atmosphere Makes Radar See Objects
Radar systems assume a fairly predictable atmosphere. Under ordinary conditions, radio waves bend slightly downward as they travel through the lower atmosphere, extending radar coverage somewhat beyond the geometric horizon. Engineers design radar systems around this expected behaviour. Problems arise when the atmosphere departs significantly from that standard model. [American Meteorological Society Journals]journals.ametsoc.orgAmerican Meteorological Society JournalsRadar Beam Tracing Methods Based on Atmospheric…by Y Zeng · 2014 · Cited by 74 — A radar beam…
The key factor is atmospheric refractivity, which depends largely on temperature, pressure, and moisture. When temperature increases with height instead of decreasing—a condition known as a temperature inversion—or when humidity changes rapidly with altitude, radio waves may bend far more strongly than normal. This excessive bending is called super-refraction. In extreme cases, the radar beam becomes trapped within a narrow atmospheric layer and follows the Earth’s curvature for great distances. This is known as ducting. [Radartutorial]radartutorial.euRadartutorialAnomalous Propagation of Electromagnetic WavesThe result of this temperature inversion is to duct the transmitted energy alo… [2Dipòsit Digital]diposit.ub.eduFig. 1. Radar beamDipòsit DigitalEffects of Anomalous Propagation Conditions on Weather…by J Bech · Cited by 12 — An extreme case of superrefraction, kn…
Radar operators have long recognised that ducting can dramatically alter what appears on their displays. Rather than travelling into open air, the beam may be forced downward toward the surface, where it illuminates terrain features that should normally be hidden below the radar horizon. The returning echoes can then be misinterpreted as airborne targets. The FAA notes that ducting may cause numerous false radar contacts when the beam is bent toward the ground. [Federal Aviation Administration]faa.govFederal Aviation Administration4-5-2Air Traffic Control Radar Beacon System (ATCRBS)Jan 8, 2015 — (b) The bending of radar pulses, often called anomalous propagation or duct…
Why Ducting Can Create Many False Blips
The most straightforward false target occurs when a downward-bent beam strikes hills, buildings, towers, coastlines, or other fixed structures. Because radar processing systems assume normal propagation, they calculate the echo’s position incorrectly. The return may appear suspended in space rather than attached to the ground. [National Weather Service]forecast.weather.govNational Weather Service NOAA's National Weather ServiceRadar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric…Rea…
This process can generate large numbers of unexplained contacts simultaneously. A single episode of strong ducting may expose dozens or hundreds of previously invisible reflectors. Instead of seeing one mysterious radar target, operators may suddenly see an entire field of blips appearing across a region. Meteorologists encounter this problem regularly. NOAA describes anomalous propagation as false radar echoes produced when non-standard atmospheric conditions redirect the beam toward the ground, contaminating radar data with returns unrelated to weather. [NOAA]noaa.govanomalous propagationNOAAJetStream Max: Anomalous PropagationAug 9, 2023 — False echoes are known as anomalous propagation (AP) - an echo that is not precipit…
What makes the effect particularly relevant to UFO investigations is that the resulting contacts can appear unexpectedly, persist for a period, and then vanish as atmospheric conditions change. To observers unfamiliar with radar propagation, the targets may look like genuine objects entering and leaving the monitored airspace. In reality, the changing atmosphere is determining whether the radar beam can reach distant surface features. [NOAA]noaa.govradar beamsJetStream Max: Radar BeamsAug 10, 2023 — Ducting often leads to false echoes also known as anomalous propagation or simply AP. When this… [NOAA]nssl.noaa.govSuper Res and CLutter RPT2It is seen from the figure that the variables from clutter have significantly wider distributions…Read more…
Researchers in radar meteorology have spent decades developing methods to identify and remove AP contamination because it can seriously degrade radar accuracy. Studies published in the American Meteorological Society’s journals describe anomalous propagation as a persistent operational problem that contaminates radar data and can resemble legitimate targets. [American Meteorological Society Journals]journals.ametsoc.orgAmerican Meteorological Society JournalsRadar Beam Tracing Methods Based on Atmospheric…by Y Zeng · 2014 · Cited by 74 — A radar beam…
How False Tracks Can Appear to Move
One common misunderstanding is that anomalous propagation can only create stationary clutter. In reality, AP events can produce apparent motion even when no airborne object exists.
Several mechanisms contribute to this effect:
- Atmospheric layers can shift in strength and altitude over time, changing where the radar beam strikes the surface.
- Different reflectors may appear and disappear from scan to scan.
- Radar tracking software may connect separate returns into a single track.
- Reflections from ships, vehicles, or waves can change position naturally while still originating from surface targets.
- Multipath effects can alter the apparent location of returns. [SkyRadar]skyradar.comSkyRadarATSEP Use Cases: Impact of False Targets on Air Traffic ControlSep 21, 2023 — Multipath propagation can cause misleading indicati…
The result can be a short-lived radar track that appears to accelerate, turn, or manoeuvre. Investigators sometimes encounter reports describing radar targets that moved erratically before disappearing. Without supporting sensor data, such behaviour does not automatically indicate a structured airborne craft. It may instead reflect changing propagation conditions and radar-processing assumptions. [SkyRadar]skyradar.comSkyRadarATSEP Use Cases: Impact of False Targets on Air Traffic ControlSep 21, 2023 — Multipath propagation can cause misleading indicati…
Military radar operators have long been aware of this danger. Naval discussions of anomalous propagation describe “ghost” targets created by unusual atmospheric conditions, as well as “radar holes” where real targets disappear because radar energy is being redirected away from the expected area. [U.S. Naval Institute]usni.orgdont fall radar holeNaval InstituteDon't Fall in the Radar Hole | ProceedingsThese false radar targets ("ghosts”) are a perplexing problem to the operational…
Conditions That Favour Anomalous Propagation
Anomalous propagation is not random. It is associated with specific meteorological environments.
Common conditions include:
- Strong overnight temperature inversions.
- High-pressure systems with stable air.
- Marine layers over oceans and coastlines.
- Sharp humidity gradients.
- Warm air moving over cooler surfaces.
- Calm weather with little vertical mixing. [radartutorial]radartutorial.euRadartutorialAnomalous Propagation of Electromagnetic WavesThe result of this temperature inversion is to duct the transmitted energy alo… Coastal areas are especially susceptible because large temperature contrasts often develop between air and water. Marine ducting can extend radar range dramatically, allowing radars to detect ships, islands, or coastlines well beyond their normal line-of-sight limits. Radar researchers have documented numerous examples of anomalous propagation over water producing unexpected ground clutter and range errors. [amt.copernicus.org]amt.copernicus.orgof anomalous propagation over waters near…by L Norin · 2023 · Cited by 7 — In this work, we present data from five Swedish weather rad…
The phenomenon frequently appears during early morning hours when temperature inversions are strongest. Weather-service guidance notes that apparent radar targets produced by AP often weaken or disappear after sunrise as surface heating breaks down the inversion layer. [Weather.gov]forecast.weather.govNational Weather Service NOAA's National Weather ServiceRadar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric…Rea…
Real-World Examples Outside UFO Investigations
One reason anomalous propagation is considered a strong explanation in some radar UFO cases is that it is repeatedly observed in non-UFO contexts.
Weather radars routinely display false echoes that inexperienced viewers mistake for rainfall. NOAA and National Weather Service educational material show how ducting can make bridges, buildings, terrain, and distant surface features appear as precipitation on radar imagery. These returns may cover large areas and resemble genuine meteorological activity until examined more closely. [NOAA]noaa.govanomalous propagationNOAAJetStream Max: Anomalous PropagationAug 9, 2023 — False echoes are known as anomalous propagation (AP) - an echo that is not precipit… [NOAA]noaa.govradar beamsJetStream Max: Radar BeamsAug 10, 2023 — Ducting often leads to false echoes also known as anomalous propagation or simply AP. When this…
Marine environments provide especially striking examples. During strong ducting events, weather radars have displayed distant ships and coastal structures as mysterious radar echoes. Meteorologists regularly identify such returns as anomalous propagation rather than actual storms or airborne targets. [Fox News]foxnews.comFox News What is radar ducting?Why weather radar sometimes…May 14, 2020 — "Ducting often leads to false echoes also known as anomalous propagation or simply AP." Suc…
The persistence of this issue has driven extensive research into automated AP detection. Multiple scientific studies have focused specifically on identifying and removing anomalous propagation returns because they are common enough to interfere with operational radar systems. [American Meteorological Society Journals+2American Meteorological Society Journals]
How Investigators Test an Anomalous Propagation Explanation
A serious investigation does not simply label an unusual radar contact as AP and move on. Instead, investigators examine whether atmospheric conditions were capable of producing the observed effect.
Several lines of evidence are commonly used:
Meteorological records. Temperature profiles, humidity measurements, and weather balloon data can reveal inversion layers or super-refraction conditions consistent with ducting. [Dipòsit Digital]diposit.ub.eduFig. 1. Radar beamDipòsit DigitalEffects of Anomalous Propagation Conditions on Weather…by J Bech · Cited by 12 — An extreme case of superrefraction, kn…
Geographical analysis. Investigators determine whether the radar beam could have illuminated terrain, buildings, offshore structures, or shipping lanes lying beyond the normal radar horizon. [Bureau of Ocean Energy Management]boem.gov4. BOEM COP ReviewBureau of Ocean Energy ManagementRadar Interference Analysis for Renewable…As such, radars that are not in LOS of offshore windfarms u…
Comparison with neighbouring radars. If a target appears on only one radar while nearby systems do not detect it, anomalous propagation becomes more plausible. Weather-service guidance specifically recommends comparing adjacent radar sites when AP is suspected. [Weather.gov]weather.govNWS Weather RadarAnomalous Propagation (AP): (Ground Clutter). Anomalous Propagation (AP) refers to meteorological situations where a sig…
Target behaviour. AP-generated contacts often appear in clusters, emerge under stable weather conditions, fluctuate in intensity, and disappear when atmospheric conditions change. These patterns differ from those expected of conventional aircraft. [NOAA]nssl.noaa.govSuper Res and CLutter RPT2It is seen from the figure that the variables from clutter have significantly wider distributions…Read more…
Independent confirmation. Radar contacts supported by transponder data, visual observations, infrared tracking, or multiple radar systems are generally less likely to be explained solely by anomalous propagation. Conversely, isolated radar returns are more vulnerable to AP explanations. [Federal Aviation Administration]faa.govFederal Aviation Administration4-5-2Air Traffic Control Radar Beacon System (ATCRBS)Jan 8, 2015 — (b) The bending of radar pulses, often called anomalous propagation or duct…
Why Anomalous Propagation Remains a Major UFO Explanation
Anomalous propagation does not explain every radar UFO report. Some cases involve multiple sensors, visual observations, or data inconsistent with known AP signatures. However, the phenomenon remains one of the most thoroughly documented mechanisms by which radar can generate apparently anomalous targets without requiring any unusual aircraft or unknown technology.
The key lesson is that radar does not merely observe reality; it observes reality through the atmosphere. When that atmosphere bends radio waves in unexpected ways, ordinary terrain and clutter can be transformed into apparently mysterious radar contacts. Because those contacts can appear suddenly, form tracks, and vanish as conditions change, they have repeatedly contributed to UFO reports throughout the history of radar surveillance. The existence of well-established physical mechanisms capable of producing such effects is why anomalous propagation remains a standard investigative hypothesis whenever unexplained radar targets are reported. [U.S. Naval Institute+3Federal Aviation Administration+3Federal Aviation Administration]
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Further Reading
Books and field guides related to When the Atmosphere Makes Radar See Objects. Use these as the next step if you want deeper reading beyond the article.
Introduction to Radar Systems
Directly covers atmospheric propagation and radar performance.
Meteorology Today
Helps readers understand atmospheric conditions that produce ducting.
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Endnotes
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Source: faa.gov
Title: Federal Aviation Administration4-5-2
Link: https://www.faa.gov/air_traffic/publications/atpubs/aim/aim0405.htmlSource snippet
Air Traffic Control Radar Beacon System (ATCRBS)Jan 8, 2015 — (b) The bending of radar pulses, often called anomalous propagation or duct...
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Source: faa.gov
Title: Federal Aviation Administration Section 5
Link: https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap4_section_5.htmlSource snippet
Section 5. Surveillance SystemsThe bending of radar pulses, often called anomalous propagation or ducting, may cause many extraneous blip...
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Source: amt.copernicus.org
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If a radar beam is subjected to atmospheric subrefraction, targets...Read more...
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Source: radartutorial.eu
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RadartutorialAnomalous Propagation of Electromagnetic WavesThe result of this temperature inversion is to duct the transmitted energy alo...
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Source: forecast.weather.gov
Title: National Weather Service NOAA’s National Weather Service
Link: https://forecast.weather.gov/glossary.php?word=APSource snippet
Radar term for false (non-precipitation) echoes resulting from nonstandard propagation of the radar beam under certain atmospheric...Rea...
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Source: noaa.gov
Title: anomalous propagation
Link: https://www.noaa.gov/jetstream/anomalous-propagationSource snippet
NOAAJetStream Max: Anomalous PropagationAug 9, 2023 — False echoes are known as anomalous propagation (AP) - an echo that is not precipit...
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Source: noaa.gov
Title: radar beams
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JetStream Max: Radar BeamsAug 10, 2023 — Ducting often leads to false echoes also known as anomalous propagation or simply AP. When this...
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Source: usni.org
Title: dont fall radar hole
Link: https://www.usni.org/magazines/proceedings/1973/december/dont-fall-radar-holeSource snippet
Naval InstituteDon't Fall in the Radar Hole | ProceedingsThese false radar targets ("ghosts”) are a perplexing problem to the operational...
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Source: weather.gov
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NWS Weather RadarAnomalous Propagation (AP): (Ground Clutter). Anomalous Propagation (AP) refers to meteorological situations where a sig...
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NWS Radar: Short-Comings of the RadarThe beam of energy transmitted from the doppler radar is no different.... Ducting often leads to fa...
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ENR 1.1: General Rulesb) The bending of radar pulses, often called anomalous propagation or ducting, may cause many extraneous blips to a...
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Title: Super Res and CLutter RPT2
Link: https://www.nssl.noaa.gov/publications/wsr88d_reports/Super_Res_and_CLutterRPT2.pdfSource snippet
It is seen from the figure that the variables from clutter have significantly wider distributions...Read more...
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WIND FARM INTERACTION WITH NEXRAD RADARThe goal of this page is to: (1) Provide examples of how radar echoes from [wind turbines]({{ 'wind-turbines/' | relative_url }}) and other...
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of anomalous propagation over waters near...by L Norin · 2023 · Cited by 7 — In this work, we present data from five Swedish weather rad...
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Source: weather.com
Title: 2025 07 02 weather words ground clutter 2025
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Weather Words: Ground Clutter | Weather.comJul 7, 2025 — Ground clutter refers to false echoes on weather radar caused by the radar beam...
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Title: Fig. 1. Radar beam
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Title: 1520 0426 1998 015 0098 oioedt 2 0 co 2.xml
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Serious contamination of weather radar data can occur in atmospheric conditions that cause anomalous propagation of the radar beam.Read more...
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Source: journals.ametsoc.org
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Part II: Real-Time Identification...by JC Hubbert · 2009 · Cited by 225 — The identification and mitigation of anomalous propagation (AP...
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Title: 4. BOEM COP Review
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Bureau of Ocean Energy ManagementRadar Interference Analysis for Renewable...As such, radars that are not in LOS of offshore windfarms u...
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Why weather radar sometimes...May 14, 2020 — "Ducting often leads to false echoes also known as anomalous propagation or simply AP." Suc...
Published: May 14, 2020
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Title: Anomalous propagation
Link: https://en.wikipedia.org/wiki/Anomalous_propagationSource snippet
Anomalous propagationAnomalous propagation includes different forms of radio propagation due to an unusual distribution of temperature...
Additional References
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Source: researchgate.net
Link: https://www.researchgate.net/publication/249603794_Identification_and_Removal_of_Ground_Echoes_and_Anomalous_Propagation_Using_the_Characteristics_of_Radar_EchoesSource snippet
(PDF) Identification and Removal of Ground Echoes...This paper explores the removal of normal ground echoes (GREs) and anomalous propaga...
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WEATHER RADAR FAQThe disadvantages of ducting are increased ground clutter and increased anomalous propagation (due to radar beams bounci...
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Source: journals.lib.unb.ca
Link: https://journals.lib.unb.ca/index.php/ihr/article/download/26697/1882519457/1882519692Source snippet
PROPAGATION IN RADARRefraction, which affects light waves, also affects radar signals, and should atmospheric conditions be such to cause...
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Source: wx.erau.edu
Link: https://wx.erau.edu/faculty/mullerb/Wx365/Radar_anomalous_propagation/false_echoes.htmlSource snippet
false_echoesAnomalous propagation of radar echoes refers to meteorological situations where a signal comes back to the radar antenna even...
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Source: medium.com
Link: https://medium.com/%40timventura/ufos-and-radar-targets-clutter-safety-and-false-certainty-c3eab7a878ad -
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Clutter and Other Stuff - What You Can See on Radar...Jan 19, 2023 — Additional ground clutter beyond what is always visible for a given...
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RADAR COVERAGE DIAGRAM & DUCT PROPAGATIONRadar Basic such as Anoprop and its effect on radar display. Lecture ends with explanation on ra...
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Source: memphisweather.net
Link: https://www.memphisweather.net/blog/2012/02/01/identifying-false-weather-returns-anomalous-propagation-using-dual-polarization-radar-data/Source snippet
Identifying false weather returns (anomalous propagation...1 Feb 2012 — In this case, the radar returns are not actually precipitation...
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Source: researchgate.net
Title: Radar beam propagation conditions (adapted from US
Link: https://www.researchgate.net/figure/Radar-beam-propagation-conditions-adapted-from-US-NOAA-National-Weather-Service_fig1_224829985Source snippet
ground clutter echoes (AP or anomalous propagation). An extreme case of super-refraction, known as ducting, occurs when the beam has a cu...
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Source: actionnews5.com
Link: https://www.actionnews5.com/2018/11/09/breakdown-why-is-there-ground-clutter-radar/Source snippet
Breakdown: Why is there ground clutter on the radar?Nov 9, 2018 — This is what we call anomalous propagation or better known as ground cl...
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