Within Atmosphere
Why Nacreous Clouds Keep Glowing After Sunset
Nacreous clouds can remain brilliantly illuminated after sunset because their stratospheric altitude keeps them in direct sunlight.
On this page
- Where nacreous clouds form in the stratosphere
- Why twilight makes them appear unusually luminous
- Clues that separate reflected sunlight from internal light
Page outline Jump by section
Introduction
Nacreous clouds are a particularly convincing example of an atmospheric phenomenon that can look like a glowing object after sunset. These rare polar stratospheric clouds form far above ordinary weather clouds, typically in the lower stratosphere. When people on the ground have already entered twilight, a nacreous cloud can still be in direct sunlight. It may therefore shine against a much darker sky in vivid pink, orange, green, blue or pearly colours, creating the impression of something illuminated from within rather than an ordinary cloud.[Met Office]weather.metoffice.gov.ukMet Office Unusual cloud formationsMet OfficeUnusual cloud formations - Met Office…
That combination matters when assessing unusual aerial observations. A nacreous cloud may be brightly coloured, sharply bounded, lenticular or patch-like, apparently almost stationary, and conspicuously brighter than nearby clouds. None of those features requires an independent light source or solid object. The unusual appearance follows from its exceptional altitude, optical properties and the geometry of twilight.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Where nacreous clouds form in the stratosphere
Ordinary weather takes place mainly in the troposphere. Nacreous clouds are different: they form in the lower stratosphere, commonly around 15–25 kilometres above the surface, although observational descriptions and classifications give somewhat different altitude ranges. The Met Office’s cloud factsheet, for example, places nacreous clouds at roughly 21–30 kilometres, while the US National Center for Atmospheric Research gives the broader polar-stratospheric-cloud range of about 15–25 kilometres.[Met Office]metoffice.gov.ukMet Office CloudsNational Meteorological Library and ArchiveOctober 31, 2023…
This is an exceptionally dry part of the atmosphere, so clouds there require extreme cold. The World Meteorological Organization’s International Cloud Atlas describes nacreous clouds as ice polar stratospheric clouds forming below the ice frost point, typically near −85°C. They occur mainly at high latitudes during winter and are especially common over Antarctica, although they are also recorded in the Arctic, Scandinavia, Scotland, Alaska, Canada and northern Russia.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Their rarity outside the polar regions is useful when evaluating a suspected sighting. In Britain, for example, the necessary stratospheric cold usually reaches sufficiently far south only when the polar vortex is displaced. That happened conspicuously in December 2023, when nacreous clouds were photographed across parts of Scotland and England. The Met Office explained that such displays are unusual in the UK because the intensely cold stratospheric air normally remains closer to the pole.[Met Office]weather.metoffice.gov.ukMet Office Unusual cloud formationsMet OfficeUnusual cloud formations - Met Office…
Nacreous clouds can also acquire surprisingly regular forms. The International Cloud Atlas notes that they are often lenticular wave clouds associated with atmospheric gravity waves downwind of mountain ranges. A visible cloud may consequently sit near the crest of a wave while air continually passes through it. To someone unfamiliar with the mechanism, a smooth luminous patch that scarcely changes position can look less like weather and more like an object hovering in the sky.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Why twilight makes them look self-luminous
The most deceptive feature is not simply their colour but their timing. Nacreous clouds are especially brilliant when the Sun has already disappeared below the observer’s horizon.
The explanation is geometric. Sunset at ground level does not mean that every part of the atmosphere has simultaneously entered Earth’s shadow. Because nacreous clouds are tens of kilometres above the observer, sunlight can still reach them while the surface and lower atmosphere are already darkening. The cloud scatters that sunlight back towards the observer. The Met Office identifies the most favourable viewing geometry as when the Sun is approximately 1° to 6° below the horizon.[Met Office]weather.metoffice.gov.ukMet Office Unusual cloud formationsMet OfficeUnusual cloud formations - Met Office…
This creates an unusually strong contrast. The Australian Antarctic Program describes civil twilight as an especially good time to identify polar stratospheric clouds because the troposphere has entered shadow while the stratosphere remains illuminated. Ordinary lower clouds therefore become dark silhouettes or grey masses while the much higher clouds can remain conspicuously bright.[Australian Antarctic Division]antarctica.gov.auOpen source on antarctica.gov.au.
That contrast can easily distort intuitive judgements about what is producing the light. In everyday experience, an object that remains bright after its surroundings have become dark often is emitting light: an aircraft, lamp, illuminated balloon or other artificial source. A nacreous cloud violates that expectation. It is merely reflecting and scattering sunlight arriving from a direction that is no longer visible from the ground.
The altitude difference produces a particularly useful diagnostic. The World Meteorological Organization notes that when cirrus and nacreous clouds coexist after sunset, the nacreous clouds can retain their brilliant colours after the cirrus has already turned grey. The apparent anomaly is therefore evidence of different altitude rather than evidence that the brighter formation contains its own source of illumination.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Why the colours can look artificial
Simple reflected sunlight does not fully describe the visual effect. Nacreous clouds are famous for strong iridescence — the shifting mother-of-pearl colours from which their name derives.
The World Meteorological Organization attributes their characteristic colours to diffraction and interference involving similarly sized particles, with ice nacreous clouds associated with particles around 10 micrometres across. The result can be much more intensely coloured than the relatively localised iridescence sometimes seen along the edges of ordinary tropospheric clouds.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
The colours also change as twilight progresses. The International Cloud Atlas describes maximum iridescence when the Sun lies several degrees below the horizon. As it sinks further, the varied colours give way to broader orange and pink coloration against the darkening sky. Older but still useful observational guidance in the Met Office cloud factsheet records a further progression towards grey as illumination diminishes.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Consequently, a witness can see something that appears to change colour, lose brightness or fade away without displaying the behaviour expected of a normal low cloud. Photographs from the widespread British display in December 2023 showed exactly the striking pastel and rainbow-like appearance that prompted extensive public attention. Contemporary reporting described displays from Edinburgh and Tynemouth and emphasised that they were best seen around sunrise and sunset.[Sky News]news.sky.comNews Rare 'rainbow clouds' spotted in UK skies | UK News | Sky NewsSky NewsRare 'rainbow clouds' spotted in UK skies | UK News | Sky NewsDecember 21, 2023…
Informal reactions also illustrate how unfamiliar the phenomenon can be. People posting photographs of nacreous clouds online have described the sky as looking as though it had “glitched”, wondered whether the formation was some unfamiliar object, or jokingly invoked aliens. Such reactions are not evidence that nacreous clouds account for any particular UAP report, but they demonstrate a relevant perceptual problem: even observers actively trying to identify a cloud may initially struggle to classify what they are seeing.[Reddit]reddit.comDo you guys know what this is?Do you guys know what this is?January 17, 2025…
Clues that separate reflected sunlight from internal light
A nacreous-cloud explanation becomes stronger when several observations fit together rather than merely when a photograph looks colourful. The most useful clues concern timing, location, motion and the relationship between the object’s brightness and the Sun.
Twilight timing is the strongest clue. Brilliant nacreous colours are expected when the Sun is only a few degrees below the horizon. A mysterious luminous formation that becomes prominent shortly after sunset, then fades as twilight deepens, fits the illumination geometry particularly well. A source that continues unchanged through the night would require a different explanation.[Met Office]weather.metoffice.gov.ukMet Office Unusual cloud formationsMet OfficeUnusual cloud formations - Met Office…
Brightness relative to lower clouds is informative. If ordinary clouds have become grey or dark while the anomalous formation remains brilliantly illuminated, that is consistent with a much greater altitude. It does not demonstrate internal illumination.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Colour should be structured rather than lamp-like. Mother-of-pearl bands, pastel patches and colours that vary across the cloud are characteristic of diffraction and interference. They differ from a compact point light, although photographs with poor focus, digital zoom or overexposure can obscure that distinction.
Very slow apparent movement is not necessarily suspicious. Lenticular nacreous clouds can remain effectively stationary at a gravity-wave crest even though air flows through them. Other nacreous formations can seem to move only slowly because of their great distance. The International Cloud Atlas specifically warns that their apparent lack of motion does not imply calm winds at their altitude.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
Season and geography provide an independent check. These clouds require exceptionally cold stratospheric conditions and are overwhelmingly associated with polar winter and high latitudes. A report from northern Europe during a period of displaced polar-vortex air is therefore much more compatible with nacreous clouds than the same visual description from a warm low-latitude location where the required stratospheric conditions were absent.[Met Office]weather.metoffice.gov.ukMet Office Unusual cloud formationsMet OfficeUnusual cloud formations - Met Office…
What a nacreous-cloud explanation can and cannot establish
Nacreous clouds should not become a catch-all explanation for colourful UFO or UAP reports. Their formation conditions are restrictive enough to make the hypothesis testable. Date, location and observation time can be compared with sunset geometry; photographs can reveal whether lower clouds were already shadowed; meteorological data can establish whether sufficiently cold stratospheric air was present; and the reported direction can be checked against the twilight sky.
Conversely, nacreous clouds are a particularly strong atmospheric candidate when a winter sighting at higher latitude involves a broad or lenticular luminous formation that persists after sunset, shows pearly or rainbow colours, moves little, and gradually changes colour or fades as the Sun sinks farther below the horizon. Those characteristics correspond closely to documented meteorological behaviour rather than requiring an unidentified source of light.[International Cloud Atlas]cloudatlas.wmo.intInternational Cloud Atlas Nacreous clouds | International Cloud AtlasInternational Cloud Atlas Nacreous clouds | International Cloud Atlas
The key distinction is therefore between appearing luminous and generating light. Nacreous clouds demonstrate how convincingly the atmosphere can blur that boundary. From the ground, the landscape and ordinary clouds may already be in shadow while a rare cloud high in the stratosphere still catches direct sunlight. For a witness without altitude information, that separation is invisible. What remains is an apparently hovering, brilliantly coloured object glowing in a darkening sky — precisely the sort of observation in which twilight geometry needs to be ruled out before the luminosity itself is treated as anomalous.[metoffice.gov.uk]weather.metoffice.gov.ukMet Office Unusual cloud formationsMet OfficeUnusual cloud formations - Met Office…
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Endnotes
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Additional References
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