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The 21-Feb-2016 BBC new article Singapore 'fire rainbow' cloud phenomenon lights up sky shows several photos of a rainbow-like iridescence in clouds on a sunny day in Singapore (about 1.3 degrees North latitude). Local time was about 17:10, 2 hours before sunset.

I've read about circumhorizontal arcs in this article, but the photos there (and other places) show a definite orientation - approximately parallel to the horizon with red on top (higher elevation angle).

However the photos from Singapore show tight circular patterns that resemble a color-coded contour map around a hill. So I think the explanation for these would not be identical to the explanation for a classic circumhorizontal arc or "Fire Rainbow".

below x2: Photo of the phenomenon seen in Singapore, from Singapore 'fire rainbow' cloud phenomenon lights up sky. Photo credit: Zhou Guang Ping.

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below: example of a circumhorizontal arc from Fire Rainbows: A Rare Cloud Phenomenon.

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uhoh
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    These are not rainbow colours but rather polarised-light interference colours. The colour sequence is identical to what you get in a polarising microscope: Michel-Levy Birefringence Chart. I'll wait for a could expert to provide an atmospherically correct answer though, not my field. – Gimelist Feb 21 '17 at 10:58
  • @Michael If I understand correctly this was seen and photographed without any polarizing filter, and the incident sunlight is not initially polarized either. While this light could so have some polarization, it does not seem to be necessary for the color pattern to become strongly visible. – uhoh Feb 21 '17 at 11:09
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    As far as I know, the clouds themselves polarise the light, but then again I'm not an expert on this. – Gimelist Feb 21 '17 at 11:10
  • @Michael OK so one large angle scattering could give some polarization, but I'm not surewhere an additional bireferingnce could come from. But I'll go take a look at the material you've linked to. Thanks! – uhoh Feb 21 '17 at 11:16
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    It's actually quite common, at least hereabouts. Usually the colors aren't nearly as bright, though, so you need (non-polarized) sunglasses or similar to see them. Otherwise the colors are washed out by the strong light of the sun. – jamesqf Feb 21 '17 at 20:44
  • the pictures do show mother of pearl clouds,they are made of ice crystals and is created twise as high in the atmosphere as normal clouds. – trond hansen Feb 22 '17 at 10:47
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    Atmospheric Optics has pages on Iridescent Clouds and Nacreous Clouds. – Keith McClary Aug 19 '19 at 01:48

2 Answers2

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The top two pictures are of iridescence in a pileus (AKA "cap cloud") above a growing thunderstorm. Pileus clouds form when a storm updraft pushes up through a layer of moist air, and lifts that layer uniformly, causing it to condense into a thin layer of cloud. Since all the cloud droplets are formed at about the same time as this moist layer is lifted, they all end up roughly the same size, which is perfect conditions for iridescence to form.

The term "fire rainbow" doesn't have a consistent definition, because it is a term invented by a journalist for phenomena that was already known and named: specifically the circumhorizon arc. This journalist apparently didn't bother to consult scientists (or even amateur skywatchers) to find out if it already had a name, and so gave it this artistic name: I'm guessing the "fire" in fire rainbow is supposed to describe the "flame" shape of the wisps of cirrus in that original display. And because journalists and others who don't know any better think all things that vaguely look the same are the same, the term is now applied to pretty much any colorful sky phenomenon, hence the embarrassing BBC article reporting on what other journalists are calling it rather than ringing up their local meteorologist. They must have at least one in-house, no?

Mike Kavulich
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  • +1 it certainly seems like they should have one! But alas they don't seem to have enough particle physicists: What are "large hadrons"? Are there also "small hadrons"? – uhoh Aug 03 '22 at 08:28
  • Honestly... it's funny... someone literally asked me what they're called last week. And the answer may shed some light... it was along the lines of I don't know. I think that may be true of many/most atmospheric scientists... not sure there's any authoritative terminology source that has spoken it (ams or journals?), nor is it a topic likely to come up too much for most meteorologists... it's more of a novelty than something with large practical use. So they may well have called the local meteorologist and gotten no help. The same reason I'd think this didn't get an answer for a long time – JeopardyTempest Aug 03 '22 at 11:36
  • You'd think the appearance, particularly the concentrated coloration atop a thunderstorm, would be unique seeming enough to have inspired more than just in . It's almost like we're all waiting for someone to come along and take it and run with a more specific name. Pileoluminescence or something... – JeopardyTempest Aug 03 '22 at 11:46
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this looks like mother of pearl clouds https://en.wikipedia.org/wiki/Polar_stratospheric_cloud they are made of ice crystals very high in the atmosphere the only clouds higher in the atmosphere are noctilucent clouds.if you google mother of pearl clouds there is a lot of pictures and information. as a result of global warming the stratosphere is getting colder this makes mother of pearl clouds or rainbow clouds visible from larger areas of our planet then before(more info is needed to dokument this statement of mine).

An other alternative is irradecent clouds,They have many of the same properties as mother of pearl clouds https://en.wikipedia.org/wiki/Cloud_iridescence

The last alternative is a pollen rainbow but they do often form in the direction of the sun or moon(it is a little hard to see if the sun is behind the clouds in your pictures)The colors of a pollen rainbow is often more vibrant than other types of optical phnomena https://soranews24.com/2019/03/09/high-pollen-levels-in-eastern-japan-create-pretty-rainbow-suns/

trond hansen
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