# What mechanism leads to these spectacular iridescent patterns in clouds labeled as a "Fire Rainbow"?

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.

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

• 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. Feb 21, 2017 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, 2017 at 11:09
• As far as I know, the clouds themselves polarise the light, but then again I'm not an expert on this. Feb 21, 2017 at 11:10
• 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. Feb 21, 2017 at 20:44
• Atmospheric Optics has pages on Iridescent Clouds and Nacreous Clouds. Aug 19, 2019 at 1:48

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?

• +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 3 at 8:28
• 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 Aug 3 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 <the generic scientific term for surface causing light color change> in <name of the cloud>. 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... Aug 3 at 11:46

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/

• Polar stratospheric clouds? Visible from the location and latitude that I've descried in the question? (double check the first sentence!)
– uhoh
Feb 22, 2017 at 12:39
• Please don't take offense. This is stackexchange and it functions a little different (and more collaborative) than other sites. If you haven't already, you might take the tour. People often add comments, discuss, and revise in order to arrive at the best possible answer. I'm suggesting that a phenomenon that starts with the word "polar" might not be visible from Singapore, 1 degree away from the equator. The pics in the question may be of the more common cloud iridescence.
– uhoh
Feb 22, 2017 at 13:10
• i gave you an answer god or bad it is my answer. Feb 22, 2017 at 20:58
• Thanks for your answer. Your linked article says those clouds are best observed just after sunset, when the sky brightness is reduced but the clouds are still illuminated. It links to this Australian Antarctic Division page as a reference. The BBC article says these clouds were photographed in the afternoon, 2 hours before sunset, and while I can't be sure from this article, I think the photos are in the direction of the sun, obscured directly behind those clouds.
– uhoh
Feb 23, 2017 at 2:47
• Related: bbc.com/news/science-environment-39697256 Oslo, not Singapore.
– uhoh
May 2, 2017 at 6:37