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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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  • $\begingroup$ 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. $\endgroup$ – Gimelist Feb 21 '17 at 10:58
  • $\begingroup$ @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. $\endgroup$ – uhoh Feb 21 '17 at 11:09
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    $\begingroup$ As far as I know, the clouds themselves polarise the light, but then again I'm not an expert on this. $\endgroup$ – Gimelist Feb 21 '17 at 11:10
  • $\begingroup$ @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! $\endgroup$ – uhoh Feb 21 '17 at 11:16
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    $\begingroup$ 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. $\endgroup$ – jamesqf Feb 21 '17 at 20:44
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this is 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).

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  • $\begingroup$ Polar stratospheric clouds? Visible from the location and latitude that I've descried in the question? (double check the first sentence!) $\endgroup$ – uhoh Feb 22 '17 at 12:39
  • $\begingroup$ 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. $\endgroup$ – uhoh Feb 22 '17 at 13:10
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    $\begingroup$ i gave you an answer god or bad it is my answer. $\endgroup$ – trond hansen Feb 22 '17 at 20:58
  • $\begingroup$ 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. $\endgroup$ – uhoh Feb 23 '17 at 2:47
  • $\begingroup$ Related: bbc.com/news/science-environment-39697256 Oslo, not Singapore. $\endgroup$ – uhoh May 2 '17 at 6:37

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