Today I saw the phenomenon in picture below. It was not raining (at least nearby me). What can that be? What is the technical explanation?

Band of colors in the sky

Edit: Just seen today in Southern Brazil another circumhorizon arc, this time together with a halo. Really beautiful.

enter image description here

  • $\begingroup$ I did a quick search for "flat rainbow". One of the first links is atlantic.ctvnews.ca/mobile/ctv-news-at-5/weather-blog/… $\endgroup$
    – Alchimista
    Dec 27, 2017 at 23:10
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    $\begingroup$ @WetSavannaAnimalakaRodVance It's of course a guess, but the roof and tiles style, the tropical looking sky, the banana tree and the other trees, the simple houses with rain damaged painting and even the old antenna, it all feels very familiar (I'm Brazilian). Then again, maybe it's somewhere else in South America or even in Asia. :-) $\endgroup$
    – stafusa
    Dec 28, 2017 at 1:52
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    $\begingroup$ Given that circumhorizon arcs can only appear when the Sun is very high in the sky, that this is Brazil is a good guess. $\endgroup$ Dec 28, 2017 at 6:10
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    $\begingroup$ @stafusa Good guess! It's in São Paulo. $\endgroup$
    – Diracology
    Dec 28, 2017 at 15:19
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    $\begingroup$ As a pedantic stickler, this seems more real than abstract to me. As a human, this looks like you had an awesome experience. $\endgroup$
    – phresnel
    Dec 29, 2017 at 10:17

2 Answers 2


This is not a "rainbow". Quoting from the linked site, "not all colored patches in the sky are rainbows". It instead is a circumhorizon arc. Rainbows are caused by internal by refraction and reflection in water droplets. Halos such as the circumhorizon arc portrayed in the question are caused by internal refraction and reflection in ice crystals.


that type of rainbow is created by light refraction within an extended cloud or layer of ice crystals, in which the crystals are similarly-shaped and have been aligned by wind shear so their principal axies are all pointing in more or less the same direction. they then behave like a cloud of microscopic prisms suspended in the air and if you are observing them from the right angle on the ground, you'll see the rainbow.


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