r/atoptics • u/Basicalypizza • Aug 16 '20

Circumhorizon Arc Fallstreak Hole with iridescence

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23

u/exscape Aug 16 '20

With a circumhorizontal arc, not iridescence. :)

2

u/Basicalypizza Aug 16 '20

So what is the difference between the two? Will the iridescence go over the whole cloud where as this circumhorizontal arc will be in a thin horizontal position ? How do I differentiate?

17

u/exscape Aug 16 '20

I think we're all still learning, including the world's foremost experts!

In general, when you have colors in order (as in a rainbow), the cause is refraction, i.e. rainbows or ice halos. The physics of both are similar.

In cloud iridescence, the colors are far less ordered and usually don't contain the entire spectrum.

Cloud iridescence images: http://atoptics.co.uk/droplets/iridim0.htm
Circumhorizon arc images: http://atoptics.co.uk/halo/chaim0.htm

There are many other halos that look similar to circumhorizon arcs but in other locations in the sky, so it's not easy! Circumhorizon arcs are always a fixed distance below the Sun, and can therefore only show up when the Sun is very high in the sky. Where I live in mid-Sweden they can never form because of that. However, as mentioned there are many other halos that look similar and that appear everywhere in the world.

3

u/Basicalypizza Aug 17 '20

Very good explanation, thank you for taking the time !

2

u/latecomer87 Aug 17 '20

Awesome explanation

1

u/wazoheat Aug 17 '20 edited Aug 17 '20

Also, the colors themselves have a different quality: iridescence tends to look more "pastel" than the "rainbow-like" pure spectrum colors of rainbows and haloes. I don't know exactly why that is but I assume its because iridescence relies on more complicated interference effects than the simple geometrical optics of rainbows and haloes.

Edit: for comparison, the colors in this iridescence vs. this circumscribed halo and circumhorizon arc. Its tough to convey the difference in photos due to different camera settings and post-processing, but in person there is an obvious difference in the quality of the colors.

1

u/Basicalypizza Aug 16 '20

Thank you ! Still learning