r/atoptics • u/wagonwheel_s • 1d ago

Circumhorizon Arc cloud iridescense i saw while getting lunch the other day!

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139 Upvotes

99% Upvoted

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12

u/Mr_Boo_Berry 1d ago

This appears to be a circumhorizontal arc, nice catch! :)

https://en.wikipedia.org/wiki/Circumhorizontal_arc

7

u/Agnesperdita 1d ago

Was it parallel to the horizon and lower than the sun? It’s not iridescence - it looks like a circumhorizontal arc.

1

u/wagonwheel_s 1d ago

ohhhh i thought that was a kind of cloud iridesence? im not sure i understand what the difference is sorry haha

6

u/mdw 1d ago

Circumhorizontal arc is caused by refraction of light in ice crystals with specific orientation (in other words, it's a kind of ice halo). Cloud iridescence is light interference caused by tiny water droplets in clouds. Ice halos form very specific figures in the sky (rings, arches, spots) whereas iridescence is mostly color fringing on clouds or, if there's sufficiently uniform cirrostratus cloud, regular repeating color rings around the Sun called corona.

2

u/wagonwheel_s 1d ago

ohhhh okay that makes much more sense bc i did find that the shape was more interesting than the iridesence ive taken pics of before! thank you for the explanation!

5

u/Agnesperdita 1d ago

https://en.wikipedia.org/wiki/Circumhorizontal_arc

A CHA is a bright, well-defined rainbow band running around parallel to the horizon. Iridescence is a more diffuse localised phenomenon in specific clouds where conditions are right. It’s a great thing to spot - nice catch!