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Posted: **Sun May 19, 2019 4:06 am**

A Circumhorizontal Arc Over Ohio

Explanation: Why would clouds appear to be different colors? The reason here is that ice crystals in distant cirrus clouds are acting like little floating prisms. Sometimes known as a fire rainbow for its flame-like appearance, a circumhorizon arc lies parallel to the horizon. For a circumhorizontal arc to be visible, the Sun must be at least 58 degrees high in a sky where cirrus clouds are present. Furthermore, the numerous, flat, hexagonal ice-crystals that compose the cirrus cloud must be aligned horizontally to properly refract sunlight in a collectively similar manner. Therefore, circumhorizontal arcs are quite unusual to see. This circumhorizon display was photographed through a polarized lens above Dublin, Ohio in 2009.

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Posted: **Sun May 19, 2019 10:06 am**

Interesting... do they NEED a polarized lens to be seen?

:---[===] *

Posted: **Sun May 19, 2019 12:20 pm**

I like the picture. It looks like a very tall and skinny candle flame that got tired and decided to lay down and have a nap.

Ann

Posted: **Sun May 19, 2019 12:38 pm**

Nice! Gonna be tomorrow's background!

Posted: **Sun May 19, 2019 1:19 pm**

: Rayleigh scattered blue light is polarized.

Boomer12k wrote: ↑Sun May 19, 2019 10:06 am
Interesting... do they NEED a polarized lens to be seen?

Ice has a low level of birefringence so a circumhorizontal arc should be relatively unpolarized.

However, the background Rayleigh scattered blue sky is polarized and the contrast improves by limiting that.

Posted: **Sun May 19, 2019 1:47 pm**

neufer wrote: ↑Sun May 19, 2019 1:19 pm
Rayleigh scattered blue light is polarized.
Boomer12k wrote: ↑Sun May 19, 2019 10:06 am
Interesting... do they NEED a polarized lens to be seen?
Ice has a low level of birefringence so a circumhorizontal arc should be relatively unpolarized.

:arrow: However, the background Rayleigh scattered blue sky is polarized and the contrast improves by limiting that.

The scattered blue sky light may be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.

Posted: **Sun May 19, 2019 1:50 pm**

So... it isn't caused by the perfect storm of a chemtrail above and the weather modification radio antenna below? Coming soon to a theater near you: When Conspiracies Collide!

Posted: **Sun May 19, 2019 2:42 pm**

Chris Peterson wrote: ↑Sun May 19, 2019 1:47 pm
The scattered blue sky light may be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.

58° should do quite nicely.

http://www.atoptics.co.uk/halo/cha2.htm wrote:
Look for the brightly coloured circumhorizon arc when the sun is very high in the sky - higher than 58°.

Posted: **Sun May 19, 2019 2:51 pm**

neufer wrote: ↑Sun May 19, 2019 2:42 pm
Chris Peterson wrote: ↑Sun May 19, 2019 1:47 pm
The scattered blue sky light may be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.

58° should do quite nicely.

http://www.atoptics.co.uk/halo/cha2.htm wrote:
Look for the brightly coloured circumhorizon arc when the sun is very high in the sky - higher than 58°.

Yes, if you want a dark horizon. But the arc itself is only 22° from the Sun, so the sky behind it is not strongly polarized. (The polarized filter still provides some contrast improvement, of course.)

Posted: **Sun May 19, 2019 3:02 pm**

Chris Peterson wrote: ↑Sun May 19, 2019 2:51 pm
neufer wrote: ↑Sun May 19, 2019 2:42 pm
Chris Peterson wrote: ↑Sun May 19, 2019 1:47 pm
The scattered blue sky light may be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.

58° should do quite nicely.

http://www.atoptics.co.uk/halo/cha2.htm wrote:
Look for the brightly coloured circumhorizon arc when the sun is very high in the sky - higher than 58°.

Yes, if you want a dark horizon. But the arc itself is only 22° from the Sun, so the sky behind it is not strongly polarized. (The polarized filter still provides some contrast improvement, of course.)

http://www.atoptics.co.uk/halo/cha2.htm wrote:
The halo is beneath the sun and twice as far from it as the 22º halo.

Posted: **Mon May 20, 2019 2:39 am**

This APOD reminded me of a halo I saw back in 2016. I mistakenly recalled it as a circumhorizontal arc but looking at the image my son took it's clear that it's a circumzenithal arc instead. Unlike the circumhorizontal arc which is parallel to the horizon, the circumzenithal arc is centered on the zenith, and the radius is variable depending on the Sun's altitude. In this case, the Sun was 18° above the western horizon which put the arc about 25° from the zenith.

Posted: **Mon May 20, 2019 6:07 am**

How can any observer identify if colouring of thin clouds is due to circumhorizontal arc or Cloud iridescence

Posted: **Mon May 20, 2019 3:26 pm**

: Circumhorizontal arc in the Nepalese Himalaya

: Cloud iridescence, seen above the sun covered with grey clouds, Pondicherry, India

Nilesh-Pune wrote: ↑Mon May 20, 2019 6:07 am
How can any observer identify if colouring of thin clouds
is due to circumhorizontal arc or Cloud iridescence

Cloud iridescence seldom produces a well ordered rainbow.

Starship Asterisk*

APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

A Circumzenithal Arc Over Redmond, WA (2016 Apr 3)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)

Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)