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

[APOD Robot]

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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[Boomer12k]

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

:---[===] *

[Ann]

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

[orin stepanek]

Nice! Gonna be tomorrow's background!

[neufer]

Rayleigh scattered blue light is polarized.

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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.

[Chris Peterson]

Rayleigh scattered blue light is polarized.

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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.

[Chris Peterson]

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!

[neufer]

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.

Look for the brightly coloured circumhorizon arc when the sun is very high in the sky - higher than 58°.

[Chris Peterson]

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.

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.)

[neufer]

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.

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.)

The halo is beneath the sun and twice as far from it as the 22º halo.

[alter-ego]

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.

[Nilesh-Pune]

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

[neufer]

Circumhorizontal arc in the Nepalese Himalaya

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

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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.