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APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 4:06 am
by 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.
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 10:06 am
by Boomer12k
Interesting... do they NEED a polarized lens to be seen?
:---[===] *
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 12:20 pm
by 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
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 12:38 pm
by orin stepanek
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 1:19 pm
by neufer
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.
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 1:47 pm
by Chris Peterson
neufer wrote: ↑Sun May 19, 2019 1:19 pm
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.
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 1:50 pm
by 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!
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 2:42 pm
by neufer
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.
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 2:51 pm
by Chris Peterson
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.
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.)
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Sun May 19, 2019 3:02 pm
by neufer
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.
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.)
A Circumzenithal Arc Over Redmond, WA (2016 Apr 3)
Posted: Mon May 20, 2019 2:39 am
by 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.
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Mon May 20, 2019 6:07 am
by Nilesh-Pune
How can any observer identify if colouring of thin clouds is due to circumhorizontal arc or Cloud iridescence
Re: APOD: A Circumhorizontal Arc Over Ohio (2019 May 19)
Posted: Mon May 20, 2019 3:26 pm
by neufer
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.