Starship Asterisk*

[float=left][img3="Circumhorizontal arc in the Nepalese Himalaya"]https://upload.wikimedia.org/wikipedia/commons/0/06/Circumhorizontal_arc_%28Fire_Rainbow%29_in_the_Nepal_Himalayas.jpg[/img3][img3="Cloud iridescence, seen above the sun covered with grey clouds, Pondicherry, India"]https://upload.wikimedia.org/wikipedia/commons/thumb/5/59/Rainbow_cloud_1.jpg/1280px-Rainbow_cloud_1.jpg[/img3][/float]

[quote=Nilesh-Pune post_id=292364 time=1558332464]

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

Cloud iridescence seldom produces a well ordered rainbow.

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

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 [url=https://www.atoptics.co.uk/halo/czasalt.htm]variable[/url] 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.
[attachment=0]Circumzenithal Halo - Redmond, 4 April 2016 0050 UT.JPG[/attachment]

[quote="Chris Peterson" post_id=292341 time=1558277460 user_id=117706]
[quote=neufer post_id=292339 time=1558276959 user_id=124483]
[quote="Chris Peterson" post_id=292337 time=1558273630 user_id=117706]

The scattered blue sky light [i]may [/i]be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.[/quote]
[list]58° should do quite nicely.[/list]
[quote="http://www.atoptics.co.uk/halo/cha2.htm"]

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

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.)
[/quote][quote="http://www.atoptics.co.uk/halo/cha2.htm"]

[b][color=#0000FF]The halo is beneath the sun and [u]twice as far from it as the 22º halo[/b][/color][/u].[/quote]

[quote=neufer post_id=292339 time=1558276959 user_id=124483]
[quote="Chris Peterson" post_id=292337 time=1558273630 user_id=117706]

The scattered blue sky light [i]may [/i]be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.[/quote]
[list]58° should do quite nicely.[/list]
[quote="http://www.atoptics.co.uk/halo/cha2.htm"]

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

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.)

[quote="Chris Peterson" post_id=292337 time=1558273630 user_id=117706]

The scattered blue sky light [i]may [/i]be polarized. It depends upon the angle of the Sun with respect to any given patch of sky.[/quote]
[list]58° should do quite nicely.[/list]
[quote="http://www.atoptics.co.uk/halo/cha2.htm"]

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

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: [i][b]When Conspiracies Collide![/b][/i]

[quote=neufer post_id=292336 time=1558271946 user_id=124483]
[float=left][img3="Rayleigh scattered blue light is polarized."]https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/CircularPolarizer.jpg/1920px-CircularPolarizer.jpg

[/img3][/float][quote=Boomer12k post_id=292328 time=1558260395 user_id=120851]

Interesting... do they NEED a polarized lens to be seen?[/quote]
Ice has a low level of [url=https://en.wikipedia.org/wiki/Birefringence#Terminology]birefringence[/url] 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.
[/quote]

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

[float=left][img3="Rayleigh scattered blue light is polarized."]https://upload.wikimedia.org/wikipedia/commons/thumb/d/d8/CircularPolarizer.jpg/1920px-CircularPolarizer.jpg

[/img3][/float][quote=Boomer12k post_id=292328 time=1558260395 user_id=120851]

Interesting... do they NEED a polarized lens to be seen?[/quote]
Ice has a low level of [url=https://en.wikipedia.org/wiki/Birefringence#Terminology]birefringence[/url] 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.

Nice! Gonna be tomorrow's background! 8-) :mrgreen: :wink:

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

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

:---[===] *

[url=https://apod.nasa.gov/apod/ap190519.html] [img]https://apod.nasa.gov/apod/calendar/S_190519.jpg[/img] [size=150]A Circumhorizontal Arc Over Ohio[/size][/url]

[b] Explanation: [/b] Why would clouds appear to be different colors? The reason here is that ice crystals in distant cirrus clouds are acting like little floating [url=http://micro.magnet.fsu.edu/primer/java/scienceopticsu/newton/]prisms[/url]. Sometimes known as a fire rainbow for its flame-like appearance, a [url=http://en.wikipedia.org/wiki/Circumhorizon_arc]circumhorizon arc[/url] lies parallel to the horizon. For a [url=http://www.atoptics.co.uk/halo/cha2.htm]circumhorizontal arc[/url] to be visible, [url=https://solarsystem.nasa.gov/solar-system/sun/overview/]the Sun[/url] must be at least 58 degrees high in a sky where [url=https://en.wikipedia.org/wiki/Cirrus_cloud]cirrus clouds[/url] are present. Furthermore, the numerous, flat, [url=http://www1.lsbu.ac.uk/water/hexagonal_ice.html]hexagonal ice-crystals[/url] that compose the [url=http://en.wikipedia.org/wiki/Cirrus_cloud]cirrus cloud[/url] must be [url=http://www.atoptics.co.uk/halo/platcol.htm]aligned horizontally[/url] to properly [url=http://youtube.com/watch?v=U98KPM2b9IA]refract sunlight[/url] in a collectively similar manner. Therefore, [url=https://apod.nasa.gov/apod/ap040804.html]circumhorizontal arcs[/url] are quite unusual to see. This circumhorizon display was photographed through a [url=http://en.wikipedia.org/wiki/Polarizer]polarized[/url] [url=http://www.youtube.com/watch?v=GTpGxIRFGoY]lens[/url] above [url=http://en.wikipedia.org/wiki/Dublin,_Ohio]Dublin[/url], [url=http://en.wikipedia.org/wiki/Ohio]Ohio[/url] in 2009.


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