(Not) An Iridescent Cloud Over Ohio (2009 May 12)

[atoptics]

This is not an iridescent cloud nor are water droplets responsible. It is a halo, a circumhorizon arc (http://www.atoptics.co.uk/halo/cha2.htm) produced by refraction in hexagonal plate shaped ice crystals in cirrus. Note the structure parallel to the horizon and the structured gradation of color from red at the top through green to blue. Both are characteristic of the halo.

Les Cowley
Atmospheric Optics - http://www.atoptics.co.uk

[neufer]

This is not an iridescent cloud nor are water droplets responsible. It is a halo, a circumhorizon arc (http://www.atoptics.co.uk/halo/cha2.htm) produced by refraction in hexagonal plate shaped ice crystals in cirrus. Note the structure parallel to the horizon and the structured gradation of color from red at the top through green to blue. Both are characteristic of the halo.

http://apod.nasa.gov/apod/ap090512.html

http://antwrp.gsfc.nasa.gov/apod/ap080610.html
http://antwrp.gsfc.nasa.gov/apod/ap040804.html

http://antwrp.gsfc.nasa.gov/apod/ap040413.html
http://antwrp.gsfc.nasa.gov/apod/ap071125.html
http://antwrp.gsfc.nasa.gov/apod/ap050928.html

<<A circumhorizontal arc (also popularly called fire rainbow) is a rare optical phenomenon. In order to be observed, the sun must be high in the sky, at an elevation of [46°] or more, and can only occur in the presence of cirrus clouds. As a result it cannot be observed at locations north of [the Arctic Circle] or south of [the Antarctic Circle], although occasionally at higher latitudes from mountains. In areas of Northern Europe it can only be observed around the time of the Summer solstice. If cloud conditions are right it is seen along the horizon on the same side of the sky as the sun.

The arc is formed as light rays enter the [near] horizontally-oriented flat hexagon crystals through a [near] vertical side face and exit through the [near] horizontal bottom face. It is the 90° inclination that produces the well-separated rainbow-like colours and, if the crystal alignment is just right, makes the entire cirrus cloud appear to shine like an undulating rainbow.

Although quite rare, it is not the rarest of all natural phenomenon. The Kern arc and Lowitz arc are even rarer.

A circumhorizontal arc can be confused with an infralateral arc when the sun is high in the sky; the former is always oriented horizontally whereas the latter is oriented as a section of a rainbow, e.g. as an arc stretching upwards from the horizon.>>

[trumpetman012]

I totally agree with Mr Cowley, these are not irridescents clouds, but circumhorizon arc. One more thing about iridescent clouds, is that the sun is not far from the direction of look (maybe a douzen a degree at maximum). For circumhorizon, the sun is at least at 46° from the optic phenomenon. If the picture poster has taken the whole scene, we will see that the sun is way high up from the phenomenon.

Most of the time, the confusion is due to the fact the circumhorizon is very bright. Another thing we can see on the pictures neufer linked, you have repetition of colors for iridescent phenomenon (due to the interferences effect by water droplets) that you have not for circumhorizon (refraction and simple color dispersion effect).

[neufer]

APOD needs to revise a header!
Is there any precedent for this?

http://apod.nasa.gov/apod/ap090512.html

This is not an iridescent cloud nor are water droplets responsible. It is a halo, a circumhorizon arc (http://www.atoptics.co.uk/halo/cha2.htm) produced by refraction in hexagonal plate shaped ice crystals in cirrus. Note the structure parallel to the horizon and the structured gradation of color from red at the top through green to blue. Both are characteristic of the halo.

[bystander]

Art, imo you missed the most spectacular one

APOD: 2003 October 14 - Iridescent Clouds Over Aiguille de la Tsa

[neufer]

Art, imo you missed the most spectacular one
APOD: 2003 October 14 - Iridescent Clouds Over Aiguille de la Tsa

Yes, those are nice iridescent water clouds...
but we are really talking about circumhorizon cirrus arcs
and getting the APOD corrected as it now has been:
http://apod.nasa.gov/apod/ap090512.html
-------------------------------------------------------------
I have a bit of a quibble about the Wikipedia argument that the sun has to be 57.8° high
since that is only technically true if the falling ice crystals are perfectly horizontal.

Allowing for some (minimal) falling ice crystal fluttering
(or, perhaps, non-vertical falling as in the case of horse hair cirrus)
the sun really only has to be about 46° high.

However, a 68° high sun (with 22° high clouds) is still optimal
so a ~58° high sun is probably a reasonable practical minimum.

[Karthik]

Any idea what is that bright object near the top of the image; the one just above what looks like a contrail? Perhaps an artifact?

Thanks

[Dr. Skeptic]

Any idea what is that bright object near the top of the image; the one just above what looks like a contrail? Perhaps an artifact?

Thanks

Look! It's a bird ... It's a plane ... No its Sup ... Aww, its just a plane's vapor trail .

[Karthik]

Any idea what is that bright object near the top of the image; the one just above what looks like a contrail? Perhaps an artifact?

Thanks

Look! It's a bird ... It's a plane ... No its Sup ... Aww, its just a plane's vapor trail .

Actually, I was inquiring about the bright, round object above the vapor trail.

Thanks

[craterchains]

This new forum stinks, , , , , , , , , , , , , , , , , , ,


Anyone else notice that these odd rainbow effects are VERY RECENT ? ? ?

Like in the past decade ? ? ? ? ? ? ? ?

Yer FUNNY ! ! ! ! ! ! ! ! ! !~

[BMAONE23]

Any idea what is that bright object near the top of the image; the one just above what looks like a contrail? Perhaps an artifact?

Thanks

Look! It's a bird ... It's a plane ... No its Sup ... Aww, its just a plane's vapor trail .

Actually, I was inquiring about the bright, round object above the vapor trail.

Thanks

I would say you have 1 of 4 choices

Choice 1) The planet Venus can appear this bright in the daytime sky especially through a polarized lens (but venus is too near the sun to be visible during the day)
Choice 2) The ISS is slightly brighter than Venus now and is also visible in the day time skies.
Choice 3) a mylar balloon reflecting sunlight
Choice 4) most likely a bad pixel on the ccd sensor in the camera.

[Andy Wade]

This new forum stinks, , , , , , , , , , , , , , , , , , ,


Anyone else notice that these odd rainbow effects are VERY RECENT ? ? ?

Like in the past decade ? ? ? ? ? ? ? ?

Yer FUNNY ! ! ! ! ! ! ! ! ! !~

The forum stinks? Funny, I can't smell anything.
Say what you mean for goodness sake, that way people can understand you better.

[Dr. Skeptic]

Any idea what is that bright object near the top of the image; the one just above what looks like a contrail? Perhaps an artifact?

Thanks

Look! It's a bird ... It's a plane ... No its Sup ... Aww, its just a plane's vapor trail .

Actually, I was inquiring about the bright, round object above the vapor trail.

Thanks

Looking at the "full view" there is a similar anomaly (not as bright) dead center of the picture just under the cloud.

[craterchains]

This new forum stinks, , , , , , , , , , , , , , , , , , ,


Anyone else notice that these odd rainbow effects are VERY RECENT ? ? ?

Like in the past decade ? ? ? ? ? ? ? ?

Yer FUNNY ! ! ! ! ! ! ! ! ! !~

The forum stinks? Funny, I can't smell anything.
Say what you mean for goodness sake, that way people can understand you better.

mmmm k, This new forum style and layout are very poor IMHO, the old one was quite adequate. Why change it?
(The Ne likes the colors better?)

There are more Rainbow pictures here;
http://asterisk.apod.com/vie ... =11&t=5334

Also a good You Tube video (there are others too if you search for them)
http://www.youtube.com/watch?v=1NqL9Ez4 ... annel_page

[aristarchusinexile]

mmmm k, This new forum style and layout are very poor IMHO, the old one was quite adequate. Why change it?

To much waste space on the right hand side, but otherise I think the format's okay.

[astrolabe]

Hello craterchains,

I've missed seeing your quote by Will Rogers, always was a wise reminder for me. Good to hear from you.

[neufer]

Any idea what is that bright object near the top of the image; the one just above what looks like a contrail? Perhaps an artifact?

I would say you have 1 of 4 choices

Choice 1) The planet Venus can appear this bright in the daytime sky especially through a polarized lens
(but venus is too near the sun to be visible during the day)
Choice 2) The ISS is slightly brighter than Venus now and is also visible in the day time skies.
Choice 3) a mylar balloon reflecting sunlight
Choice 4) most likely a bad pixel on the ccd sensor in the camera.

I would have guessed Venus except for the fact that it is circular and Venus would be crescent shaped.

Looking at the "full view" there is a similar anomaly (not as bright) dead center of the picture just under the cloud.

How about
Karthik's bright object is from Mars.
Dr. Skeptic bright object is from Venus.

[grump]

This is not an iridescent cloud nor are water droplets responsible. It is a halo, a circumhorizon arc (http://www.atoptics.co.uk/halo/cha2.htm) produced by refraction in hexagonal plate shaped ice crystals in cirrus. Note the structure parallel to the horizon and the structured gradation of color from red at the top through green to blue. Both are characteristic of the halo.

Les Cowley
Atmospheric Optics - http://www.atoptics.co.uk

The explanation of the apod says that the photo was taken using a polarising lens. Would the colours appear if it was not polarised?

- Jim

[neufer]

This is not an iridescent cloud nor are water droplets responsible. It is a halo, a circumhorizon arc (http://www.atoptics.co.uk/halo/cha2.htm) produced by refraction in hexagonal plate shaped ice crystals in cirrus. Note the structure parallel to the horizon and the structured gradation of color from red at the top through green to blue. Both are characteristic of the halo.

The explanation of the apod says that the photo was taken using a polarising lens. Would the colours appear if it was not polarised?

Sure, but they would be more washed out:

The circumhorizontal arc is due to (near Brewster angle) ice crystal prisms refracting mostly vertically polarized light.

The background blue sky color is from little horizontal dielectric dipoles creating horizontally polarized noise light near the circumhorizon arc.

A vertical polarizer helps us to see other planets during the day (as well as circumhorizontal arcs) by cutting out singularly scattered blue Rayleigh scattering.

[Chris Peterson]

A vertical polarizer helps us to see other planets during the day (as well as circumhorizontal arcs) by cutting out singularly scattered blue Rayleigh scattering.

Or more simply, it improves contrast.