APOD: Colorful Arcs over Buenos Aires (2015 Dec 15)

[APOD Robot]

Colorful Arcs over Buenos Aires

Explanation: What are those colorful arcs in the sky? Like rainbows that are caused by rain, arcs of sunlight broken up into component colors can also result when ice crystals floating in Earth's atmosphere act together as a gigantic prism. The top color arc is more typical as it is part of the 22 degree halo surrounding the Sun when hexagonal ice crystals refract sunlight between two of the six sides. More unusual, though, is the bottom color arc. Sometimes called a fire rainbow, this circumhorizon arc is also created by ice, not fire nor even rain. Here, a series of horizontal, thin, flat ice crystals in high cirrus clouds refract sunlight between the top and bottom faces toward the observer. These arcs only occur when the Sun is higher than 58 degrees above the horizon. The featured sky occurred to the northwest in the early afternoon last month over a street Diagonal of La Plata City, Buenos Aires, Argentina.

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[Ann]

That's really beautiful!

And congratulations to Sergio Montúfar, one of the stalwarts of the Recent Submissions forum, which he graces with so many fine pictures!

Ann

[saturno2]

Beautiful image, yes, yes.

[Peter Smith]

Here, a series of horizontal, thin, flat ice crystals in high cirrus clouds refract sunlight between the top and bottom faces toward the observer.

The refraction is not between the top and bottom faces: they are parallel so light refracted between them would not change direction. The refraction is through a 90 degree corner of the crystal formed between a vertical side face and a horizontal bottom face.

[neufer]

Here, a series of horizontal, thin, flat ice crystals in high cirrus clouds refract sunlight between the top and bottom faces toward the observer.

The refraction is not between the top and bottom faces: they are parallel so light refracted between them would not change direction. The refraction is through a 90 degree corner of the crystal formed between a vertical side face and a horizontal bottom face.

• Peter has a point!

index of refraction of ice squared = 1.716
critical sun angle = 57.8º = sin^-1(sqrt[1.716 - 1])
circumhorizon arc angle = cos^-1(sqrt[1.716 - sin²(sun angle)])

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Code: Select all

sun angle    circumhorizon arc angle
-------------------------------------
   57.8º         0.00º
   58º           3.24º
   59º           7.87º
   60º          10.63º
   61º          12.78º
   62º          14.61º
   65º          18.94º
   67.86º       22.14º (max. bright!)
   70º          24.12º
   80º          30.25º
   90º          31.20º

[Boomer12k]

Awesome!!

:---(===) *

[starsurfer]

Very nice and colourful!

[KryptoCleric]

Why are some of the Power Lines that cross the image bent?

Was this a panorama of some type?

[geckzilla]

Why are some of the Power Lines that cross the image bent?

Was this a panorama of some type?

The whole image is bent. The street, all the wires, and the buildings are all distorted by the wide-angle lens. That one line crossing the middle is just the most apparent. One might guess that the thick drooping lines at the right side appear to droop less due to the distortion.

[Tirth03]

I did not clearly understand the reason behind the formation of bottom arc and the 58 degree value for Sun to be above the horizon. Any reference on it will be very helpful to me!!

[neufer]

I did not clearly understand the reason behind the formation of bottom arc and the 58 degree value for Sun to be above the horizon. Any reference on it will be very helpful to me!!

Ideally, the Sun is 22.14º from zenith (i.e., 67.86º from the horizon) such that the 90º circumhorizon arc ice prism is symmetrically positioned (with its minimal 45.72º refraction angle) to produce a circumhorizon arc 22.14º above the horizon.

http://asterisk.apod.com/viewtopic.php? ... 38#p251538

However, if the Sun is at zenith then the
entrance angle of the asymmetrically positioned ice prism is 90º (i.e., parallel to the side entrance surface).

Contrariwise, if the Sun is 32.2º from zenith (i.e., 57.8º from the horizon) then the
exit angle of the asymmetrically positioned ice prism is 90º (i.e., parallel to the bottom exit surface).

Thus the Sun is limited to being <32.2º from zenith (i.e., >57.8º from the horizon) for the fixed circumhorizon arc ice prism (with its maximal 57.8º refraction angle) to work.

[whiskybreath]

I looked out of my office at Orapa, Botswana, one day, and saw a triple cross in the sky. Photos taken with a blackberry (including the 9:00 BA Flight from Joburg (!)) but I don't know how to upload to this site without having uploaded to some other website!
A beautiful sight.

[neufer]

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I looked out of my office at Orapa, Botswana, one day, and saw a triple cross in the sky. Photos taken with a blackberry (including the 9:00 BA Flight from Joburg (!)) but I don't know how to upload to this site without having uploaded to some other website!
A beautiful sight.

Below the "Save draft" "Preview" "Submit" buttons tap on "Attachments" & "Add files"

[bystander]

Below the "Save draft" "Preview" "Submit" buttons tap on "Attachments" & "Add files"

That's only available to established members.

[geckzilla]

Below the "Save draft" "Preview" "Submit" buttons tap on "Attachments" & "Add files"

That's only available to established members.

Or to anyone who signs up for an account and then simply asks me. No need to spam for the 10 post minimum!

[Astromontufar]

That's really beautiful!

And congratulations to Sergio Montúfar, one of the stalwarts of the Recent Submissions forum, which he graces with so many fine pictures!

Ann

Thank you for your kind words Ann!

Sergio

[Astromontufar]

Why are some of the Power Lines that cross the image bent?

Was this a panorama of some type?

It's a panorama, sadly we have a lot of power lines all over the place, almost impossible to get rid of them.

[DavidLeodis]

This nice image was used as the Earth Science Picture of the Day (ESPOD) on December 6 2015. For anyone like me that likes to know when a photo used an APOD was taken the information with the ESPOD states it was taken on November 21, 2015.

A quibble of mine is that the photographers name/coloured icon to the top left is distracting and would be much better placed on the bottom left or right corner. Sorry for that quibble Sergio as it is otherwise a nice photo.

[Whiskybreath1]

Many thanks those above! Now engaging the process...