Mystery images; what is going on here?

[owlice]

Our gracious host and astrophysicist RJN forwarded the photos below to me. Both of them puzzle me a bit, so I ask for your help in figuring out -- or informing me if you already know -- just what the heck is going on here!

Mystery Image #1
Double Halo in Hong Kong
Copyright: Wah!

Click to view full size image

In this image, the doubling of the halo strikes me as unusual. Usually when there's a double, the inner is the common 22-degree halo, and the outer the rarer 46-degree halo. These halos don't look spaced far enough apart to be 22-degree/46-degree halos. Often a halo will have a reddish inner edge, and the inside of the halo will be a bit darker than the rest of the sky. In this image, both halos have a reddish inner edge, and the "bit darker" part strikes me as being between the inner and outer halo, not inside the inner halo.

So... what's the deal with this image? Is this one halo that is split in some way by a dark band, or is it two halos, and if so, what's making that second halo?

Mystery Image #2
Cloud Shadow
Copyright: Ken Chaney

[attachment=0]C_9123.jpg[/attachment][/i]

The photographer of this image says:

"I took this photo last Saturday afternoon (8/7/10) just as the sun was approaching the horizon at sunset. It appears the cloud with the silver lining is casting a shadow or reflection on the brightly illuminated clouds further west. The photo is not retouched except for minor color/contrast adjustment.

It's not the best of pics, maybe not suitable for APOD, but I thought I'd send it along - I just have no explanation for
the "shadow."

Owl says: I don't, either, though I'm sure one exists. What is it?

[Ann]

Well, next time you see a cloud with a silver lining, remember that it might cast a shadow!

Fascinating image!

Ann

[Beyond]

As to image #1 -- Perhaps the density of the cloud the sun is shinning through has a direct bearing on the Halo? I've never seen a sun halo. I've only seen moon halo's.

As to image #2 -- it looks just like the darker bottom cloud just plain fell out of the higher brighter cloud.

[Henning Makholm]

Cloud Shadow
Copyright: Ken Chaney
The photographer of this image says:

"I took this photo last Saturday afternoon (8/7/10) just as the sun was approaching the horizon at sunset. It appears the cloud with the silver lining is casting a shadow or reflection on the brightly illuminated clouds further west. The photo is not retouched except for minor color/contrast adjustment.

The explanation must be that the rosy-illuminated clouds are between the observer and the dense cloud with the silver lining. They are being illuminated from behind by the sun, except where the sunlight is blocked by the one further away, and thin enough to appear transparent when not illuminated.

[owlice]

I still don't know what's going on with the halo.

I think I get mystery image #2, though; it strikes me as likely that it is a double cloud shadow: http://www.atoptics.co.uk/atoptics/rayim13.htm

[Chris Peterson]

I still don't know what's going on with the halo.

I suspect it isn't really a double halo, but just a normal halo that has a dark internal band. Still cogitating on the optics of such a thing, though.

I think I get mystery image #2, though; it strikes me as likely that it is a double cloud shadow...

I don't see a double shadow. I think it is just a plain, ordinary single cloud shadow. I've shot double cloud shadows before; they look like this:

[owlice]

I still don't know what's going on with the halo.

I suspect it isn't really a double halo, but just a normal halo that has a dark internal band. Still cogitating on the optics of such a thing, though.

Both rings have red inner edges, though, which strikes me as more like a double halo than a single one with a dark internal band. If it weren't for the reddish inner edge on the external halo, I'd be right there with you on the single halo with a dark internal band!

I don't see a double shadow. I think it is just a plain, ordinary single cloud shadow.

Ooo, I'll buy that! Yes, there's just one shadow there.

That double shadow image is very cool; thanks for posting it!

[alter-ego]

I still don't know what's going on with the halo.

I suspect it isn't really a double halo, but just a normal halo that has a dark internal band. Still cogitating on the optics of such a thing, though.

Both rings have red inner edges, though, which strikes me as more like a double halo than a single one with a dark internal band. If it weren't for the reddish inner edge on the external halo, I'd be right there with you on the single halo with a dark internal band!

Yes, there are two halos
The double halo is likely explained by pyramidal ice crystals . A 9°halo can accompany the "double halo" but not necessarily. Allowable crystal shapes can preclude both a 9°and 22°halo. Also, other halos can be visible depending on crystal shape and orientation. These pryamidal crystal halos are often referred to as odd radius halos.
Wah! claimed the inner halo in his picture is a standard 22º halo, but I think he is wrong. I've analyzed the relative diameters of the halos in his picture and found they overlap very well (within approx. ±0.5º) with the ~20º and ~24º (radius) halos generated by pyramidal crystals (See simulation below, I used HaloSim software). This correlation indirectly eliminates the 22º halo. Real pyramidal halos can swamp a 22°halo with their nearby components, but that does not appear to be the case here. I believe Wah! made an eyeball estimate, or just assumed, that the inner halo is 22º because it looks close. Given that Wah!'s picture details, and post, lack any direct data from which the halo radii can be directly determined (photo metadata incomplete), the logical solution is a straight forward, partial pyramidal crystal halo set.
The simulation below uses pyramidal crystals (inset shows crystal side view). Normally, a pyramidal ice crystal has a middle section that is the source of the 9º halo - so no middle section, no 9°halo (and no 22°halo).

Halo simulation having same approximate FOV as Wah!, FOV ~80º x ~54º
- Red scale shows 10º (major) and 1º (minor) tick marks
- Inset shows edge-on view of crystal shape used in model
NOTE: Halo behavior is more sensitive to certain crystal shape parameters than others.
Displayed crystal shape may not be the exact, but I believe the central section must be absent

Picture by Wah!

Similar to APOD, Atmospheric Optics has a searchable OPOD data base, here are a few samples (also showing crystals and light paths):
Lunar Halo Set
18º to 24º Halos
Full Pyramid Crystal Halo Set

Here are other examples from another site:


Archive, Friday, February 08, 2008
<<The image above is stacked from 118 images which were taken on a frozen river in Rovaniemi. Between each shot the camera, which was on tripod, was moved some 20-30 cm sideways. This serves to decrease noise of the snow surface in a similar manner as cloud movement decreases the noise in stacking of sky halos.>>

Keep a look out for odd radius halos as they are most likely caused by pyramidal ice crystals.

I want to acknowledge Les Cowley and his Atmospheric Optics web site for, over the years, providing insightful help in understanding opto-atmospheric phenomena and great pictures.

ae

[owlice]

ae, thank you!!

Wah! sent this in as a "double halo." I was the one speculating as to what kind of halo; the more usual are 22° and 46° (which I know from the Atmospheric Optics site ), and this image just didn't look as though it was those halos.

Odd radius halos it is! Or rather, they are. Thanks!!

[guess who]

It appears the cloud with the silver lining is casting a shadow or reflection on the brightly illuminated clouds further west... I just have no explanation for the "shadow."

Owl says: I don't, either, though I'm sure one exists. What is it?

Why do you think "brightly illuminated clouds" are actually "further west"? The fact that they have shadow on themselves should have made you think they are on the east side.

[Chris Peterson]

Why do you think "brightly illuminated clouds" are actually "further west"? The fact that they have shadow on themselves should have made you think they are on the east side.

I think we all agree that the bright clouds must be to the east (in front of) the dark clouds- at least, the shadowed part. But if the shadow wasn't there as evidence, would you think that? There is a powerful illusion that all of the bright clouds are behind the dark clouds. In fact, I suspect some are- but the clouds appear fairly "flat", so we don't really see their actual 3D structure and position very well. It is often difficult to determine when you have layered clouds which are in front and which behind- unless you get shadows like this, or shadows from contrails, to put things in perspective.