owlice wrote:Chris Peterson wrote:owlice wrote: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
![Smile :)](./images/smilies/icon_smile.gif)
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).
![Double Halo Solution.JPG (72.38 KiB) Viewed 457 times Halo simulation having same approximate FOV as Wah!, FOV ~80º x ~54º<br />- Red scale shows 10º (major) and 1º (minor) tick marks<br />- Inset shows edge-on view of crystal shape used in model<br />NOTE: Halo behavior is more sensitive to certain crystal shape parameters than others.<br />Displayed crystal shape may not be the exact, but I believe the central section must be absent](./download/file.php?id=989&t=1&sid=60d92d34953e34d716fa85e5c04dfdc9)
- 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
![DoubleHalo1.jpg (171.82 KiB) Viewed 457 times Picture by Wah!](./download/file.php?id=990&t=1&sid=60d92d34953e34d716fa85e5c04dfdc9)
- 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:
![Image](http://www.ursa.fi/blogit/media/blogs/halos/2010mixed/07241438small.jpg)
![Image](http://www.ursa.fi/blogit/media/blogs/halos/elmarschmidt/or2usmsmall.jpg)
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.
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