Wednesday, May 10, 2006

Lower 23 degree plate arc observed in Ohio USA 5-9-06

While on break time at work I decided to check the sky and saw a 22d halo with a rather suspicious brightening that encircled the lower half of the halo. As the cloud moved on a rather bright arc appeared and I thought circuscribed halo no it was a lower 23 degree plate arc!! I could tell because there was no circumscribed halo which goes all the way around while lower 23 degree plate arc tails off away from the circular halo. weak 18 degree halo was also observed. The photo has been given an unshap mask to show the halo better. You can clearly see the arc tailing away from the halo while diminishing in intensity

15 Comments:

Blogger Marko Riikonen said...

Good catch Michael. Can you give us also the sun elevation, it would be nice to know.

10 May, 2006  
Blogger Michael Ellestad said...

Thanks Marko I'll do some sims with halosim and see if I can get an elevation match. What would this observation be number 2 or number 3?

11 May, 2006  
Blogger Michael Ellestad said...

I did a simulation with halosim and judging by the way the arc tailed away from the cirular halo the elevation: 67 degrees which is quite high.

11 May, 2006  
Blogger Marko Riikonen said...

You got the time the photograph was taken? Knowing your geographical coordinates you can actually calculate the sun elevation. Coordinates you get from here and calculator is here. Actually, as you are residing in the US, you need only the latter link, it will automatically get your coordinates by just typing in the town you live.

This was the third observations of lower 23°, I think.

11 May, 2006  
Anonymous Frank Nieuwenhuys said...

The 'lower arc' might be the lower suncave Parry arc as well. When using (non-pyramidal) hexagonal prisms in Parry orientation, while solar elevation is 67 deg., HaloSim shows the same display....

11 May, 2006  
Blogger Michael Ellestad said...

That is no lower parry arc Frank because there is no circumscribed halo present or else you would see circumscribed halo as a uniform oval all the way around 22d halo.


Marko The time I got photograph was 1:15 PM.

11 May, 2006  
Anonymous Anonymous said...

Here, we have an interesting and important point, in my opinion. Michael, why has the circumscribed halo to be uniform and complete, to identify the 'tailing away arc part' as a lower suncave Parry arc?
The majority of the halos I observe here in Holland are partial and incomplete.
The 'lower arc' you observed and photographed on May 9th last, could be interpreted as a combination of: 22 deg.halo, 22 deg.lower tangent arc, lower part of the circumscibed (22 deg.)halo and ... lower suncave Parry arc.
When solar elevation is 67 deg. the circumscribed halo is very close to the 22 deg.halo, and hence difficult to identify (see HaloSim). But: the 'tailing away arc' could easily be identified as Parry.
And, if we go on with the presuming of the importance of uniformity and completeness: there is a disadvantage, the upper 23 deg. plate arc was not present.
Michael, it's not, I would envy you the lower 23 deg.plate arc, but we have to be very cautious in interpretation of displays.

12 May, 2006  
Blogger Marko Riikonen said...

I got 66-67° for sun elevation. Your estimate was good Michael!

Yeah, the cloud in Michael's display was not uniform, because all hypothesis for the lower arc (22° tangnet arc, Parry arc, 23° plate arc ) would call for explaining the missing upper part.

22° tangent arc is out of question because at this sun elevation it's very close to circular in shape. You can see Michael's arc is "tailing away" in manner that would not fit tangent arc.

So we are left with Parry and 23° plate arc.

If it were Parry arc there should be hell of a bright lower 22° tangent arc. You can always make simulations with little column crystals and plenty Parry crystals to make the tangent arc weak, but any dedicated observer should know that Parry arcs occur with brighter tanget arcs. (There are some exceptions in diamons dust displays from Antarctic and Finland, but I think we can leave these special cases aside now).

Lower 23° plate arc appearance is only a question of sun elevation. There are not much observers residing at locations where sun rises high enough for lower 23° to be seen.

Michael said he had also 18° plate arcs in the display. If so, case for 23° plate arc strenghtens. And considering the great number of solitary upper 23° plate arcs we seen in Finland - some of them rather bright - Michaels interprepatation seem all the more correct (though of cource lower 23° plate arc needs a crystal with upper pyramid prism that can make also the 18° stuff depending on the sun elevation and pyramid prism sizes).

Frank, please don't post your comments twice. Or if you do, you can delete the other. When signed in, trash can appears in the bottom corner of the comment (and check the box "remove forever").

12 May, 2006  
Blogger Jarmo Moilanen said...

I would like to remind that at that high sun elevation (over 65°) ray path for 23° upper plate arc and 18° plates arcs are not very favourable any more. Those will be faint at sun elevation 70° and disappears totally when sun is 75°. Of course, this is not fully true since crystals are not very often perfectly oriented.

Case would be the same with (upper suncave) Parry arc. Although Parry can be identified in simulations still at 75° sun elevation.

So the lack of upper part of display does not have to be caused by uneven distribution of clouds. In matter of fact, both 23° upper or upper suncave Parry arc should be already so much higher at this sun elevation that they would be outside Michael's photo.

Michael have some halo activity in upper part of photo and that seems to be a circular halo (22° or 23° halo). Since circular halo seems to be quite complete, we can not claim that uneven clouds are causing lack of upper part of the display. This means that we can say that there is no traces of circumscriped halo.

This support 23° lower plate arc explanation strongly, but we always have possibility that we have Parry without tangent arcs. Then we have to remember that Parry oriented columns can produce parhelic circle too (not always thou) and there is no traces of that either.

I think that Michael got 23° lower plate arc. However, some acccurate measurements would be nice to do with that arc if possible (we should know the sun location exactly)...

12 May, 2006  
Blogger Marko Riikonen said...

Good point Jarmo about the upper Parry and 23° plate arc being out of the photo. At solar elevation of 67° they are about 27-28° from the sun.

See also another observation of 23° lower plate arc.

You got more photos of the display Michael? Maybe you could put them in your web page?

12 May, 2006  
Blogger Michael Ellestad said...

Whoa I said there was fragment of 18d halo not 18d plate arcs. Later on around 4:00PM while at the Wal-Mart I observed upper 23 degree plate arc and faint 18d plate arcs but they faided to rapidly

Yes I have more photos of display but I have used up all my 15 MB of free space on my Geocities account. I need to get domain that will have a lot of or unlimitied space for a yearly fee.

12 May, 2006  
Blogger Marko Riikonen said...

Should read more carefully what people write, but somehow sometimes my brain just does not get it right no matter how many times I read. Anyway, seems like the case is settled. One last point about the identification. Sharpness of the arc can also tell about whether the halo is Parry or 23° plate arc. If the halo arc is really thin and sharp it's very likely Parry arc, but less well defined cases can rather be 23° plate arcs.

13 May, 2006  
Anonymous Frank Nieuwemhuys said...

Marko, you made the two following remarks, concerning the relationship between Parry arcs and 22 deg. column arcs: "If it were Parry arc there should be hell of a bright lower 22° tangent arc."
"but any dedicated observer should know that Parry arcs occur with brighter tanget arcs."

In my long halo observing experience, I noticed several times here in Holland, the 22 deg. column arc was (remarkably) less bright than the Parry arc, and there are even a few cases, Parry was visible without the 22 deg. c.a. The last time I catched Parry arc without column arc was on October 17th 2005. I quote from my observing report of that day:

"14u30-15u25 bijzonnen (omstreeks 15u00 opvallend helder en met 'staarten'; foto-opnamen gemaakt), bov.raakboog, Suncave
Boog van Parry (om circa 14u35 duidelijk en vrij wijd zichtbaar
op (naar schatting) 3 gr. boven de bov.raakboog; gedurende enige
tijd werd de Parryboog zonder de bov.raakboog opgemerkt;"
The last sentence says: 'during a short time, the Parry arc was visible without the upper tangent arc (=22 deg.column arc)'

According to my observing experiences here in Holland, it's not systematic, the Parry arc should be visible, together with a bright 22 deg. column arc.

14 May, 2006  
Blogger Günther Können said...

Just when a next alleged 23 degree arc appears: checking the polarization of its inner edge can help to decide whether it is a 23 arc or a 22 Parry arc. For the 23 one, the polarization should be perpendicular to the scattering plane; for the 22 arcs it should be in the scattering plane. So keep your polarizers with you.

15 May, 2006  
Blogger Jarmo Moilanen said...

Frank, are you really sure that all those Parry arcs you are refering are really Parry arcs? How you can be so sure? Have you measured them accurately from photos or have you check their polarizity?

As we all should know, Parry arc and 23° upper plate arc behave similar way. They even looks alike each others. 23° plate arc is located only degree a so further away from the sun and it is usually a little bit shorter and more often it may have diffuse tips.

23° plate arc can be visible with 22° tangent arcs too. If 22° tangent arc disappears and Parry stays on the sky as in Frank's observation, I think that it probably suggest that the Parry was in matter of fact a 23° plate arc.

We have to take this down to ice crystal level to understand these arcs. Parry and tangent arcs ar made by columns. That is why they usually are seen together and only few really convincing exceptions are known.

23° plate arc is from plate like pyramidal crystals. So it can be present without any hint of 22° tangent arcs or even 22° halo or parhelia. Because it has quite simple and favourable near wedge ray path, it can be seen alone or with 23° halo only.

I think that all Parry arcs I have seen have been fainter than 22° upper tangent arc as Marko claimed. And I have seen some good ones too. I have not seen any Parry arc without 22° tangent arcs as far as I can remember now. Unfortunately, I can not be sure that all Parry arc I have seen are really Parry arcs and same with 23° plate arcs since in many cases good measurements are almost impossible and polarization is not always so easy to check.

There is lots of Parry observations which should re-evaluate since many of them may be 23° plate arcs. Maybe some observed 23° plate arcs may be Parry arcs but that is not so common error...

23° upper plate arc is probably the most common odd radius halo and one of the most common rare halos in general.

17 May, 2006  

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