2006. május 9., kedd

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

5 megjegyzés:

  1. Frank Nieuwenhuys2006. május 11. 13:07

    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....

  2. 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.

  3. 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.

  4. Frank Nieuwemhuys2006. május 13. 23:16

    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.

  5. 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.