Doubled Halos?
This image was taken in Fukushima pref. in Japan, by Mr. Tasan (handle), May 5, 2006. There are two halos -- the lower one seems a 22 degree halo, but what is the upper one? I think it can be a 24 (or 23?) degree halo, a circular Lowitz arc or a 23 degree plate arc. What are your opinions?
posted by Anonymous at 16:57
3 Comments:
Interesting there I think by the looks of that diffuse arc I think its upper 23 degree plate arc.
Yeah, looks like a upper 23 plate arc. Parry or Lowitz would be expected to be sharper. The halo below can be either 23 halo or 22 halo. Pyramid crystals usually have such a large tilt that 23 halo is produced below 23 plate arc. But here it looks like there is bit of a pooly developed tanget arc look in the halo below, so I think it's 22 stuff. Knowing sun elevation might give us more definitive answer.
As Können point out, one way to check if the arc is 22° or 23° stuff is to check its polarization. However, polarization may be hard to see, so if you have digital camera and tripod, you can photograph polarization the shift of an halo.
Remember to make notes about the orientation of the filter while turning it in front of the lens between photos. Do not use wide angle lens since the polarization shift may be too small to be seen in wide angle photo.
I made an animation of 23° plate arc polarization in 6 May display. You can find the animation here. Polarization shift is not very big. Two photos were taken with Nikon digital SLR with 50 mm lens while turning polarization filter 90° between photos.
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