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Sunday, March 15, 2009

Two, two, two haloes in one!

Note: I originally used the soelling "haloes" throughout this post. My Webster's New World Dictionary (Fourth Edition) lists the plural of halo as "halos" or "haloes." Minnaert uses "halos" throughout his book, so I revised my spelling to match his...and inadvertantly missed the title. Since that spelling is also in the url for this entry, changing the title might revise the url. So I'm leaving it as "haloes."

I dropped off my tax information at an accountant's today - my tax situation has become too complex in recent years for me to do my taxes on my own, something I actually used to enjoy. On the way out I noticed a hazy wash across the sky. I knew then to block the Sun with my fist and look in its general direction through by polarized clip-on sunglasses. And what I saw was...odd.

I was hoping for a halo, and maybe a circumzenithal arc. What I saw was something I've never seen before: a halo, all right, but with some other arc touching it, like a hyperbola with the Sun at one focus.

I only had a camera phone with me. I grabbed a photo, but since I live just a few minutes from the accountant's office, I decided to try to get more images when I got home, if the halos were still present.

They were.

These images were taken between 1:11 PM and 1:13 PM on Saturday, March 14, 2008. I used a hoary old evergreen to block the Sun. Blinded as I was by the glare in the sky, I did my best to capture the halos. I took several images, but most of them came out looking nearly identical. Wispy cirrus clouds are visible, probably providing the ice crystals which formed the halos. Oddly, the point where the two halos touch seems to be offset very slightly counterclockwise from a point directly above the Sun.

Here's another image, using an old gaslight to block the Sun. The deviation of the two arcs on the right is more obvious here. Note the trees reflected in the glass.

So the question is, what were these halos?

M.G.J. Minnaert provides a comprehensive list of halo phenomena in his classic and essential Light and Color in the Outdoors. The lower halo, a segment of a circle that went completely around the Sun, is the 22 degree halo. The other halo appears to either be Parry's arc, or the upper tangential arc, or possibly both.

While trying to image the right side of the 22 degree halo, I actually captured something quite odd:

This appears to be a nearly-vertical white line in the sky, just to the right of the evergreen. Remember that I was mostly blinded by the glare from the sky, so I didn't really see this at all - I thought I was photographing the edge of the halo. So what is this? None of the optical phenomena that I can find on Minnaert's list match this at all. It also isn't an artifact from a defect in the lens or the imager, because it also showed up when I rotated the camera 90 degrees.

My best guess is that this is a cirrus cloud, since that would make it parallel with the cirrus clouds seen in the previous images. Or perhaps it's the edge of a larger cloud, or a linear indentation in a cloud. It's also possible that it was an otherwise unnoticed contrail. Or perhaps it is an artifact, an internal reflection of some sort - I guess if I took a picture with the camera held at an angle to the horizontal, we would have known for sure.

For now we will have to write it off as just one of those things. But keep a camera handy, and keep your eyes open. You never know what you might see! And if you do get any cool halo images, please share them with the rest of us!


whimsical brainpan said...

Very cool! Thanks for sharing.

Missy said...

I love that effect!