Wednesday, October 31, 2012

Circumzenithal Arc

Driving back from work the other day, Sunday October 28 to be precise, I noticed a bright sundog on the right side of the sun as we came around the corner at Button Point. I looked straight overhead, hoping to see a Kern arc, and there was the so-called circumzenithal arc, which does not make a complete circle around the zenith. When I got home, I ran in, grabbed my camera, took one quick shot of a fairly faded arc before the batteries died and before the arc faded almost completely in fickle cirrus clouds. After changing the batteries, and after the arc came back a couple minutes later, I took the above photo, where the arc is still faint.

By the time I took this picture, two sundogs had formed on opposite sides of the sun (the right one is not visible behind a building to the right; the left one is the bright, washed out spot on the left), and a nice halo had started to develop.

Unenhanced circumzenithal arc looking straight up.
I repeatedly looked straight overhead to see if the rare Kern arc, which completely circles the zenith, would develop. One never did.

Circumzenithal arc looking straight up, somewhat enhanced.
A view of just the circumzenithal arc.
A zoomed in view of the same arc, taken a moment later.
The halo around the sun continued to develop.
Halo mimics the shape of the tree.
Sundog to the left of the sun.
The sun was fairly low in the sky between 4:32 and 4:47 pm PST when I took these photos; it was probably between 10 and 15 degrees above the horizon, a little lower than the ideal 22 degrees.

The following day, I ran outside with my Brunton at about 4:50 pm to measure the sun angle, which was at 10 degrees above the horizon. The days are getting shorter by only 1 to 2 minutes each day, with minimal time difference in sunset between the two days in question, so the sun was probably at 10 degrees or slightly higher.

In fact, photos in which the ground isn't cropped out show that the height of the sun above the horizon is less than half of the 22 degree halo around the sun, although some distortion by the camera cannot be ruled out.

All photos except the one described as unenhanced, were enhanced by increasing color saturation and contrast, and by adjusting the brightness up or down as needed. They were then exported to a smaller size (1000px long dimension). After export, they were minimally sharpened to diminish blurriness imparted during export.

The first two photos both consist of two photos that were stitched together and cropped prior to enhancement, export, and sharpening. My semi-wide angle lens won't show a circumzenithal arc and the lower part of the sky in one shot.

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