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Handouts and illustrations:

A flock of sundog(pdf)


Strange Irridescent Clouds
Chasing Circumzenuthal arcs

Parhelia: Chasing the circumzenuthal arc




I've seen three circumzenuthal arcs. One was fabulous, brighter than any rainbow. Unfortunately, upon seeing the arc, I had to decide whether to enjoy it or get it on film. By the time a got my camera, the arc was fading, and I snapped the picture (shown left) just at the arc was disappearing (I snapped three photos in succession and the arc was on only the first).

Being on color slide film, this arc proved difficult to scan. I finally succeeded by using a slide duplicator lens on digital camera. Then, I combined two of these exposures in Photoshop using its high dynamic range features. In short, I had to manipulate the computer image in order to show what is clearly shown on the film.

On Mar 6, 2008, I observed similar atmospheric conditions, so I expected an arc to form.

The conditions that say "Circumzenuthal arc approaching" are captured in this photo:

1. Sun near horizon

2. cool temperature

3. Wispy cirrus clouds (contrails from jets work too)

4. and sundogs:

So I waited with my camera.....(see below)

And here it was:

....OK. Not exactly the arc I was hoping for. Notice that I had to manipulate the image to enhance the arc, but it IS THERE! Look carefully in the center and you will see what I saw and what I had to persuade the computer to bring out. The arc did occur as predicted. The conditions were close, but not ideal. I suspect the larger cloud bank low on the horizon was dampening the effect.

A better one, photographed on 1 January 2008 is shown below. I had my 50-200 mm zoom lens on the camera and later wished I had a wider field offered by 18-30mm focal lenth settings typical of general purpose lenses on digital cameras. Or, if you have a standard format camera (e.g., film, 35 mm), I suspect a full circumzenuthal arc will fit within the field of view of a 50 mm focal length lens.