by Dr. Jeffrey L. Kretsch

Circum Zenithal Arc 400px

Circumzenithal Arc

I have watched and logged various atmospheric phenomena for over forty years and have enjoyed pointing them out to others. Going beyond rainbows, for the first ten years I noted rings (halos) and sundogs, and I had read about the many other atmospheric phenomena. Although I kept a log, I never noted more than rings and sundogs until a day in October 1981 when I saw a full display of rings, arcs, and the circumzenithal arc. Upon seeing those I started keeping a closer look at the sky and starting seeing other sunlight phenomena, some fairly frequently.

You learn to see. I hope in this brief article to help others to see these more often. This isn’t intended to be a comprehensive review of the phenomena as so many have done before (see references for some examples). I hope by this short article to have people use those moments when walking outside to briefly scan the sky and maybe catch a glimpse of sun arcs and other parahelic phenomena. Looking at what nature has to offer is inspiring. Take interest in the open air sunlight and see what beauty it has to offer.

Ring and Circumzenithal Arc 400px

Upper part of 23 degree Ring (bright ring near date mark) and Circumzenithal Arc (faint rainbow arc)

With a few guidelines you can look for these objects ,enjoy them, even photograph them. Most common are rings that can seen almost anytime with a cloud haze when then sun is over 40 degrees from the horizon. When the sun is low in morning or evening look for sundogs. These "false suns" occur at the quadrants of a ring 23 degrees in radius. Many times only two sundogs (left and right) of the sun are visible. Sometimes a third sundog north of the sun will be seen. If the sun is high in the sky, all four quadrant sundogs may be visible as well as a connecting ring.

Circumzenithal arcs are seen when the sun is below 45 degrees in the sky.  If you search the sky about 45 degrees above the sun, nearly overhead you might find a ring of bright colors centered about the point overhead, the zenith.  Hence the name circumzenithal arc. The colors can be quite strong.

Another kind of arc can be found in the high summer months when the sun is high along the southern horizon. It is called the circumhorizontal arc. Its colors can be more pastel in nature, and I find it more rare than the circumzenithal arc.


Circum Horizontal Arc 400px

Circumhorizontal Arc

To get an idea of the frequency of some of these phenomena I totaled up I total up the number of my sightings of each of these phenomena over the past two years (Sep 2014 to Sep 2016): Sundogs 31 sightings, Rings 54 sightings, Circumzenithal Arcs 13 sightings, Circumhorizontal Arcs 6 sightings. Based on my records from the 1990s here is a frequency chart of different parahelic and similar phenomena. As you can see they can occur anytime but favor colder months since these arcs are created by ice crystals in the upper atmosphere.

Seasonal Frequency

I find that these are more easily seen if you use polarized sunglasses. So for a little investment of time looking at the sky may provide unique views of nature at work ... and make a stress reducing walk even more enjoyable.

M. Minnaert, “The Nature of Light and Color in the Open Air”, Dover Publications, 1954
Robert Greenler, “Rainbows, Haloes, and Glories”, Cambridge University Press, 1980
Tim Herd, “Kaleidoscope Sky”, Abrams Books, 2007
Google Search “Atmospheric Optics”