Circumhorizontal arc, or something else?
Posted January 14, 2013
It's always nice to receive photos of weather phenomena, interesting clouds, and optical effects from people in the community, and to identify the type of phenomena and the processes behind it. On December 28th, we got an e-mail from Cassie Mentha, who sent along the first couple of photos you see here, with this note:
"I took these photos this morning around 9. Thought you might like to see them! I have included the originals and edits - a little contrast makes the arc more visible. I assumed it to be a circumhorizontal arc, although the bits I read about those arcs say the sun must be at an angle of 58° or greater to create a true circumhorizontal arc, so I'm not certain. Any thoughts?"
As it turns out, there is no circumhorizontal arc in the photos Cassie sent, but there are certainly a couple of other nice looking effects visible here. The thin layer of cirrus clouds in the pictures, composed of tiny ice crystals, had spread into the area the night before, and some of you may have seen the "haloed moon" photo that we pasted on our WRAL Weather Facebook Page around that time. That was taken the night before Cassie shot these images. By the time she did so, some subtle changes had occurred in the type and distribution of ice crystals in the clouds. A fully formed and reasonably bright halo is typically associated with large numbers of six-sided, columnar ice crystals that are "poorly oriented, " meaning they are tumbling such that the individual crystals are tilted in a variety of directions. There is still some of that evident in Cassie's pictures, as you can make out a faint 22-degree halo that completely encircles the sun. However, there are much brighter arcs of rainbow color directly to the left and right of the sun that are caused by a concentration of six-sided, but plate-like ice crystals that are oriented with their wider dimension parallel to the ground (kind of like millions of tiny crystalline frisbees). That orientation causes the refracting effects of the crystals to be concentrated at the locations you see in the photo. Because of the location and brightness of these arcs, they have come to be known as "mock suns," "sun dogs," and more technically, "parhelia."
Sun dogs are most easily formed and seen when the sun is low in the sky not too long before sunset or after sunrise, and are visible rather frequently. The circumhorizontal (also called "circumhorizon") arc that Cassie mentions is caused by the same plate-like crystals that create the sun dogs, but is much less frequently seen, in part because as she notes in her message, they are only visible when the right kind of clouds are in place AND the sun is at an elevation of 58 degrees above the horizon or more. At our latitude, the sun never reaches that height at all from about mid-September until late March, so seeing a circumhorizontal arc around here is a Spring or Summer event. I happened to see one during a summer in the mid 1980s in eastern NC, not far from Pamlico Beach, and caught a picture of it, which is the third image you see here.
For those of you interested in more details about how these phenomena occur, along with some galleries of nice photos of each type, I've included some links to the halo, sun dog and circumhorizon arc sections of the Atmospheric Optics web site. Thanks again to Cassie for her photos and message...