What looks like a rainbow but doesn't bend?
Posted June 14, 2016
I got a question last week from a viewer named Gail Sievert, who had seen what struck her as a straight-line rainbow, with no sides, on Sunday, June 5, over the Kenly area.
A little follow-up with Gail led to some added information about the colored band she's seen, and better yet, she had a photo, which I've included here.
Although it sports the spectrum of color, this is not a rainbow, but a form of halo that can occur when the sun is very high in the sky and there are some thin cirrus clouds made up of well-formed, six-sided, plate-like ice crystals that are mostly oriented with their flat sides parallel to the ground.
The result is called a "circumhorizon arc," and they are typically visible several times a year, though you have to happen to be looking in the right area at the right time. Because the sun has to be at least 58 degrees above the horizon, they can only occur here in central NC between late March and mid-September (in fall and winter the sun never climbs that high over us).
You can read more about them, see some other photos in a gallery and find some diagrams that explain how they form, at the circumhorizon arc page of the "Atmospheric Optics" web site, a really nice resource to check when you see almost any interesting play of light and color in the sky. I've included links here to both their main web page and the page specific to the type of arc in Gail's photo.