Ice crystals color the skies Saturday afternoon
Posted August 24, 2015
This past Saturday brought a few hours during which we had some upper-level moisture pass across the region from the west, leading to periods of high cirrus clouds and contrails from aircraft, all composed of ice crystals. Some of those turned out to be concentrations of well-formed plate-like hexagonal crystals with their faces oriented parallel to the ground, while some others formed into a mixture of crystal types that likely included some hexagonal columns that were more randomly oriented.
At times, these crystals acted to refract sunlight in ways that, while not exceedingly rare, nonetheless aren't something we see all that often. The plate-shaped crystals produced a rainbow-colored band called the "circumhorizontal arc" while the less uniform mix of columns and crystals produced a "22-degree halo" at times. Quite a few people in the area saw it (wish I'd been out at that time, but I missed it) and sent us photos or posted them to the WRAL Weather Facebook page. I've included two of them here.
The first is from Chas Armstrong, who nicely captured both the circumhorizontal arc in the lower part of the picture and a portion of the 22-degree halo in the upper portion. You'll notice both band of color involve greater amounts of refractive bending of shorter wavelengths compared to longer. This leaves the inner portion (closest to the sun) with red and orange colors, while the outer portion is blue to violet. Only the circumhorizontal arc is visible in the pool photo contributed by Austin Brooks. Thanks to both for sharing their pictures.
While the 22-degree halo is a display that can occur any time of year, the circumhorizontal (also called "circumhorizon") arc requires the sun to be high in the sky (58 degrees or more above the horizon), and the sun at our latitude only gets that high between about late March and mid-September, so it isn't something we can see during the "cooler half" of the year.
There is a related colorful band called the "circumzenithal arc" that can occur anytime of year - that one is produced by the same kinds of crystals as the circumhorizon, but requires the sun to be fairly low in the sky (less than about 32 degrees) and is seen much higher in the sky. I've included a link to a great reference for these and other ice-crystal related displays at the "Atmospheric Optics" web site. The links there have some good descriptions and diagrams describing how these phenomena work, along with some photo galleries to illustrate how they appear under a variety of conditions.