Roger's SkyPix

Visual Effects

Circumzenithal Arc

Circumzenithal Arc

(above 22 Degree Halo and Parhelia)

We so often miss interesting visual effects, especially on days of placid weather, because they're going on almost directly overhead and with little fanfare. I'm glad, therefore, someone ran inside and told me of this as I was leaving work! The "inverted rainbow" feature at the top is the circumzenithal arc, so named because, if it were an exceedingly rare full circle, it would enclose the zenith. These appear through the same mechanism as 22 degree halos and parhelia (sundogs): six-edged, disc shaped ice crystals high in the troposphere. That's why they're often seen together, as was true here. For the arc, the crystals bend sunlight entering their broad sides and out their edges like millions of tiny prisms, creating magnificently separated colors even more "pure" than those of a rainbow.

Norman OK (10 Feb 5), looking WSW.