Roger's SkyPix

Visual Effects

Circumhorizon Arc

Circumhorizon Arc

We had arrived in the general target area of marginal afternoon storm potential, and decided to explore parts of the ironically named and almost wholly anthropogenic Nebraska National Forest. Right after leaving, while cruising toward Thedford, an odd color effect that I couldn't quite peg appeared among thin cirrus and cirrocumulus in the southern, midday sky. It turned out to be a segment of a circumhorizon arc -- so named because it theoretically could form an annulus inside the full 360o of horizon at the same elevation angle above ground. These highly uncommon apparitions happen mainly near the early summer noontime, in the presence of cirrus, nearly always as segments instead of a fully formed circle around the sky. They are caused by refraction through the edge, then out the flat side, of each of countless billions of ice crystals shaped like hexagonal plates.

3 WNW Halsey NE (3 Jun 6), looking SSW.