Sunday, April 19, 2015

Circumzenithal Arc

A few days ago I photographed this circumzenithal arc in the sky above the Hudson Valley. These halo phenomena are sometimes called "smiles in the sky" or "upside down rainbows."

Unlike a normal rainbow, which describes a circle centered around the antisolar point (directly opposite the sun), this light effect curves around the zenith. The colors appear on the section of the circle closest to the setting sun.

Whereas the regular rainbow is the result of sunlight bouncing back to the eye in suspended raindrops, this effect occurs when sunlight refracts through plate-shaped hexagonal ice prisms floating in a horizontal position in cirrus clouds. Therefore, it often appears interrupted as it intersects the parallel tendrils of the clouds.
Wikipedia on circumzenithal arc
More at Atmospheric Optics
Classic book on light/atmosphere phenomena: The Nature of Light and Colour in the Open Air


Eugene Arenhaus said...

Nice catch, James!

Melle Ferre said...

Indeed !

Steve said...

Beautiful photo. I think I've simply known these as "sun dogs".....or is this different?

Willow's Quiet Corner said...

Awesome! I wonder how long they stick around?

James Gurney said...

Steve, Sun dogs are related, because they're also spectral halos caused by ice crystals that you see in the direction of the sun. They're both part of a whole array of halo phenomena that includes sun pillars, parahelic circles, and supralateral arcs, 22 degree circle, and Parry arc (Handy terms to drop at cocktail parties). More here:

Willow's: The circumzenithal arc is strongest when the sun is about 22 degrees above the horizon. It all depends on the relationship of angles between the sun and those floating ice crystals.