We’re locked down under a dome of bitter cold air here in the Upper Midwest with temperature barely reaching zero (-18 C) and lows of 15-20 below (-26 to -29 C) at night. While that makes night sky viewing a man vs. nature challenge, days are filled with brilliant sunshine and often invisible ice crystals. We know they’re there because of their wonderful refractive abilities. Light passes through them and comes out the other side bent into halos or spread in a rainbow smile of color near the zenith (overhead point) called a circumzenithal arc.
Yesterday morning, with the temperature still below zero, a pair of sun dogs stood obediently at either side of the sun. Sun dogs form when sunlight enters one side of a 6-sided, plate-shaped crystal, gets bent or refracted and then exits a different face. The crystal acts like a prism and spreads the exiting light into a small rainbow of color with red on the inside and blue on the outside.
My work Friday sent me 80 miles north of Duluth to near Aurora, Minn. There the air was visibly foggy with the sun still shining brightly.
When I finally had a chance to stop the car for a look, I was blown away by the sight. Two brilliant sun dogs anchored a nearly complete halo with a fabulously colorful circumzenithal arc crowing it all.
Halos around the sun and moon form when light is bent or refracted by column-like hexagonal ice crystals. Light strikes one face of the crystal, gets bent and then exits through the other side where it’s bent again. When billions of crystals get involved, the combined effect is a ring of light 22 degrees in radius or a little better than four ‘fists’ (44 degrees) in diameter.
A circumzenithal arc forms when downcoming rays from the sun strike the horizontal sides of 6-sided, flat, plate-shaped crystals and get refracted through a vertical side face. Click HERE for a nice visual aid.
With winter underway, keep watch on the daytime sky for these amazing works of crystal art made of ice crystals so small you need a microscope to see them.