Skywatching knows no hours. Wherever and whenever you tip your head back, a sight awaits. As I was dashing off to an appointment yesterday, I noticed high clouds and looked up toward the sun to see a brief but lovely display of atmospheric colors. Besides the often-seen halo, there were two sundogs on either side the sun — each with a tail! — a Parry arc, an upper tangential arc and a portion of the parhelic circle.
After all the recent cold weather, a welcome warm front was on its way. Warm fronts announce their arrival with high clouds that gradually yield to lower-altitude types over the next 12 to 24 hours. Whenever you hear of one on the way, keep an eye on the daytime sky. Those high clouds often make for beautiful refractive displays that range from a simple circle around the sun to a dozen or more arcs and patches.
Cirrus and cirrostratus clouds, the ones that make the sun look smeared, are best. They’re composed of six-sided (hexagonal) ice crystals shaped like plates and columns. Light refracted or bent through the columnar crystals spreads to make the circular halo as well as the Parry and upper tangent arcs. Sundogs form when sunlight is refracted by flat, plate-shaped crystals floating with their broad faces horizontal to the ground. Sundog tails form when plate crystals wobble as they drift downward from the clouds.
Red light is refracted less strongly than blue, so the inner edges of halos, sundogs and Parry arcs appear subtly red while their outer edges glow pale blue. Thursday’s display was brief though parts and pieces of it lingered for at least a half-hour. Ice crystals are very tiny and deceptively simple, yet the ways in which they interact can produce stunning visual effects.
The parhelic circle is a rarer sight and forms when sunlight reflects from the near-vertical faces of both types of crystals. Most of the time it’s just an arc near the sun, but if you spy that arc, be sure to turn around and look opposite the sun. If you’re lucky, you might see a complete circle all the way around the sky.
One of the best things about daytime skywatching is how easy it is, and there’s plenty of light to find your way. Temperatures are usually warmer, too!