We had just finished our coney dogs at a local restaurant and were talking in the parking lot. I happened to look up, something I happen to do a lot. Hazy clouds smeared the sun into a brilliant, featureless glow accompanied by a pair of bright sun dogs two fists to either side. Sun dogs or mock suns aren’t too unusual and often accompany a big ring around the sun called a halo. They’re caused by sunlight streaming through tiny, six-sided plate-like ice crystals floating with their broad sides parallel to the ground. A sunbeam enters one side of a crystal, gets bent or refracted, and exits the opposite side at a 60° angle.
As we learned with frost crystals the other day, different colors of light are bent different amounts — blue the most and red the least. As the light leaves the cloud crystal it spreads into a tiny rainbow and colors the inner edge of a sun dog red and the outer edge blue. You can also see a faint halo in the photo above also caused by sunlight refracted by hexagonal crystals but with a difference. Instead of plates, they’re pencil or column-shaped — long instead of flat. Most halos are big with a radius of 22°. That’s the distance between the sun and the ring’s inner edge, equal to a little more than two fists at arm’s length.
The upper tangent arc that touches the top of the halo directly above the sun looks like it has wings. It was beautiful and very bright. Like a halo, pencil-shaped ice crystals are behind its creation but only those crystals that are drifting through the air with their long sides horizontal to the ground.
Walking to my car now I gave one last look sunward before starting the engine. What?! A vividly colorful circumzenithal arc seemed to come out of nowhere. The “circum” part of the name means “circle,” while “zenithal” refers to its position near the zenith or the point in the sky directly overhead. Often called the most beautiful of all the halo phenomena this curl of evanescent eye-candy occurs infrequently. As with sundogs, plate-shaped crystals horizontal to the ground are at play. But this time sunbeams strike the top, flat faces of the crystals and leave through a vertical side face. The refraction is a strong one, spreading out the colors to make a compact rainbow right over your head.
Sometimes sun dogs grow tails. These are part of a much larger and rarely seen complete circle of light that runs around the entire sky called the parhelic circle. To our amazement Greg and I looked in opposite the sun and saw the circle — band of pale light across the sky (photo at the very top) a little wider than an airplane contrail, the cloud of condensation that trails behind high-flying planes.
My phone could only capture a segment of this grand circle that matches the sun’s altitude in the sky, but you get the idea. I’ve seen parts of the parhelic circle before but never the bizarre glowing disk within the ring called a 120° parhelion. It resembled a sun dog but was more diffuse and located — you guessed it — 120° to the left of the sun or one-third of a complete circle.
Plates and columns combine forces with multiple refractions and reflections to create this astoundingly large circle strung with a fuzzy bead of light at the 120° point. I looked for but did not see its companion parhelion 120° to the right of the sun. They typically come in pairs and require thick plate crystals with alternating long and short sides.
So here’s the point. If you eat hotdogs slathered in coney sauce and topped with chopped raw onions then look up, there’s a good chance you’ll see a spectacular halo display. Of course if that were true I’d be back there right now armed with a fisheye lens. No, the point is to make a habit of looking up because you never know what beautiful things nature will put on the table.
To learn more about halo phenomenon highly recommend Les Cowley’s Atmospheric Optics site. It has many helpful diagrams to visualize how light travels through ice.