It was just after lunch on Sept. 25 when I stepped out onto the rear deck of my home in Ohio. What a gorgeous autumn afternoon. The pale blue sky was streaked with wispy white cirrus clouds. The sun was high and bright.
I glanced up. The sun was surrounded by an extraordinarily bright, rainbow-colored halo. Flanking it both left and right were two brilliant, comet-shaped rainbow-colored sun dogs or mock suns (technically known as parhelia from Greek words meaning “beside the sun”). Wow!
I dashed to the front yard, which has a better view of the sky, and began turning to see how far the “comet tails” of the sun dogs reached. I turned 360 degrees, accidentally unbalancing myself and falling onto the grass.
Not only was there a halo around the sun – the so-called “22 degree halo,” which sky watchers often see – but also there was an enormous ring of light running parallel to the horizon at the same altitude as the sun. It was like a giant angel’s halo suspended above my town, interrupted every 120 degrees by a brighter splash of light (more “mock suns”).
“That’s the complete parhelic circle!” I exclaimed aloud to the empty street.
Looking up. All that morning I had been stepping outside hourly to look up, because I knew that thin cirrus clouds plus bright sunlight almost guaranteed seeing something wonderful. Cirrus clouds are made of millions of hexagonal ice crystals three to six miles up in the troposphere where jet airplanes fly – each crystal acting as a tiny prism refracting (bending) the sun’s light and throwing it elsewhere into the sky.
Because the troposphere is almost always below freezing, ice-crystal displays can be seen year-round (I’ve seen weak sundogs even in July). But truly good displays in the United States are most common in the fall, winter, and spring when the northern jet stream descends southward, drawing down Arctic air masses with their treasure-trove of jewel-like ice prisms.
Spreading the news. Just then, my neighbor Cindy backed her van out of her drive. I called to her and pointed upward. She stepped out of her idling van and looked up. Her eyes widened and her jaw dropped.
“Was this predicted?” she asked eagerly. “Did people know this was going to happen? How do you find out when to look?”
No, I explained, atmospheric displays cannot be predicted the way astronomers can pinpoint the dates and times of meteor showers and eclipses. Sighting such a light show is more akin to spotting an unusual migratory South American bird in Ohio: knowing generally the right weather conditions and time of year, you simply must trust to luck.
Kaleidoscope of patterns. Not only that, but every ice-crystal display is as different as every pattern seen through a kaleidoscope – and for similar reasons. Displays in the daylit sky depend on the tilt of the ice crystals in the air and the altitude of the sun. They depend on whether the ice crystals are flat plates or long pencils. They depend on the crystals’ size (at least 0.1 millimeter across) and optical quality.
Crystals too tiny or imperfect can’t act as prisms. But if the crystals are of exceptional gem-like quality, the entire dome of the daytime sky may be festooned with exotic halos, loops, arcs, and crosses – or the full parhelic circle now glowing overhead in silent glory.
“And to think,” Cindy remarked, climbing back into her van, “I wouldn’t have noticed any of this if you hadn’t gotten me just to look up!”
After she drove off, I pondered her questions and comment.
A reward awaits. Although I’ve been an avid amateur astronomer and lover of the nighttime sky since 1965, only in the last five years have I also become a devotee of the daytime sky. During that time, my experience has revealed that even supposedly rare atmospheric-optics displays are more common than meteorology books imply – plainly visible to anyone who simply thinks to tilt back the head.
Now that I’ve cultivated the simple habit of looking up a dozen times a day, I’ve found scarcely a week passes without some reward – be it solar halos, sun dogs, crepuscular rays (shafts of sunbeams and shadows from behind puffy cumulus clouds), circumzenithal arcs (a rainbow-colored “ice bow” arc half-encircling the zenith), sun pillars, or… now, the complete parhelic circle.
Yes, the daytime sky abounds with unexpected gifts, which can be yours for taking a moment – just to look up.
(Trudy Bell is a science journalist who lives in Lakewood, Ohio.)
