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Rings around the Sun

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Rings around the Sun

Whenever both sun and clouds are in the sky, be sure to look up--you may behold rings, arcs, and other marvels!

NASA

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see captionOct. 24, 2002: It was just after lunch on Sept. 25th 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!

Above: This scene, recorded in Finland by Pekka Parviainen using a wide-angle lens, is similar to the one author Trudy E. Bell saw in Ohio last month. A football-shaped "circumscribed halo" surrounds the Sun. A fainter "parhelic circle" rings the horizon. "I had never seen anything so huge and so perfectly circular," says Bell.

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 360o, accidentally unbalancing myself and falling onto the grass.

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Not only was there a halo around the Sun--the so-called "22o 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 120o by a brighter splash of light (more "mock suns").

"That's the complete parhelic circle!" I exclaimed aloud to the empty street.

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 3 to 6 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 upper 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.

see captionLeft: Wispy high-altitude clouds like these harbor ice crystals that cause sun halos. Credit: Pekka Parviainen

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.

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!"

see captionRight: Author Trudy Bell made this 3D sketch of the parhelic circle she saw on Sept. 25, 2002. Click to view a larger version of the sketch or a more colorful one. She also made a pastel drawing.

After she drove off, I pondered her questions and comment.

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!

Editor's note: Ice crystals in Earth's atmosphere cause not only rings around the Sun, but also rings around the Moon, Moondogs and even Venus pillars. If you spot a Sun pillar or halo not long before sunset, be alert for rings and pillars around objects in the night sky a few hours later.

More information

Atmospheric Optics -- a comprehensive guide to sundogs and halos, pillars. These sections are of particular interest: getting started; photography tips; How infrequent are some halos and why?

Sun halos, pillars and sundogs can happen during any season. "The icy crystals that cause them form in high altitude clouds 5 km or so above Earth's surface where it is always freezing," says Bruce Wielicki, an atmospheric scientist at NASA's Langley Research Center.

see captionRight: Cirrus clouds and hexagonal ice crystals. [more]

A gallery of solar halos -- captured by sky photographer Pekka Parviainen.

Polar Halos: Complex ice-crystal displays are especially common in the Arctic and Antarctic, even being produced by the full moon during the months-long winter night! Here's a description of one seen by Robert Peary on his last voyage toward the North Pole: "On the evening of November 11 [1908], there was a brilliant paraselene, two distinct halos and eight false moons being visible in the southern sky. This phenomenon is not unusual in the Arctic, and is caused by the frost crystals in the air. On this particular occasion the inner halo had a false moon at its zenith, another at its nadir, and one each at the right and left. Outside was another halo, with four other moons." - Robert E. Peary, The North Pole: Its Discovery in 1909 Under the Auspices of the Peary Arctic Club, 1910; Dover Publications, 1986, pp. 175-176.

more references: Trudy E. Bell "Skyscapes," League of American Bicyclists magazine, 37 (3): 12-15 (Summer) -- photos of different common ice-crystal, water-droplet, and dust-mote phenomena you can see in the daytime sky

Robert Greenler, Rainbows, Halos, and Glories, with a comprehensive catalogue of ice crystal forms and the displays they produce, plus highly useful color photos of many kinds of displays one can see. (This is a classic book now out of print, but widely available at libraries.)


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