Sun dog

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A sun dog or sundog (scientific name parhelion) is a relatively common atmospheric optical phenomenon associated with the reflection/refraction of sunlight by the numerous small ice crystals that make up cirrus or cirrostratus clouds. Sundogs typically appear as a bright and sometimes colorful spot in the sky at a position 22 degrees to the left and/or right of the sun. They are closely related to, and sometimes appear together with, a halo. Less common associated phenomena, collectively called "ice halos," are the circumzenithal arc, upper tangental arc, parhelic circle, and lower tangental arc. There are 14 other named ice halo phenomena that could be seen given optimal conditions.

The ice crystals responsible are tiny (less than 0.5 micrometers in diameter) and hexagonal. These ice crystals refract the sunlight at an angle of about 158º, resulting in the appearance of sundogs about 22º from the sun. (180º is line of sight; subtract the 158º of refraction and the image appears at 22º.) The amount of refraction is dependent on size of the crystals, so instead of a precise, neat rainbow, stretched, pale images can appear.

Sundogs are seen in short arcs in a horizontal plane with the sun because the short, hexagonal "plate" ice crystals that make up them predominantly tumble through the sky with their flat axis oriented horizontally. The faint halo seen with the sundogs is from longer, pencil-like crystals that are at the correct orientation for their location to refract light to the eye.

Because it occurs due to internal reflection in ice crystals, it is produced only by clouds cold enough to consist entirely of ice particles rather than liquid water droplets. Such clouds are usually found only at temperatures below about -20°C. These can be seen frequently during winter cold snaps in North America and Europe, as well as at high altitudes and in the arctic regions.

Although often less vivid and more diffuse than the one depicted in the photograph, sundogs are actually rather common; they are often overlooked by amateurs because one must look in the general direction of the bright sun in order to spot them.

In remote stretches of Western Texas, sundog refers colloquially to a segment of a common rainbow.

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Quotations

My beloved children, I want to tell you that on the day after the departure of our brothers Kuntz and Michel, on a Friday, we saw three suns in the sky for a good long time, about an hour, as well as two rainbows. These had their backs turned toward each other, almost touching in the middle, and their ends pointed away from each other. And this I, Jakob, saw with my own eyes, and many brothers and sisters saw it with me. After a while the two suns and rainbows disappeared, and only the one sun remained. Even though the other two suns were not as bright as the one, they were clearly visible. I feel this was no small miracle . . . .

(Possibly the earliest clear description of a sun dog, from pages 20-1 of Jakob Hutter, Brotherly Faithfulness: Epistles from a Time of Persecution, Rifton, NY: Plough Publishing, 1979. ISBN 0874861918 The observation most likely occurred in Auspitz (Hustopeče), Moravia in very late October or very early November of 1533. The original was written in German, and is from a letter originally sent in November 1533 from Auspitz in Moravia to the Adige Valley in Tirol. The Kuntz Maurer and Michel Schuster mentioned in the letter left Jakob Hutter on the Thursday after the feast day of Simon and Jude, which is October 28. This quote is also referenced by Fred Schaaf on page 94 of the November 1997 and December 1997 issues of Sky and Telescope.)

. . . All around them, too, were signs that the Antarctic winter was fast approaching: there were now twelve hours of darkness, and during the daylight hours petrels and terns fled toward the north. Skuas kept up a screeching clamor, and penguins on the move honked and brayed from the ice for miles around. Killer whales cruised the open leads, blowing spouts of icy spray. The tricks of the Antarctic atmosphere brought mock suns and green sunsets, and showers of jewel-colored ice crystals.

(From page 76 of Jennifer Armstrong, Shipwreck at the Bottom of the World: the extraordinary true story of Shackleton and the Endurance, NY: Crown, 1998. ISBN 0375810498)

A visit to the cabin showed him the glass lower still: sickeningly low. And back on the poop he saw that he was by no means the only one to have noticed the mounting sea – an oddly disturbed sea, as if moved by some not very distant force; white water too, and a strange green colour in the curl of the waves and in the water slipping by. He glanced north-west, and there the sun, though shining still, had a halo, with sun-dogs on either side. Ahead, the aurora had gained in strength: streamers of an unearthly splendour.

(From page 279 (chapter 9) of Patrick O'Brian, Desolation Island, NY: Norton, 1978. ISBN 0393308138)

Hastings laughed, too, and shook his head. "Men do make their luck, Lady Margaret, and never have I seen that better proven than at Mortimer's Cross. For ere the battle, there appeared a most fearsome and strange sight in the sky." He paused. "Three suns did we see over us, shining full clear."

(From page 60 (chapter 4) of Sharon Kay Penman, The Sunne in Splendour, NY: Ballantine, 1982. Footnote from original: Phenomenon known as a parhelion, generally caused by the formation of ice crystals in the upper air. Two pages later, again mentioning the English king Edward IV, she writes: Many, she saw, flaunted streaming sun emblems to denote her son's triumph under the triple suns at Mortimer's Cross. ISBN 0345363132)

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