Sun dog

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For other uses, see Sun dog (disambiguation).

Pronounced sun dogs on both sides of a setting sun in southern Minnesota. Note the halo arcs passing through each sun dog.

An unusually pronounced sundog produced by sunlight passing through thin cirrus clouds. The true sun is located outside of the picture to the right.

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

Halo with sundogs visible to the left and right (NOAA)

The ice crystals responsible are hexagonal plate shapes 0.05 - >1mm in size. These ice crystals refract the sunlight in many directions but with a minimum deviation angle of about 158°, resulting in the appearance of sundogs about 22° from the sun. The refraction depends on wavelength and so sundogs have a red inner edge and more muted colours further from the sun as colours increasingly overlap. Solar altitude is important and sundogs draw away from the sun at increasing solar altitudes.

Sundogs are seen in short arcs always at the same altitude as the sun because the plate crystals are preferentially aligned by aerodynamic drag effects with their large basal faces approximately horizontal.

Although often less vivid and more diffuse than the ones depicted in the photographs, sundogs are actually rather common; they are often overlooked 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.

[edit] See also

Moon dog

[edit] Quotations

Well then, Africanus, since you give me a sort of invitation, and encourage me in my hope regarding yourself, shall we not first inquire, before the others arrive, what the facts are in regard to that second sun that has been reported to the Senate? For those who claim to have seen two suns are neither few nor untrustworthy, so that we must rather explain the fact than disbelieve it. [From Cicero, On the Republic, Book 1, section 15, one of many passages in Greek and Roman authors that refer to sundogs and similar phenomena.]
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 0-87486-191-8 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 0-375-81049-8)

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 0-393-30813-8)

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 0-345-36313-2)