May 02, 2013


Some thought they predicted the death of kings.  

Before the Battle of Mortimer's Cross in 1461, the future King Edward IV tried to convince his troops, frightened by the appearance of three suns, that they represented the Holy Trinity and presaged a great victory.  (He won).

And during the past days several of you have seen this apparition here in the Northwest and sent me pictures and questions.

What am I talking about?   Sundogs.  Also known as mock suns or parhelia.

To begin, here are two pictures sent to me today by Lief Zimmerman (the second a blow-up of a portion of the first)

And here is a dramatic example in which the sun and two "dogs" are clearly visible.

The two sundogs, or mock suns, frequently show a rainbow-like effect and often appear as brighter spots in a partial or total halo around the sun.

Why do sundogs exist?

Sundogs are the result of the refraction of light through ice crystals, normally ice crystal clouds such as cirrostratus or cirrus.    Light from the sun is bent or refracted by the ice crystals, which are six sided (or hexagonal).  The sun's rays are generally bent by 22 degrees when they goes through the ice crystal from one side to the other.   If the ice crystals are randomly oriented one sees a halo around the sun.   But if the ice crystals are shaped in plates, they tend to orient themselves in one way (wide side up and down).  The result is then a partial halo, with two "suns" on both sides (see schematic).

Why the rainbow colors?   Turns out that the ice crystals can act like little prisms since the bending of light (refraction) depends on the wavelength of light.  This causes a split of the sun's ray into various wavelengths or colors, a process known as dispersion (see schematic below).

Why don't we get sundogs all the time?.  Well, first you need sun (a relative rarity in these parts).  Then you need ice crystal clouds in which the shapes of the ice crystal are right (the relative flat plates shown above).  That requires that the crystals form in  a certain temperature change.  Then you need the right geometry of the sun relative to the clouds.  I probably see sun dogs around here 3-8 times a year...not rare, but not an everyday affair.

Shakespeare wrote about sundogs at length, here is a portion from Henry IV, Part I:

Dazzle mine eyes, or do I see three suns?
Three glorious suns, each one a perfect sun;
Not separated with the racking clouds,
But sever'd in a pale clear-shining sky.
See, see! they join, embrace, and seem to kiss,
As if they vow'd some league inviolable:
Now are they but one lamp, one light, one sun.
In this the heaven figures some event.
'Tis wondrous strange, the like yet never heard of.
I think it cites us, brother, to the field,
That we, the sons of brave Plantagenet,
Each one already blazing by our meeds,
Should notwithstanding join our lights together
And over-shine the earth as this the world.
Whate'er it bodes, henceforward will I bear
Upon my target three fair-shining suns.

I wish I could write like that!  Whether sundogs have anything to do with longevity of kings or the propects on the field of battle, I will leave that to philosophers and writers.

 Want to help find out the effects of coal trains on the local environment?

Atmospheric Chemistry Professor Dan Jaffe is ready to begin a study on air pollution from trains, especially freight  and coal trains, in the Puget Sound region.   Several state and local agencies told him that this work needs to be done, but that it is too politically hot for them to fund.

Considering the importance of this work, Dr. Jaffe is going to depend on crowd funding to support this effort.  For more information and perhaps to help, please check out this website:


  1. Hi Cliff,

    Thanks for the piece on Sundogs.

    I have been reading about sting jets, in the UK media. Apparently a sting jet caused the record high winds in the great storm of October 16th 1987 (which I lived through).

    I was curious if you feel this research is relevant to our part of the world, as we are at a similar latitude to the UK storms.


  2. Interesting. I've never noticed this phenomenon.

    Cliff I'm loving the warmer, sunny temps of the last few days but I've noticed recently we seem to get a lot of "haze" on sunny days. Yesterday we had beautiful blue skies in Seattle until around 3:00 when it started getting hazy. This morning its already a bit hazy.

    Is this the marine layer, or pollution, or something else? What causes it? Thanks.

  3. Finally! I saw one of these about a month or so ago and was completely confused as to how there could be a rainbow "spot", when the sun was shining light and there were just extremely thin clouds dotting the sky. I took a picture because I knew no one would believe me when I told them about it. Thank you Cliff!

  4. Hi Cliff,

    This isn't related to this post, but I wanted to suggest a possible blog or at least something you may want to take a look at. The upper level low moving slowly across the Mid and Eastern U.S. this weekend and early next week was predicted by the GFS several days ahead of formation. Interestingly though, the ECMWF either showed the low not-forming or staying much farther north until about 36 hours ago.

    I thought with all your past posts about the ECMWF's superiority it might make an intersting blog entry to discuss a system that the GFS actually "beat" the ECMWF at.

  5. Thanks for mentioning the microryza study! I've sent the link around to my friends and everyone is excited, both about the coal study and microryza, which is based out of Seattle.

    Crowdfunding scientific research is an excellent idea, as it reminds our government where the people really want research funds to go.

  6. Used to see these in Montana a lot. Haven't seen them much here, as you say.

  7. I remember a time when I was in college (in Gunnison, Colorado) I observed sundog formed by snow crystals probably within a hundred feet of the ground. It was either gently blowing snow or crystals forming in the air by condensation. The sky was clear but there were suspended crystals of snow. The air temperature was probably well below zero F. that morning. There were two clear sundogs as a result.


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