Sunday, December 26, 2010

Sun Dogs

Yesterday, before dawn, an icy fog rolled in.  Most of it dissipated as the sun rose.  High cirrus cloud remained.  The result was the glorious phenomenon known as Sun Dogs (or Parahelia).  It wasn't bitterly cold, as is usually the case when we see Sun Dogs here.

Sun dogs happen when sunlight passes through ice crystals in the air.  The ice crystals are flat hexagonal in shape and act as prisms, bending the light rays at 22°.  If the crystals are randomly aligned in the air, the 22° Halo forms.  If the crystals are aligned horizonally as they fall, the refracted light is concentrated into Sun Dogs.

I'll turn it over to The Weather Doctor who explains:

"Ice crystals in the atmosphere are hexagonally shaped. Crystals forming most optical phenomena in the air are typically hexagonal rods, shaped like pencils, or flat, hexagonal plate patterns, like microscopic stop signs or dinner plates. When plate-shaped ice crystals fall unimpaired, drag forces automatically orient them horizontally so that their larger, flat surface parallels the earth like a large maple leaf drifting down from a tree.
Sun dogs emerge when sunlight passing through the ice plate's thin sidefaces is refracted. The more perfectly aligned the falling crystals are to the horizontal, the more compact the resulting sun dog. Crystal misalignment from true horizontal will spread the sun dog vertically — its angular height being approximately four times the maximum crystal angular tilt.
Sun dogs frequently display a reddish tint on the side facing the sun and may sport bluish-white tails which stretch horizontally away from it. The degree to which colours are visible depends on the amount of wobble in the ice crystal's fall: the more wobble, the more colour. The sun dog's tail is formed by light passing through the crystal at angles other than the optimal deviation angle."

A second bit of refraction is happening at the horizon from air-bourne snow crystals - the colours are just visible to the left of the trees in the photo below

High Noon and Due South at the 49th Parallel on December 25, 2010

Auspicious times to come?  Throughout the ages, people have ascribed omens and portents to the appearance of Sun Dogs.  Wikipedia has a collection of associated past events.


  1. Yes Kathy, I too was amazed that you captured sun dogs on relatively warm day. My dad's omen for sun dogs (which usually came true) was that sun dogs forecast continued cold weather, as apposed to a Chinook arch which forecast more warm weather. I really appreciate you digging out the technical explanation as well. My dad's explanation to me was simply that the cold messed up the sun's rays and formed the sun dogs, which appears to be technically correct but just a little simplistic. We have beautiful arch today, and the temp is already 0 degrees.

  2. Me too, always associating sun dogs with super cold winter air temps, so I was a tad surprised to see the phenom happening on a -10C mid-day.
    I like finding out how stuff really works; the science of. Equally, I think it's neat to learn about the folklore and commonsense take on such events as well. Our parents and older ancestors had to use their wits to survive and prosper. Reading the weather signals out here on the prairies is as crucial as it is to the mariners (red sun at night, sailor's delight - I think that's how it goes - red sun in morning, sailors take warning).

  3. Oh, and I do so miss the chinooks.

  4. These sun dogs were especially neat as it wasn't very cold outside. The pics turned out better than I thought they would, shooting straight into the sun and all.