Sunday, January 23, 2011

Sundogs on a Brilliant, Bitterly Cold Day

Two sundogs greeted us this morning when we looked out the window. It had plunged to about -26 overnight, and the air was crisp and cold, but not a cloud in the sky, a brilliant winter morning. There was a slight haze though, as if tiny frost crystals were floating through the air; you could actually see them sparkling as they drifted around.

Then I saw the sundogs, bright flares shining on either side of the morning sun, for awhile after 9 a.m. (and perhaps earlier).

Later I headed for the Internet and learned that sundogs are caused by tiny hexagonal ice crystals, either at high altitudes in cirrus clouds, or floating in the lower atmosphere as 'diamond dust' - this is what I think we had this morning. The crystals float horizontally, like tiny hexagonal dinner plates, drifting to the ground like a leaf in the fall, and reflect the light at a 22 degree angle, causing the bright rainbow-like flares, 22 degrees out on either side of the sun. The sundog colours range from red nearest the sun to blue on the outside. These two pictures show the left and right sundogs, with the reddish colour nearest the sun in each case.

Larger crystals develop more wobble as they fall, generating a taller sundog, which is how I would describe these. In the right hand sundog you can actually see the faint reddish colour extending up and down, almost forming part of a more complete halo around the sun. Today I guess we had large diamond dust in the air!

Thanks to Wikipedia, The Weather Doctor and Atmospheric Optics for what I've learned about sundogs today!

Interestingly, The Weather Doctor refers also to atmospheric crystals forming in hexagonal rod shapes, much like a pencil, in other situations. I've observed that melting ice in a pond also forms pencil-like rod shapes. In those 24 to 48 hours between being solid and vanishing in early spring, pond ice forms vertical pencil-like rods of ice that will fall apart in your hand before melting altogether.

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