Saturday, February 19, 2011

Sundogs, tangent arcs and double solar halos

Double solar halo -- a huge outer halo. Photo © SB
Regina, Saskatchewan, Canada: Atmospheric ice crystals shifted yesterday, and I saw and partially captured a brief, rare, second solar halo, two hand-spans out from the sun.

This display dominates the sky -- far too wide a range for my pocket camera. I wish I'd had a fish-eye lens.

I had to remove my gloves to use the camera for these pictures, and  a day later, my fingertips still have a phantom frostbite throb. Minus 37 degrees seeps through clothing, flesh, muscles, bone -- viciously cold in either Centigrade or Fahrenheit. 

But these winter solar displays, formed by ice crystals reflecting and refracting light, entrance me.

A single halo, like the one shown in Sundogs and Solar Halos over Wascana, is a fairly common extreme cold phenomenon. These appear about one hands-width (with your palm held near your face) away from the sun. Sundogs often flash out on either side of these halos at set positions along what's called the parhelic circle. (The other name for sundogs is parhelia.) At times, you can also see a rainbow glow at the top of the halo; that's the upper tangent arc -- apparently there is a lower one, as well, but I don't remember ever seeing one. 

It's always difficult to photograph this display without the sun at least partially obscured. And almost impossible without a wide angle lens. These images filled the width of my screen. 

So many wonders. It almost makes me wish for more extreme cold weather.

Solar halo with sundogs and upper tangent arc
(Sun partially obscured by winter branches) 
About 10 a.m., February 18, 2011.  Photo © Shelley Banks

Update, April 16, 2011: The image of the sun and halo partially obscured by branches, above, is the Optics Picture of the Day at Atmospheric Optics.


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