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While driving west along Lake Benmore on a spectacular day, my cell phone alerted me to my first ever view of a solar halo.

22 Degree Solar Halo over Lake Benmore 26 Oct 2008

While driving west along Lake Benmore on a spectacular day, my cell phone alerted me to my first ever view of a solar halo.  By now I had some idea about such halos and moved decisively to secure a series of valuable shots before it was over in a wink.  The people we had been visiting saw it and by some chance we were not too far away and within phone coverage to receive their message.  The result is the image above, assembled from four separate photos taken to define the sun and avoid lens flare.  A similar image of mine was published in the Timaru Herald on 30 October 2008.

 
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High above, the 'upside down 'rainbow' CZA dominates over faintly detectable 46 degree and supralateral arcs.

22 Degree Solar Halo over Oamarma

I was utterly astonished to not only see this phenomenon again three hours later but to clearly see bright sundogs as well.  I was previewing my first image for a commercial photo shoot and there the two extra suns glowed out of the back of my camera.  Other lines were discerned and I considered their fortuitous use for my shoot.  Soon I was told of yet another band of light glowing like a broad slice of rainbow far overhead.  Intuitively grabbing at shots, there was little time to think of finding a clear horizon: the high cloud was passing on, the halos and arcs were fading. The perfect landscape shot eluded me.

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Parhelia (Sun Dogs) forming 'false suns' left and right of the sun. Less common Tangential Arc form the flat wavy 'm' on top of the 22 degree Halo.

From my little knowledge, I assumed the rainbow effect was part of a secondary 46 degree circle and sent images to James McGregor of metvuw.co.nz and was taken aback when he said they ‘created quite a stir’. His and Ian Coopers comments here prompted me to investigate further.

I was to learn the extra features where not so common, the rainbow effect even less so. Using a simulator for comparison, the upside down rainbow proved to be a Circumzenithal Arc (CZA) a quite rare phenomenon reported to occur only 13 times annually in Germany).

Sun Halo and Arc Simulation
The Omarama sun elevation for the simulation was calculated from a London Metropolitan University website on low energy design in architecture here

HaloSim3 the ice crystal halo simulator software was downloaded from the Atmospheric Optics website here and used to create the ideal view for the sun in the same place and time. The upside down rainbow at the top is the Circumzenithal Arc (CZA) sitting on a very faint 46 Degree Halo and Supralateral Arc. Lower down Tangential and Parry arcs sit on the 22 Degree Halo which is intersected by the Parhelic Circle with Parhelia (Sundogs) on either side.

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Image processing helped reveal unseen parts of the 46 degree halo and infralateral arcs as predicted by HaloSim3.

Again I considered my slim chance of ever seeing this. The CZA.only forms when the sun is less than 33 degrees up from the horizon and at its brightest 22 degrees up, close to the height when the cirrus cloud thinned to a passing wisp covering enough of the sky with its water crystals critically shaped and orientated.  I just happened to be pointing my camera with a super-wide angle lens towards the sundogs which are a big hint that a CZA may also be lurking around the centre of the sky.

I now know better of these clues and will be more watchful. Next time I would like to make enough photos to cover the entire sky and stitch up a 180 degree fisheye view.

Such rare phenomena makes life as a photographer so amazing.

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