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Circumzenithal Arc, Perth, Western Australia imaged by Graham Watts on 21st December. ©Graham Watts, shown with permission.

"The second pic was taken 2 mins after the first. That band of cirrus was moving through very quickly."

Circumzenithal arcs - 'upside-down rainbows' - 'smiles in the sky' are ethereal beasts often changing from second to second as the cirrus that makes them moves at its fastest when overhead.

Their colours are the purest of the halos. Those of 22° halos and sundogs overlap and smudge because those halos are minimum deviation arcs formed by crystals in many rotational positions. In contrast, the rays of the circumzenithal arc, but for a slight divergence produced by the sun's disk, enter crystals at the same angle and are as neatly dispersed by the 90° ice prisms as in the best of laboratory spectrometers.

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