Rainbow cloud spotted above Chch

Cloud above Christchurch an 'optical illusion'

ANNA TURNER
Last updated 14:45 17/12/2012
rainbow cloud std
Hamish Stroud

RARE CLOUD: A 'fire rainbow' was spotted above the skies of Christchurch today.

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A ''rainbow cloud'' was spotted in the skies above Christchurch today.

The unusual cloud was visible for about 30 minutes just after noon.

Many Press readers sent in images of the phenomenon.

One reader, Michael MacAskill, said he believed it was an example of a circumhorizontal arc, colloquially known as a fire rainbow.

"They form when the sun is high in the sky and shines through ice particles in high-level cirrus clouds. The ice particles act like tiny prisms," he said.

"They can only form in Christchurch for about 400 hours a year in mid-summer when the sun is high enough in the sky.

"But even then, there has to be cirrus cloud in just the right place too, so they are quite rare."

Blue Skies meteorologist Tony Trewinnard said the unusual cloud was caused by "iridescence". 

"It's an optical illusion that results when light is reflected by droplets in the cloud," he said.

"The sort of effect is seen in bubbles, or on wet surfaces or on the surface of a compact disc," he said.

Iridescent clouds were an optical phenomenon caused by small water droplets or small ice crystals individually scattering light, he said.

Canterbury was a good place to see iridescence in clouds as the atmosphere was often dry, which meant clouds were more often thin and semi-transparent, he said.

"Newly forming clouds [as in the example here] often produce the brightest and most colourful iridescence," he said. 

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- The Press

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