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Incredibly rare cloud formation paints the sky above New Zealand

The rainbow that looked as if it had been painted on New Zealand's sky on Tuesday turned out to be what's called a circumhorizontal arc.

MetService meteorologist Mmathapelo Makgabutlane​ had explained it was cloud iridescence but upon further analysis, clarified it was actually another rare phenomenon.

"The tell-tale is if it gets big enough, it almost has a bit of a curve to it," Makgabutlane said.

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Regardless of its official name, many said they'd never seen anything quite like it before.

The two meterological phenomenons are easily mistaken for one another, but the process was slightly different, Makgabutlane​ said.

A circumhorizontal arc or a circumzenithal arc both occurred with ice crystals which made up high clouds that were in the skies above Wellington on Tuesday.

The scattered clouds were cirrus clouds which were covering most of the South Island along with a cold front and some had drifted over to Wellington with a south-easterly wind, Makgabutlane​ said.

A circumhorizontal arc appeared when the sun was high in the sky and therefore formed low in the sky (like yesterday), while a circumzenithal arc formed high in the sky when the sun was low in the sky.

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Cloud iridescence on the other hand, could occur with both rain or ice droplets, Makgabutlane​ said.

As for the difference in process, both phenomenons involved the bending of light.

Cloud iridescence involved a process called defraction where light bent in the gaps around ice or rain particles, whereas circumhorizontal and circumzenithal arcs involved a process called refraction where light bends passing through one medium to another; from the air to inside the ice crystal and out again.

"Ice crystals form in different orientations high in the sky, and when sunlight refracts, or bends, through the top, horizontal face of the crystal, and leaves through a vertical side, it causes the light to separate into the different colours we are able to see."

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So, how do you tell the difference between the two when you're lucky enough to spot them again?

Firstly, in both cases, seeing them would depend on the composition of the cloud, angle of sun and the time of day, Makgabutlane said.

"We're quite lucky everything lined up quite nicely."

One of the ways to tell the difference, was how the colours appeared.

The process that formed iridescence could bend the light in ways that made them appear scattered, whereas in the arcs, the colours were more uniform – similar to what most people are familiar with in a rainbow.

"We could see that the colours were more uniform," Makgabutlane said.

The confusion between the two was possible when the arc was beginning to form, but "once it's big enough, you can kind of see the shape," she said, also hence the names of the arcs.

"Those are probably the things to look out for."

While the arcs also depended on having the right shape of crystals, a circumhorizontal arc like yesterday's was also relatively short-lived.

"We were pretty lucky to catch it when it happened."

This story originally appeared on Stuff and has been reproduced with permission.