Circumhorizontal arc: Incredibly rare cloud-formation painted in the sky above Wellington

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 is explained Makgabutlane​ is explained had explain it was cloud iridescence but upon further analysis , clarify 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.

The phenomenon over Wellington of a rainbow cloud was not cloud iridescence, but rather a circumhorizontal arc on Tuesday. (Supplied)

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 cloud were cirrus cloud which were cover most of the South Island along with a cold front and some had drift over to Wellington with a south – easterly wind , Makgabutlane​ is said say .

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

Karaka Bay resident Nick Tyler is happened happen to look up at the right moment to see the short – live ” cloud – bow ” formation . ( supply / Nick Tyler )

cloud iridescence on the other hand , could occur with both rain or ice droplet , Makgabutlane​ is said say .

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

cloud iridescence is involved involve a process call defraction where light bend in the gap around ice or rain particle , whereas circumhorizontal and circumzenithal arc involve a process call refraction where light bend pass 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.”

Stephanie Turgoose is enjoying was enjoy a cup of tea on her friend ‘s balcony in Wellington ‘s Karaka Bay when she notice a ” horizontal rainbow ” . ( supply / Stephanie Turgoose )

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 is bend that form iridescence could bend the light in way that made them appear scatter , whereas in the arc , the colour 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.

What ‘s call a circumhorizontal arc appear in the sky above Wellington on Tuesday . ( Supplied )

The confusion is was between the two was possible when the arc was begin to form , but ” once it ‘s big enough , you can kind of see the shape , ” she say , also hence the name of the arc .

“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 is were were pretty lucky to catch it when it happen . “

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