Cloudy with a chance of rainbow: iridescent clouds brighten the sky over parts of Victoria

Rainbow clouds shone over parts of Victoria over the weekend, in an unusual visual phenomenon known as iridescent clouds.

Photographs of the spectacle were taken by onlookers in the Melbourne suburbs of Mount Waverley and Caroline Springs.

Sarah Scully, a senior meteorologist at the Bureau of Meteorology, said the visual phenomenon was not uncommon in Australia but was rarely captured.

“People don’t often notice them, as they’re really fleeting,” she said.

Iridescent clouds often form from cirrus clouds, which were seen over Victoria on the weekend. Cirrus clouds are made up of ice crystals and are commonly found in the higher levels of the troposphere – the level of the atmosphere where most weather occurs – where the temperatures are coldest.

The clouds get their rainbow appearance when light is diffracted – bent – around ice crystals. The phenomenon often occurs in newly formed clouds where the ice crystals are the same size.

The smaller the cloud, the more rays of sunlight can pass through and be diffracted by a single ice droplet, according to the BoM.

Iridescent clouds gain their appearance from the diffraction of sunlight, which causes the colours to spread into rings or waves.

They differ from rainbows, in which both the refraction (bending) and reflection (bouncing back) of sunlight result in colours in a specific order with neater bands.

Iridescent clouds are sometimes mistaken for sun dogs, another type of visual phenomenon with similar characteristics.

“Sun dogs are a different type of optical phenomenon,” Scully said. “They occur almost like a pillar of light that’s either on both sides of the sun or just on one side of the sun.”

“That is due … to water crystals being in the atmosphere, and … it sits at low levels of the horizon. You need it to sit even lower [than an iridescent cloud],” Scully said.