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, an unusual visual phenomenon known as iridescent clouds. The spectacle was captured in photographs 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 phenomenon is not uncommon in Australia but is rarely captured. She explained that people do not often notice them because they are fleeting.

Iridescent clouds form from cirrus clouds, which were seen over Victoria on the weekend. Cirrus clouds are made of ice crystals and are found in the higher levels of the troposphere, where temperatures are coldest. The clouds acquire a rainbow appearance when sunlight is diffracted around the ice crystals. The effect is strongest in newly formed clouds where the crystals are the same size, and the smaller the cloud, the more sunlight can pass through and be diffracted by a single ice droplet, according to the Bureau of Meteorology.

Unlike true rainbows, which involve both refraction and reflection of sunlight to produce neat, ordered bands, iridescent clouds display colours spread into rings or waves. They can also be mistaken for sun dogs, another optical phenomenon that appears as a pillar of light on either side of the sun. Scully noted that sun dogs occur at lower levels of the horizon and involve water crystals rather than ice.