Rainbow cloud? Rare natural phenomenon baffles locals in China

A stunning cloud spotted in China (Image credit: limchiseng/Twitter)

A stunning iridescent cloud mesmerised and puzzled locals in one Chinese province earlier this week.

Footage filmed in the city of Haikou in the Hainan Province of China shows a cloud coloured in the hues of a rainbow. This natural phenomenon is rarely spotted in the skies, and it therefore goes to reason that most who witnessed the cloud were left baffled by it.

Photos of the ‘rainbow cloud’ spread like wildfire on Chinese social media sites, and from there to Twitter and Instagram.

虹彩幞狀雲 彩雲之南 —— 雲南 8/16，一朵積雨雲萌發在雲南普洱，同時還帶來了千層的虹彩幞狀雲。 幞 (fú) ㄈㄨˊ，包東西的布，古代男子用的一種頭巾，也稱 “幞頭”。 這種雲頂的 “小帽子” 被形象的稱為幞狀雲（Pileus）。 孫嘉琪 雲南普洱 2022/8/16https://t.co/BlIigX5Agfpic.twitter.com/MV0njBqqbL

— limchiseng (@limchiseng) August 30, 2022

“This is insane. Like a rainbow and a cloud had a baby,” one person commented after seeing the stunning visual. “This is wow,” another concurred.

The cloud in question is actually a pileus cloud, which is also called a scarf cloud or cap cloud for its peculiar shape – it looks a cap or hood attached to the upper part of another cloud. This distinction is clearly visible in pictures of the cloud from China that have gone viral online.

But pileus refers to a type of cloud that may or may not be coloured. So what caused this one in particular to have an iridescent effect?

Well, according to Slate, pileus clouds are thin and have tiny water droplets in them – conditions perfect for the formation of iridescence.

“In iridescence, the light actually bends (diffracts) around the droplets. Different colours bend by different amounts, splitting the colours apart,” astronomer Phil Plait explained in the Slate article.

For iridescence to occur, droplets have to be really tiny – around a micron in size. For comparison, the width of a human hair is roughly 100 microns!

Besides that, the cloud has to be “optically thin” or almost transparent so that light hits only one droplet and gets bent only once, unlike rainbows in which light gets bent twice to create colours.