Michael Anderson was trekking in the Himalayas with his wife and friends when they caught sight of a spectacular cloud developing in the winds over Mount Manaslu. The cloud diffracted the sunlight as it shined through to produce shimmering colours known as cloud iridescence. “We got our first clear look at Mount Manaslu as we arrived at a local village to stay in a hostel,” remembers Michael. “But when we saw this cloud develop, our view went from spectacular to surreal.”
The main body of the cloud is known as an Altocumulus lenticularis, a disc-like cloud formation that appears over mountain ranges in the rising and dipping flow of winds. The cloud’s droplets had formed where the air cooled as it rose to pass over Mount Manaslu and evaporated away again where the airflow warmed again as it dipped back down beyond the peak. In these high winds, the droplets didn’t hang around for long. They remained very small and of a consistent size in their short lifespan. Tiny, evenly sized droplets like this are perfect for producing cloud iridescence when bright sunlight shines through a cloud.
The village where they were staying was made up of two monasteries, each with just 40-50 inhabitants. “There couldn’t have been more than 100 people who saw this cloud,” remarked Michael.
In addition to its shimmering colours, the cloud also had lots of eyebrow-like features that formed and dissipated in the turbulent winds. These are examples of a cloud formation we’ve been arguing deserves an official classification of its own. Although it is yet to be accepted as an official type of cloud, we have tentatively been calling this type of cloud feature that forms in turbulent mountain winds supercilium after the Latin for an “eyebrow”.
Michael and his companions weren’t the only ones excited by the iridescent eyebrows. The villagers were amazed too. “I asked the Nepali owner of the tea house where we were staying if he’d seen this sort of thing before,” Michael remembered. “He said he’d never seen anything like it in the 30 years he had been living there.” Another of the villagers explained that it was likely a Hindu god expelling some of his excess power. Whether formed by a hyperactive deity or by mountain airflows, the iridescent eyebrow cloud captivated the whole village. “We all stood there with mouths agape,” said Michael, “and we felt ridiculously lucky.”