Asperitas clouds over Burnie, Tasmania.
Asperitas clouds over Burnie, Tasmania, Australia. A new study suggests that this type of cloud feature may form from the interaction between air circulation and gravity. Credit: © Gary McArthur, Cloud Appreciation Society Member 5353

Giles Harrison, a cloud aficionado, is always hoping for overcast skies. “I’m required to reject ‘blue-sky thinking,’” jokes Harrison, an atmospheric physicist at the University of Reading in Reading, UK. He and Gavin Pretor-Pinney, the founder of the Cloud Appreciation Society (CAS), recently investigated the science behind “asperitas,” a new cloud feature that the World Meteorological Organization added to its International Cloud Atlas earlier this year. The team suggests that the new feature, which was first reported by citizen scientists 11 years ago, owes its rough and chaotic appearance to oscillating streams of moving air contained within it.

The collaboration between scientists and enthusiastic members of the public allowed asperitas to be documented, said Matthew Igel, an atmospheric scientist at the University of California, Davis. “This is a great example of how crucial members of the public can be to the scientific enterprise,” he said.

Standing Up for Clouds

Pretor-Pinney, a British graphic designer, founded the Cloud Appreciation Society in 2005. “I’ve always thought that clouds were an underappreciated aspect of nature,” he said. “I thought someone needed to stand up for them. That’s why I started CAS.” Today, the society has over 43,000 members and constantly receives photographs of clouds from its members and the general public. Eleven years ago, the young club received a few astonishing images that triggered an international investigation.

It’s as if you’re “looking up at the surface of the sea from below on a really stormy day.”

Those images, snapped by paralegal Jane Wiggins from Cedar Rapids, Iowa, showed choppy clouds filling the sky. It’s as if you’re “looking up at the surface of the sea from below on a really stormy day,” said Pretor-Pinney. Such cloud features hadn’t been documented before, and “this formation did not easily fit within the existing naming system,” the CAS website reported. Pretor-Pinney suggested calling the formation “asperatus,” a form of the Latin verb “aspero,” meaning to make rough, and the name stuck.

A Strict Cloud-Naming System

Getting the new cloud feature (it’s not a new cloud type because it can be observed on known varieties of clouds, notes Igel) officially recognized would require permission from the World Meteorological Organization (WMO), the United Nations agency that maintains the International Cloud Atlas. WMO is notoriously strict about adding entries to its definitive cloud-naming system, which was last updated in 1987.

Meanwhile, cloud aficionados continued reporting asperatus sightings in the United States, Scotland, Belgium, and Norway. In 2011, social media accounts flooded with news of asperatus over Chicago. “It’s a rare phenomenon, but people are clearly observing it,” said Harrison.

Earlier this year, WMO announced that it would be including asperatus in its revised International Cloud Atlas, which was being published online for the first time. Asperatus would be filed as a cloud “supplementary feature,” and its name would be altered slightly to “asperitas” because of WMO convention: Supplementary cloud features are always named using a Latin noun.

The Science of Asperitas

Now, cloud researchers, including Giles Harrison, have studied why asperitas forms. “From a photograph, you see nothing—you just see a picture,” said Harrison. “If we want to analyze why these clouds form, we need to have some meteorological information.” Harrison, Pretor-Pinney, and their colleagues collected reports of European sightings of asperitas. They correlated the images with meteorological data such as temperature, relative humidity, wind speed, and wind shear collected from weather forecasting models.

Using estimates of cloud height, the team found that asperitas was a low-lying feature: It was typically no higher than 1000 or 2000 meters. Because the feature is relatively close to the ground and well below the height where freezing occurs, asperitas is likely composed of water droplets rather than ice particles, the team reported this month in Weather, a publication of the Royal Meteorological Society.

Asperitas’s characteristic wavy look might be caused by atmospheric gravity waves propagating through the cloud.

Harrison and his colleagues also suggested that asperitas’s characteristic wavy look might be caused by atmospheric gravity waves propagating through the cloud. These waves are caused when air is displaced upward and then falls back down because of gravity. “They’re quite common in the atmosphere,” said Harrison. Atmospheric gravity waves might oscillate within a cloud, the team proposes, resulting in a rough and chaotic appearance—asperitas—that persists for several hours.

The case isn’t closed on this new cloud feature, however. “The paper suggests several possible formation mechanisms, and I think many more are plausible,” said Igel. Cloud watchers will continue to appreciate asperitas for its beauty and its demonstration of citizen science in action. “Life would be dull if we had to look up at cloudless monotony day after day,” reads the CAS manifesto.

—Katherine Kornei (email:; @katherinekornei), Freelance Science Journalist

Correction, 13 June 2017: An earlier version of this article incorrectly identified the likely state of matter of the water within the cloud feature known as asperitas. This article has been updated with the correct information.


Kornei, K. (2017), Science explains “rough and chaotic” cloud feature, Eos, 98, Published on 06 June 2017.

Text © 2017. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.