Say Hello to the 11 New Cloud Atlas Inductees

There's more to identifying clouds than you might think. anna.a/Shutterstock

The next time you look up in the sky and wonder what kind of cloud is hovering above, you can look down at your phone for the answer.

Last revised in 1987, the World Meteorological Organization's (WMO) International Cloud Atlas has been the professional standard for identifying clouds since the late 19th century. Now the latest edition of the atlas is available online for free, and it includes 12 previously unlisted cloud formations, including roll and asperitas clouds.

"Advances in science, technology and photography prompted WMO to undertake the ambitious and exhaustive task of revising and updating the Atlas with images contributed by meteorologists, cloud watchers and photographers," WMO Secretary-General Petteri Taalas said in a statement.

But it wasn't just scientists who contributed. Citizen cloud watchers played an important part in getting at least one "new" cloud added to the mix.

Stormy wave clouds

Asperitas clouds in Missouri
Asperitas clouds often look like choppy waters during a storm, but they rarely produce storms themselves. Agathman/Wikimedia Commons

The asperitas cloud question has been hanging over the WMO almost 10 years. According to the BBC, the clouds were first recorded in 2006, looming in the Iowan sky. As more images began to pour in, the Cloud Appreciation Society president, Gavin Pretor-Pinney, and the Royal Meteorological Society began lobbying the WMO in 2009 for asperitas clouds to be recognized by the organization.

"Back in 2008, I thought the chances of this becoming official were really minimal," Pretor-Pinney told the BBC.

"At first the WMO were saying they had no plans to do a new edition, but over time I think they began to realize there is an interest among the public in clouds and there is a need for that interest to be an informed one, there's a need for this authoritative work."

While asperitas clouds — whose name means "rough" in Latin — look quite scary, they rarely produce storms.

Cloud classification

Photo: Daniela Mirner Eberl [CC by 3.0]/Wikimedia Commons

Some clarification is necessary, however, because how we talk about clouds and how the WMO talks about clouds can be, well, a little cloudy in and of itself.

The WMO has 10 basic genera of clouds, all defined based on where they form in the sky and their basic size. These genera are then divided into species that provide a more refined description of the cloud's shape and its internal structure. From species, the WMO drills down further for varieties to describe a cloud's transparency and arrangement. Some clouds have multiple varieties, and then there are supplementary features to clouds that appear with the clouds, or sometimes merge with them. An asperitas, for example, is a supplementary feature associated with stratocumulus and altocumulus clouds, so it's a newly recognized feature of two established genera of clouds.

Only one new species of cloud was added to the atlas, and that was the volutus, or roll cloud. This long, tube-looking cloud seems to move slowly across the sky. Unlike an asperitas cloud, a roll cloud is often the herald of approaching storms.

The other new clouds added to the atlas were either supplementary features, accessory clouds or "special clouds." Here's what they look like and a brief description of each:


A cavum cloud over Hong Kong.
A cavum cloud over Hong Kong. Lee Tsz Cheung/World Meteorological Organization

A supplementary feature of altocumulus and cirrocumulus clouds, this is basically a big hole in a thin layer of clouds


Fluctus clouds over Laukvik, Norway
Fluctus clouds over Laukvik, Norway. June Grønseth/World Meteorological Organization

These wavy clouds are very short-lived supplementary features of cirrus, altocumulus, stratocumulus and stratus clouds.

3. Murus

Murus cloud in Cheyenne, Wyoming.
Murus cloud in Cheyenne, Wyoming. Eric Van Lochem/World Meteorological Organization

Commonly known as wall clouds, murus are a lowering of a cumulonimbus cloud. If you're familiar with supercell storms, you've probably seen one of these.

4. Cauda

Like, murus clouds, caudas are a supplementary feature of supercell cumulonimbus clouds. While murus clouds are the massive wall, the cauda is a cloud that forms at the bottom of the wall cloud. In the image above, you can see thee cauda cloud tail.

5. Flumen

A wall cloud with a cauda cloud tail forms over Elmer, Oklahoma.
A wall cloud with a cauda cloud tail forms over Elmer, Oklahoma. The lowest middle section of this cloud is the flumen. Steve Willington/World Meteorological Organization

A flumen is an accessory cloud to our now familiar supercell cumulonimbus clouds. If you look at the picture above for cauda clouds, you'll see a long tail of clouds right at the bottom. That's the flumen.

6. Cataractagenitus

Clouds form near Niagara Falls in Canada
Clouds form near Niagara Falls in Canada. courtel/World Meteorological Organization

One of the newly inducted "special clouds," cataractagenitus clouds form near waterfalls, the result of water broken up into spray by the falls.

7. Flammagenitus

Flammagenitus cloud forms near Serra de Alvorge, Portugal, the result of a wildfire.
Flammagenitus cloud forms near Serra de Alvorge, Portugal, the result of a wildfire. Jan Knight/World Meteorological Organization

As their name might suggest, these special clouds may develop due to naturally occurring heat-related incidents, like forest fires or volcanoes.

8. Homogenitus

Cumulus and stratocumulus clouds generated by the thermal plumes from a coal-using power plant in Espoo, Finland.
Cumulus and stratocumulus clouds generated by the thermal plumes from a coal-using power plant in Espoo, Finland. Jarmo Koistinen/World Meteorological Organization

Homogenitus are special clouds that cover a range of man-made clouds. This includes clouds from industrial practices and contrails that are formed from airplanes. So the next time a child suggests there's a cloud factory nearby, you can tell her that she's not entirely wrong!

9. Silvagenitus

Silvagenitus clouds form over a forested area of Switzerland.
Clouds form over a forested area of Switzerland. Franz Feldmann/World Meteorological Organization

These fleeting special clouds develop over forests, the result of evaporation along the canopy.

10. Homomutatus

Persistent contrails form over Wokingham, England.
Persistent contrails form over Wokingham, England. George Anderson/World Meteorological Organization

Contrails get special-special-cloud treatment! Homomutatus clouds are multiple, persistent contrails that form over a period of time and seem to spread out over time.