Cloud Atlas leaps into 21st century with 12 new cloud types

Asperitus?! Flammagenitus?! Fluctus?! No, those aren't new spells from the world of Harry Potter. They're three of the dozen new cloud types added to the WMO Cloud Atlas, in its first update in over 30 years!

Cloud Atlasmay be best known as a big-budget 2012 movie, staring Tom Hanks, but the International Cloud Atlas is a guide that every meteorologist in the world can turn to when looking at the clouds we see in the sky. Established in 1939 by the World Meteorological Organization (WMO) and last updated in 1986, this invaluable resource has now been moved into the digital realm, so that it is available to us all, and it adds a dozen new cloud types - both naturally-occurring and human-caused - to its lexicon of weather terms.

"This is THE world reference for observing and classifying clouds and other weather phenomena. The Atlas contains pictures, definitions, and explanations that are accepted and used by all WMO's 191 Member countries and territories," Bertrand Calpini, President of WMO's Commission for Instruments and Methods of Observation (CIMO), the group that oversaw the revision process, said in a statement on Wednesday.

"This new edition brings together for the first time all types of measurements, including very high-tech surface-based, in situ and space observations and remote sensing, thus giving to the human observer a revolutionary tool to understand clouds."

So, let's meet these new cloud types!

Clouds are classified in a way very similar to what's used in biology - genera for the overarching classes, based on their height and thickness (cirrus, cumulus, stratus, etc), species for specific types under those broad classes (fractus, congestus, etc), varieties that describe the organization and transparency of the clouds, and then supplementary features, which are smaller clouds associated with and attached to larger clouds, and accessory clouds that are smaller clouds associated with larger clouds, but are mostly separate from them.

So, for mid-level puffy clouds that form a near-continuous sheet across the sky, that is opaque enough to block out the Sun and has a chaotic, rolling base caused by wind shear, they end up with the name Altocumulus stratiformis opacus asperitas (see the 2nd entry, below!).

Volutus

Credit: Daniela Mirner Eberl/Wikimeda Commons

Volutus is a new species of cloud that encompasses the various roll clouds. These form as long, horizontal tubes, detached from any other clouds in their vicinity, which are caused by differences in wind speed and direction between the surface and higher up (aka wind shear).

Asperitus

Formerly known as undulatus asperatus, when the supplemental feature Asperitus shows up, it's almost like looking at waves on the surface of the water, but from a vantage point under the water. The Atlas separates this from "undulatus" clouds, since undulatus are much more organized (often into bands).

Although the exact way that these cloud form isn't understood yet, according to the Cloud Appreciation Society, there's some evidence that they are similar to mammatus clouds - from sinking, moisture-laden air, but with wind shear (its change in direction/speed over some distance) along the cloud base that causes the wind to flow in waves, carving the cloud base into these bizarre-looking shapes.

Cavum

Credit: H. Raab/Wikimedia Commons

Cavumis now the formal name of what's been called a "fallstreak cloud" or "hole punch cloud".

This supplemental feature is caused when ice crystals are introduced into a thin cloud comprised of super-cooled water droplets, usually due to an aircraft passing through the cloud on takeoff or landing. The water droplets remain liquid well below freezing, since there is nothing present for them to freeze onto. With the sudden appearance of the ice crystals, the water droplets in the vicinity all rush in - creating a clear spot in the cloud - where they freeze and grow the crystals into snowflakes, which then fall out of the sky. The wispy clouds in the middle of the cavum, in the image above, are virga produced by that falling snow.

Murus

Credit: NOAA

A Murus, or wall cloud, is a supplemental feature that takes the shape of bank of cloud that lowers from the base of a supercell, is associated with strong updrafts, and can indicate the presence, or impending development, of a tornado.

Cauda

Credit: NOAA

A Cauda, also known as a tail cloud, is another supplemental feature, which is seen off to the right of the wall cloud, above, and it looks a bit like a dinosaur's tail trailing behind the storm. Caused by air flowing into the storm, its tube-like shape can sometimes be mistaken for a tornado.

Fluctus

Credit: GRAHAMUK/Wikimedia Commons

These supplemental features, now called Fluctus, are widely known as Kelvin-Helmholtz waves or Kelvin-Helmholtz instability. These features are caused by the winds above the cloud top blowing faster than the winds inside the cloud. This difference in speed (aka velocity shear) creates vortices, resulting in the top of the cloud being pulled upward into these wave forms.

Flumen

A Flumen, aka "a beaver's tail", is an accessory cloud caused by warm, humid air flowing into the based of the storm. 

These are often mistaken for tornadoes, but it only takes a moment to tell them apart, as a flumen will not be rotating.

And then there are the "special clouds", which prompted the WMO to add two new cloud "suffices" - genitus, which indicates a cloud that formed or grew due to local conditions, and mutatus, which is used for when local conditions cause one cloud type to morph into another type.

Cataractagenitus

Credit: Billy Hathorn/Wikimedia Commons

Cataractagenitus are clouds generated by the spray from large waterfalls, as seen above in this photo from Niagara Falls.

Flammagenitus

Flammagenitus form due to the heat from wildfires. Stormhunter Mark Robinson spotted these above the Fort McMurray wildfire, in May 2016.

Homogenitus and Homomutatus

Credit: Wikimedia user "La Responsable"

Homogenitus are clouds that are produced by human activities. Aircraft contrails, clouds that form from industrial emissions, or that form over top of power plant cooling towers, are just three examples.

Homomutatus are clouds that formed due to human activities, and then underwent a change to a different kind of cloud. The best example of this is persistent contrails, which is when aircraft contrails form in air that contains enough humidity for them to persist and grow, spreading out into cirrostratus or cirrocumulus clouds.

Both of these types can be seen in the image above. The thinner, more distinct contails are the more recent homogenitus. In the distance, and higher up (the contrail closest to the top of the image), are the homomutatus as the contrails are developing into more widespread cirrus clouds.

Silvagenitus

Silvagenitus, is a type of cloud that forms as humidity is added to the air above the forest canopy from the soil and plants that make up the forest.

The International Cloud Atlas can be accessed via the WMO website, which includes hundreds of cloud classifications and amazing images from people around the world.

"The International Cloud Atlas task team had to choose from thousands of images from meteorologists, cloud lovers and photographers around the world," Chi-ming Shun, Director of the Hong Kong Observatory, which is hosting the web portal, said in the WMO press release. "We selected the best of the best. We feel honoured to be involved in such a visible product of WMO. We applaud WMO for updating the International Cloud Atlas which provides a unique platform for engaging the public in better understanding clouds, weather and climate."

Sources: WMO International Cloud Atlas| WMO | Iowa State University of Science and Technology

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