Cloud Atlas may 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.
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, let’s meet these new cloud types!
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).
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).
Cavum is 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.
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.
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.
For more amazing cloud pictures, head over to the International Cloud Atlas image gallery.
Please like, share and tweet this article.
Pass it on: Popular Science