cloud atlas

All posts tagged cloud atlas

© Tsz Cheung Lee – Tap for larger

Cloud of the Day – Cavum

Cavum is one of the new clouds that show up in the latest edition of the World Meteorological Organization’s International Cloud Atlas. I reported on the release of the new edition in this post. Cavum is really just a new name for a cloud type previously known as a fall streak hole, which I reported on here. There are more great pictures in that post. The full name for the example shown in this post is altocumulus stratiformis perlucidus translucidus cavum. That is, the middle etage cloud altocumulus (my previous post on altocumulus) which is in a layer thin enough to allow light through, and which has gaps between its elements and a great big hole with virga in it. Here’s how cavum is described in the International Cloud Atlas.

A well-defined generally circular (sometimes linear) hole in a thin layer of supercooled water droplet cloud. Virga or wisps of Cirrus typically fall from the central part of the hole, which generally grows larger with time. Cavum is typically a circular feature when viewed from directly beneath, but may appear oval shaped when viewed from a distance.

When resulting directly from the interaction of an aircraft with the cloud, it is generally linear (in the form of a dissipation trail). Virga typically falls from the progressively widening dissipation trail.

Occurs in Altocumulus and Cirrocumulus and rarely Stratocumulus.

And here’s the description of the image from the International Cloud Atlas.

This thin, translucent and extensive layer of cloud is Altocumulus stratiformis translucidus. In the top part of the picture it also displays the variety perlucidus, as there are the gaps between the cloud elements. However, the most striking feature is the large, roughly circular hole beneath which there is virga. The large hole is the supplementary feature cavum, popularly known as a “fallstreak hole” or “hole-punch cloud”. The full classification for the cloud is therefore Altocumulus stratiformis perlucidus translucidus cavum.

Also of note is a linear gap in the cloud between the fallstreak hole and the horizon. This is an aircraft dissipation trail, or distrail, formed as a result of an aircraft flying through the cloud layer. Informally this is sometimes known as a “canal cloud”. It later transformed into a circular-type hole.

The supplementary feature cavum is formed when glaciation occurs in a thin cloud layer consisting of supercooled water droplets that are in a liquid state and at a temperature below 0 °C. As the supercooled water drops glaciate, the resulting ice crystals fall from the cloud layer to a lower level as virga, or fallstreaks. The resulting cloud hole typically grows larger with time while the glaciation process continues.

There’s not much I can add to that, except to invite you to visit the International Cloud Atlas website.


Steve Willington – WMO

Cloud of the Day – Flumen

Flumen, from the Latin flumen, a noun meaning river, is another of the new cloud types included in the World Meteorological Organization’s fifth edition of their venerable Cloud Atlas, published in 2017. It’s the first new edition in thirty years so there aren’t many opportunities to get a new cloud in.

Flumen are described as bands of low clouds associated with a ‘supercell severe convective storm’ — a form of cumulonimbus — arranged parallel to the low level winds blowing toward the supercell. So the powerful convection of the supercell is drawing air into its base, and the high speed airflow causes a drop in air pressure with the concommitant drop in temperature, allowing condensation and cloud-forming. This explains why flumen form parallel to wind direction.

Kelvinsong – CC-BY-SA

Flumen are associated with supercells, a particularly vigorous form of thunderstorm featuring a mesocyclone, a rotating updraft. These are the clouds that spawn tornadoes. Flumen are very likely to be associated with precipitation, and then some.


Cloud of the Day – Asperatus

Asperatus, also known as undulatus asperatus and altocumulus undulatus asperatus, is biblical. It’s the kind of cloud that makes people think that the world might be at the mercy of supernatural forces. They might fall to their knees and beseech their wrathful gods for mercy. Other people might look at them and say, “So, that’s where van Gogh got it from.”

Photo credit - Ken Prior

Photo credit – Ken Prior

Asperatus, loosely meaning “roughened waves,” is thought to form under the same kind of conditions as mammatocumulus, only with winds strong enough to shear the mammatus bulges into wave-like undulatus forms. This cloud hasn’t yet been officially named and added to the World Meteorological Organization’s definitive International Cloud Atlas. The Atlas was most recently published in 1975. The last time a cloud was added was 1951. The jury is out on whether asperatus will be added to the Atlas, and no one expects it to be soon if it is.

Note: The World Meteorological Organization has added asperatus — renamed asperitas — to the International Cloud Atlas (see this Green Comet post) in its fifth edition published in 2017.

Photo credit - Agathman - CC-BY

Photo credit – Agathman – CC-BY

The Cloud Appreciation Society has been very important in the discovery of this new cloud type, especially its founder, Gavin Pretor-Pinney. The society has thousands of members who send in beautiful photographs of clouds, and it has a nice selection of asperatus. Because of the way their site is set up, I can’t link directly, so you’ll have to search on “asperatus.”

Photo credit - Ave Maria Mõistlik - CC-BY-SA

Photo credit – Ave Maria Mõistlik – CC-BY-SA

Asperatus is not a harbinger of stormy weather, more often appearing as the weather abates.

Photo credit - NASA - PD

Photo credit – NASA – PD