Cavum

Copyright Tsz Cheung Lee

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.

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