cavum

A supplementary feature incorporated as a novelty into the WMO International Cloud Atlas in its 2017 edition. It is a well-defined hole, usually circular or elliptical, that pierces a layer of cirrocumulus or altocumulus, allowing the blue sky of the upper atmospheric level to be seen through it. Occasionally, those cavities present elongated shapes and it is common for there to be a small, frayed cloud of the genus Cirrus inside them, from which a virga hangs. For some time, these striking holes gave rise to different theories about their formation. In 2011 the puzzle was resolved, by linking most of them to the passage of aircraft. A significant fraction of those cloud layers is made up of water droplets in a state of supercooling, which, subjected to a sudden change in pressure, freeze immediately and precipitate. When an aircraft in its takeoff phase crosses one of those cloud layers, a chain-reaction freezing process of those droplets is set in motion, resulting in the hole, also known internationally as a hole punch cloud.