Description & Characteristics. The ‘cavum’ cloud feature can be found amongst three cloud types: cirrocumulus, altocumulus, and stratocumulus. Translated from latin, meaning cavity, the three types of cavum clouds are respectively abbreviated as ‘Cc cav’, ‘Ac cav’, and ‘Sc cav’. The cavum cloud formation is fairly easy to identify because of its head turning appearance. It looks like a giant hole in a cloud layer accented by a virga wisp falling underneath the hole.
The cloud feature cavum have a number of nicknames: fallstreak hole, hole punch cloud, cloud cavity, skypunch cloud, cloud canal, cloud hole, and probably others. The term fallstreak hole is the most popular of the bunch and describes the formation best, as a fallstreak (another name for a cirrus or virga wisp) is found below a hole in a cloud layer.
Generally speaking, the more prolific of fallstreak holes in altocumulus layers are formed when an aircraft flies through a thin, cumuliform cloud layer and triggers glaciation, which is a fancy word for cloud particles transforming from water droplets to ice particles. This domino effect creates a gap in the cloud, where ice particles descend in altitude, and sometimes curl due to different wind speeds at varying altitudes. There are instances where fallstreak holes form without the help of aircraft, particularly in cirrocumulus layers.
Because of their strange appearance and rarity, the cavum cloud feature is a favorite amongst cloudspotters. They’re most often found in altocumulus clouds, followed by cirrocumulus and then stratocumulus. It’s also worth noting that fallstreak holes are often paired with the stratiformis cloud species. You might increase your chances of seeing one if you position yourself near an airport where its flight paths have aircraft ascending and descending through these clouds.