Cavum (formerly hole-punch cloud or fallstreak hole) was officially recognised by the WMO in 2017. It consists of a circular or elliptical gap several kilometres across in a layer of altocumulus or cirrocumulus, with ice-crystal filaments (virga) falling from the centre.
Formation mechanism
It occurs when supercooled water droplets begin freezing at one point, typically triggered by an aircraft whose wake induces glaciation. Freezing releases latent heat that warms the surrounding air, causing adjacent droplets to evaporate and expanding the hole radially. The opening can persist for up to an hour as ice crystals fall as virga.