Description

A fallstreak hole (also known as a cavum, hole punch cloud, punch hole cloud, skypunch, cloud canal or cloud hole) is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. It forms when part of the cloud layer forms ice crystals that are large enough to fall as a 'fallstreak'.

Mechanism

The ice crystals form in clouds of supercooled water droplets (water below 0 °C but not yet frozen). These water droplets need a tiny particle, a nucleus, to freeze or to be cooled below -40 °C.