A fallstreak hole — officially named cavum — is a clean, often circular gap in a thin sheet of supercooled water-droplet cloud. The droplets are below freezing yet still liquid; when something triggers a few to freeze, ice crystals grow at the expense of the surrounding droplets, which evaporate and open a hole. Wisps of falling ice often trail through the gap. Aircraft punching through the layer are a frequent trigger, leaving circular or linear holes.
Worth knowing
The WMO formally added cavum to the International Cloud Atlas in 2017 — the atlas's first major update in roughly 30 years — recognising that many of these tidy 'hole-punch' clouds are inadvertently created by aircraft passing through supercooled cloud.
Note
Still unsure it's the same cloud? Our app, Cloud Point— on iOS and Android — reads a photo and returns the likely type with altitude, useful when a formation is fading faster than you can look it up.
