photo #5614701

Holger Karsten Schmidt spotted this distinctive hole forming in a layer of mid-level cloudlets known as Altocumulus over Fuseta, Portugal. Sometimes known as a fallstreak hole, this Altocumulus cloud feature has the Latin name cavum, meaning ‘cavity’ or ‘hole’. The Altocumulus layer here is made up of supercooled water droplets, which means the liquid droplets are well below the freezing temperature of water in normal conditions down at ground level, but they’re stubbornly refusing to freeze into ice crystals.Water often remains in a supercooled liquid state like this when it is in tiny droplet form up in the low pressure of the higher atmosphere. But it is an unstable state of affairs, and something can set off the freezing by disturbing the cloud as it passes through. This is generally an aircraft climbing or descending through the cloud. The cooling in the low air pressuring of its wing vortices can cool the cloud just enough to start its droplets freezing. The ice crystals that form soon grow large enough to fall below and leave a hole. They also encourage any surrounding supercooled droplets to freeze also, and so a chain reaction of freezing spreads outwards and the cloud hole grows.