Cavum – The Icy Hole in the Sky

What caused this hole, shaped like a horseshoe crab, in the Altocumulus layer of cloud spotted by Anne Ducret over Lot, Occitanie, France? It is known as a cavum, or fallstreak hole, and it is a feature that appears in layers of the mid-level, clumpy cloud Altocumulus more than any other.

This is because the water droplets in Altocumulus clouds tend to be ‘supercooled’. Clouds like these, high up in the mid-levels of the troposphere, are at temperatures well below 0°C (32°F), yet their droplets remain unfrozen. In fact, they often stay liquid at temperatures as low as –15ºC (5ºF) or even colder. Becoming supercooled like this, water droplets up in the atmosphere behave rather differently from water down at ground level. But it is not a stable state of affairs. The supercooled droplets in an Altocumulus cloud can start to freeze. This is when a cavum can appear, and it begins when something acts as a trigger to get them started.

That something is usually an aircraft. As it climbs or descends through a cloud of supercooled droplets, the drop in pressure at an aircraft’s wing tips can cool the air just enough to trigger freezing. And once it starts, the freezing process spreads. The ice crystals that develop in the wake of the aircraft soon grow and act as the seeds onto which the surrounding supercooled droplets readily freeze. The crystals grow and splinter, and the resulting tiny ice shards act as more seeds encouraging more droplets to freeze. The aircraft sets off a chain reaction of freezing.

As the freezing spreads outwards through the cloud layer, the ice crystals soon grow large enough to fall below, appearing as a fallstreak. They dissipate away in the drier air below, leaving behind a circular hole in the cloud – or, in the case of Anne’s cavum, one that was shaped like a horseshoe crab trailing a short strip of contrail as its tail.

Cavum, or fallstreak hole, spotted in a high layer of Altocumulus by Anne Ducret over Lot, Occitanie, France. View this in the photo gallery.