Cavum – Cookie Clouds and Icing Sugar

A cavum cloud feature, like this one spotted in Altocumulus clouds by John Henderson over Scarborough, North Yorkshire, England, looks like a large hole made with a cloud cookie cutter with a dusting of icing sugar thrown in. The feature, which is also known as a fallstreak hole, results from the strange behaviour of water droplets in a cloud that have become ‘supercooled’.

We describe cloud droplets as supercooled when they’ve refused to freeze into ice crystals even though they are at temperatures below 0°C (32°F). They end up like this surprisingly often. Many clouds are made of supercooled droplets – in particular, mid-level clouds like Altocumulus, which are the ones that most often produce cavum features. When it is in the form of tiny droplets in a cloud, water doesn’t freeze in the same way as it does in the ice tray of your freezer. Cloud droplets only tend to freeze when there’s something for them to get started on: tiny airborne particles like dust, ash or salt that are the right shape and size to act as seeds onto which the tiny crystals of ice can start to grow. Without enough of these icing nuclei floating around, the droplets will just drift along in the clouds in a supercooled liquid state in temperatures as low as –40°C (–40°F) until something happens to get the freezing started. Once it starts in one part of the cloud, the freezing process spreads in a domino effect, which causes the cavum to appear with a streak of ice crystals falling from it.

The freezing can be triggered by an aircraft climbing or descending through it. This disturbance of the cloud layer can start the freezing process when the low air pressure in the vortices around the aircraft wings cools the air enough to make a few of the supercooled droplets start to freeze. And once a few get going, a chain reaction starts in that region of the cloud. This is because the very best icing nuclei are ice crystals themselves. They are the perfect seeds for supercooled droplets to start freezing into. The moisture in the supercooled droplets around the newly formed ice crystals readily jump ship and start to freeze onto the crystals, which soon grow and splinter to form more tiny pieces of ice that, in turn, act as seeds for more droplets to freeze. Over 15 minutes or so, the freezing can have spread out in a huge circle. As they form, the ice crystals grow large enough to fall below. They appear as the fallstreak beneath the hole, usually dissipating away in the warmer air below long before reaching the ground. 

While aircraft are the most common triggers for the freezing that forms cavum in supercooled clouds, it can also be set off by the ice crystals of Cirrus clouds above falling into the layer and acting as the icing nuclei. Sometimes, more typically in higher clouds like Cirrocumulus, just a natural drop in air temperature at the cloud level is enough for the cloud’s supercooled droplets to become less fussy about which specks of dust they freeze onto. This can result in multiple cavum appearing in the cloud layer. Whether there is just one or a whole tray, cavum is a feast for the sweet-toothed cloudspotter.

A cavum, or fallstreak hole, spotted in Altocumulus clouds over Scarborough, North Yorkshire, England by John Henderson.