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Fallstreak Hole

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Article Information
Category: Weather Weather
Content source: SKYbrary About SKYbrary
Content control: SKYbrary About SKYbrary
WX
Tag(s) Cloud Formations

Description

A fallstreak hole (also known as a cavum, hole punch cloud, punch hole cloud, skypunch, cloud canal or cloud hole) forms when part of the cloud layer (cirrocumulus or altocumulus) forms ice crystals which 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.

Aircraft, which often have a large reduction in pressure behind the wing- or propeller-tips, punching through this cloud layer can cause air to expand and cool very quickly as it passes over the aircraft wings or propeller. This change in temperature can be enough to encourage the supercooled droplets to freeze and can produce a ribbon of ice crystals trailing in the aircraft's wake. The ice crystals then fall from the cloud layer. When ice crystals do form, a domino effect is set off due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular or elliptical, hole in the cloud.


Further Reading