A fallstreak hole (also known as a cavum, hole punch cloud, punch hole cloud, skypunch, cloud canal or cloud hole) is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. It forms when part of the cloud layer forms ice crystals that are large enough to fall as a 'fallstreak'.
A Hole Punch Cloud (or Fallstreak Hole), observed on 2008 August 17 about 20km south of Linz, Austria. - image byH. Raab. Source: Wikicommons.
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 with streaks of ice crystals below the hole.
Fallstreak holes over Louisiana and Texas as seen by NASA's Terra satellite on 29th January 2007. Source: Wikicommons.
The image above shows hole-punch clouds captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on 29th January 2007. The cloud blanket on 29 January 2007 consisted of supercooled clouds, with cloud-top temperatures ranging from -20 to -35 degrees Celsius. As aircraft from the Dallas-Fort Worth airport passed through these clouds, tiny particles in the exhaust came into contact with the supercooled water droplets, which froze instantly. The larger ice crystals fell out of the cloud deck, leaving behind the "holes". A few of the “holes” are elongated, with what appear to be smaller clouds inside them. This strange phenomenon resulted from a combination of cold temperatures, air traffic, and perhaps unusual atmospheric stability.