A fallstreak hole (also known as a cavum, hole punch cloud, punch hole cloud, skypunch, cloud canal or cloud hole) is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water, in a supercooled state, has not frozen yet due to the lack of ice nucleation. 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, hole in the cloud.
It is thought that the introduction of large numbers of tiny ice crystals into the cloud layer sets off this domino effect of fusion which creates the hole. The ice crystals can be formed by passing aircraft, which often have a large reduction in pressure behind the wing- or propeller-tips. This cools the air very quickly, and can produce a ribbon of ice crystals trailing in the aircraft's wake. These ice crystals find themselves surrounded by droplets, and grow quickly by the Bergeron process, causing the droplets to evaporate and creating a hole with brush-like streaks of ice crystals below it.
The articles by Westbrook and Davies (2010) and Heymsfield et al. (2010) explain the process in more detail, and show some observations of their microphysics and dynamics. Such clouds are not unique to any one geographic area and have been photographed from many places.
- Sutherland, Scott (March 23, 2017). "Cloud Atlas leaps into 21st century with 12 new cloud types". The Weather Network. Pelmorex Media. Retrieved 24 March 2017.
- "Cloud Appreciation Society | Fallstreak Holes (February '06)". www.cloudappreciationsociety.org. Retrieved 2017-07-29.
- Westbrook, Chris; Davies, Owain (July 2010). "Observations of a glaciating hole-punch cloud". Weather. 65: 176–180. arXiv:0907.4302. Bibcode:2010Wthr...65..176W. doi:10.1002/wea.504.
- Heymsfield, Andrew J.; Kennedy, Patrick C.; Massie, Steve; Schmitt, Carl; Wang, Zhien; Haimov, Samuel; Rangno, Art (2010). "Aircraft-Induced Hole Punch and Canal Clouds: Inadvertent Cloud Seeding". Bulletin of the American Meteorological Society. 91: 753–766. Bibcode:2010BAMS...91..753H. doi:10.1175/2009BAMs2905.1.
- 'UFO cloud formation' filmed in Romania
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