A fallstreak hole, also known as a hole punch cloud, punch hole cloud, skypunch, canal cloud or cloud hole, is a large circular or elliptical gap 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 has not frozen yet due to the lack of ice nucleation particles (see supercooled water). When ice crystals do form it will set off a domino effect, 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 believed that the introduction of large numbers of tiny ice crystals into the cloud layer sets off this domino effect of evaporation 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, 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.
- The cloud appreciation society
- 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
- Many images and explanations from LiveScience
- Weather.com images and data
- Description and photographs of several fallstreak holes in the United States and Russia October 2009
- BBC News: Fallstreak hole clouds over Derbyshire
- Fallstreak Hole: Photographers capture rare cloud formation in Victoria's east. Australian Broadcasting Corporation News, 3 November 2014
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