Fallstreak hole

A fallstreak hole visible over Omarama, New Zealand in May 2006 Fallstreak hole over Austria, August 2008 Satellite image of canals in clouds over east Texas in January 2007

A fallstreak hole, also known as a hole punch cloud, punch hole cloud, canal cloud or cloud hole, is a large circular 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 a portion of the water does start to freeze it will set off a domino effect, due to the Bergeron process, causing the water vapor around it to freeze and fall to the earth as well. This leaves a large, often circular, hole in the cloud.

It is believed that a disruption in the stability of the cloud layer, such as that caused by a passing jet, may induce the domino process of evaporation which creates the hole. Such clouds are not unique to any one geographic area and have been photographed from the United States to Russia.

Because of their rarity and unusual appearance, fallstreak holes are often mistaken for or attributed to unidentified flying objects.

See also

• List of cloud types
• Distrails

External links

• Description and photographs of several fallstreak holes in the United States and Russia October 2009
• 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. 
• Two photographs of a fallstreak hole in Northern Germany October 2009