A
fallstreak hole, also known as a hole punch cloud, punch hole cloud,
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.
