A fallstreak hole is a large gap, usually circular or elliptical, that can appear in cirrocumulus or altocumulus clouds. The holes are caused by supercooled water in the clouds suddenly evaporating, and may be triggered by passing aircraft.
These Fallstreak or ‘hole punch cloud’ gives the appearance of Bifrost like in Thor movies. Because of their rarity and unusual appearance, fallstreak holes have been mistaken for or attributed to unidentified flying objects.
How do they form?
High to mid level clouds, such as altocumulus, are often composed of tiny water droplets that are much colder than freezing, but have yet to freeze. These “supercooled” water droplets need a “reason” to freeze, which usually comes in the form of ice crystals. Planes passing through the cloud layer can bring these ice crystals.
Once the ice crystals are introduced, the water droplet quickly freeze, grow and start to fall. A hole is left behind, which will start to expand outward as neighboring droplets start to freeze.
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 Wegener-Bergeron-Findeisen 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-tip 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. An early satellite documentation of elongated fallstreak holes over the Florida Panhandle that likely were induced by passing aircraft appeared in Corfidi and Brandli (1986). Fallstreak holes are more routinely seen by the higher resolution satellites of today (e.g., see third example image in this article).
The articles by Westbrook and Davies (2010) and Heymsfield et al. (2010) explain the processes behind the formation of fallstreak holes in greater 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.