Around once or twice a year during the coolest months specific atmospheric conditions occur to create fallstreak holes. This is a circular gap or channel gap seen in cirrocumulus and altocumulus clouds which occur both naturally and are also influenced by jet aircraft flying through the cloud layer.
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 (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.
We hope you all find this very interesting and now have a better understanding how and why this amazing phenomenon occurs!
Glenn Reed – Fallstreak Channel caused by an aircraft flying directly at the same level of the cloud layer around 25,000ft over CQLD
Renee Rudd from Rockhampton showing a classic natural Fallstreak hole cloud
Rite Gawne aerial photo from above, of the distinct cloud layer in CQLD
Rockhampton’s actual weather balloon data from BOM with text details applied to scientifically prove and show why these clouds occurred