Punch hole clouds

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Often mistaken for UFOs these rarely seen punch hole clouds or fallstreak holes 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.

I was lucky enough to see one of them forming in San Francisco near Land’s End.

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