Fallstreak hole clouds.

A fallstreak hole (also known as a holepunch cloud) forms when part of the cloud layer forms ice crystals which are large enough to fall as a 'fallstreak'.
They form in clouds of supercooled water droplets, water below 0 °C but not yet frozen. These water droplets need a tiny particle, a nucleus, to freeze or to be cooled below -40 °C.

Have you ever left a water bottle in your freezer overnight only to find it miraculously still unfrozen in the morning? That's because the water has been supercooled, meaning its temperature is below freezing but it remains in a liquid state because there was nothing in the water to trigger the formation of ice crystals. All it takes is the introduction of a single ice crystal to set off a chain reaction that turns your liquid water to solid ice. The exact same process occurs with fallstreak holes, but on a much larger scale. When something disturbs the supercooled water vapour in alto- or cirrocumulus clouds -- such as a plane passing through -- the sudden introduction of ice crystals creates a domino effect. As the crystals grow, they fall through the center of the hole, creating wispy trails of ice.

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