Fallstreak holes form when a part of a cloud falls out, leaving a hole. This happens when part of a cloud made of very cold water droplets turns to ice crystals and drops out of that layer.
Here is the explanation from the book, The Secret World of Weather, by Tristan Gooley:
“Sometimes when a hole is punched through the cloud, the broken pieces come back together and form a new cloud. When an aircraft passes through clouds that are made of water but near freezing, the aircraft can cause the clouds to freeze into ice crystals and then fall out of the cloud, leaving a hole. This creates a beautiful pattern: a blue-sky hole in a layer of cloud, with a feathery cirrus pattern inside it.”
It is particularly common in layers of altocumulus clouds.
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When Clouds Appear Like Rocks and Towers…
The Secret World of Weather – The Book
