This crazy circular hole in a cloud goes by many names — skypunch, fallstreak hole, punchole cloud, or sometimes, when it's a line instead of a circle, a canal cloud. Whatever it's called, it's the result of a weird domino effect that sometimes happens in the air above us.

This effect is called the Bergeron Process after Tor Bergeron, a Swedish meteorologist who studied how clouds turn into rain. The first step is the formation of the cloud itself, which happens when vapor in the air condenses into tiny droplets. These droplets have to grow, which seems like an inevitable process as the temperature drops, but is actually fairly hard to get going. Existing droplets will collide, but each droplet has a kind of "skin" on it, created by the surface tension of the water. Perhaps you've observed a form of this, when pressing two water droplets together. Instead of immediately adhering to each other, they sometimes retain their shapes for a bit before combining. Because of this resistance, considerable pressure has to be exerted to get tiny water droplets to build.

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Ice, oddly enough, requires lower temperatures to form but not as much pressure to grow. Water vapor will sublime — go from vapor to ice without becoming liquid first — onto crystals more readily than it will coalesce into drops. The Bergeron process kicks into gear when ice crystals form in the atmosphere, and water sublimes onto them. They grow, and the water vapor available for liquid water droplets shrinks. A cloud will suddenly disappear into a tiny shower of ice crystals. (This shower will not necessarily reach the ground, since it will encounter warmer air as it drops.)

So a skypunch happens when a plane goes through a cloud. There is extremely low pressure in the area behind the wing. That low pressure area causes a temperature drop which causes little seed ice crystals to form, which suck up the cloud around them and fall down, leaving a hole.

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[Sources: Sun Dog Inside a Hole Punch Cloud, Cloud Physics, The Bergeron Process]

Top image: H. Raab, Second Image: Croq, Last Image: NASA Earth Observatory.