After looking at a picture of a sky punch or sun hole (apparently formed when ‘Ice crystals form above the high-altitude cirro-cumulo-stratus clouds, then fall downward, punching a hole in the cloud cover’) by Terry Boswell on Facebook, I knew it couldn’t be the first time this ominous-looking phenomenon has been captured and preserved on the Internet. These pictures are evidence.
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.
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.