Dear Tom, 

One day, shortly before sunset in the western sky over Chicago there was a layer of clouds that looked like it had a hole punched through it, with wispy clouds falling through. What causes that?

Leanne C. L.

Dear Leanne,

What you observed is referred to as a fallstreak. The photograph that you provided shows a hole in a deck of altocumulus clouds (mid-level clouds occurring at a height of about 7,000 to 15,000 feet). I estimate the height of this particular altocumulus layer was about 8,000 feet. Jet aircraft flying through such clouds are usually responsible for causing the holes. Altocumulus clouds are composed of water droplets that, in cooler environments, become super cooled. That is, they remain in a liquid state at temperatures below the freezing point of water. Such droplets, when disturbed by an aircraft, can change state and initiate ice crystal growth. These ice crystals then grow by scavenging water vapor from neighboring droplets causing the cloud to dissipate as the ice crystals fall out, creating the hole and wispy fallstreak that you observed.