fallstreak

A fallstreak, also known as a hole punch cloud or skypunch, is a large circular or elliptical gap that appears in cirrocumulus or altocumulus clouds. These phenomena are caused by the rapid freezing of water droplets in the cloud. When ice crystals form, they grow by drawing in supercooled water vapor from the surrounding droplets, causing them to evaporate. This process leaves a hole in the cloud layer.

Fallstreaks are often accompanied by wispy trails of ice crystals, known as virga, that fall from the hole. These trails are the remnants of the evaporated water droplets and are what give the fallstreak its characteristic appearance. The formation of fallstreaks is a common occurrence in mid-latitude regions and can be observed when the atmospheric conditions are right.

The temperature within the cloud must be below freezing, but the water droplets must remain in a supercooled liquid state. The introduction of ice nuclei, which can be naturally occurring or man-made particles, triggers the rapid freezing process. Aircraft flying through supercooled clouds can also initiate this phenomenon by creating the necessary ice nuclei. Fallstreaks are an interesting atmospheric optical phenomenon that demonstrates the dynamic processes occurring within clouds.