Some people in Oman and the United Arab Emirates had a chance to see a particularly stunning atmospheric phenomenon this week: a fallstreak, otherwise known as a punch hole cloud. This curious event only happens when certain conditions are met because it requires supercooled water in the clouds and something to make it crystallized.
Having supercooled water in clouds is not unusual. High-to mid-level clouds, such as cirrocumulus or altocumulus, are often made by droplets at a temperature way past the freezing point of water but that haven’t turned into ice crystals yet. That is essentially what allows the formation of the fallstreak hole.
Throwing some ice crystals in a supercooled water cloud layer produces a domino effect. Water droplets quickly interact with the crystals freezing and produce more ice crystals, affecting even more droplets. Suddenly most of the droplets have crystallized and have started to fall to the ground. The only thing left behind in their wake is the circular or sometimes elliptical hole.
Ice crystals are the culprit, and these fallstreak clouds have about 10 times more ice than the average altocumulus. But for a long time, it wasn’t clear how they formed in a supercooled water cloud. Usually, supercooled water needs something to make it freeze – it could be an impurity, it could be interaction with the environment, or it could very low temperatures.
The cause of the crystals in the case of punch hole clouds is airplanes. Airplanes create the ice in the wake of their wing tips, flaps, gears, and even propellers. Pressure in the wake is much lower than in that particular layer of air. Thermodynamically, as air expands due to the pressure, it also cools down to temperatures where even supercooled water has to freeze.
So airplanes can leave a trail of ice crystals behind and these crystals are then responsible for affecting clouds, changing their properties and creating the beautiful fallstreak hole.