First recorded during the 1940s, a fallstreak hole forms when water droplets in a cloud freeze into ice crystals and refract light to form a rainbow, while at the same time dropping down below the cloud layer to form a hole.
These unique formations - also known as 'holepunch clouds' - can materialise on their own, but researchers have found that aircraft flying through super-cooled clouds can also set off the freezing process. This is because when air flows around the tips of an aeroplane's propellor and over its wings, it can cool in a localised area, causing the surrounding water droplets to drop to temperatures as low as -40 degrees and freeze.
"A fallstreak hole is a circular or elliptical gap that can appear in high- to mid-level clouds," US-based meteorologist Linda Lam told Weather.com. "There are water droplets in the cloud that have yet to freeze, and once they do - when ice crystals are introduced - the water droplet freezes and grows. As it grows, it begins to fall and a hole is left behind."
While the process of fallstreak hole cloud formation is entirely natural, it mimics the formation of jet contrails - no, not chemtrails! - which are the long, thin trails of clouds sometimes produces by an aircraft.