News > What are fallstreak or punch hole clouds?
Image: A fallstreak hole that was seen over Oklahoma City on 2nd January 2010 (By Pfranson [GFDL, via Wikimedia Commons])
There have been lots of 'fallstreak' or 'punch hole' clouds appearing in the news and on social media channels recently, notably in Australia and the US, but what are they?
Essentially they form when a high- or mid-level cloud of supercooled cloud droplets - liquid water that exists below freezing point - are disturbed by something like an aeroplane that introduces ice crystals into the cloud structure. These act as 'ice nuclei' causing the supercooled droplets to spontaneously freeze in a process known as 'seeding'. These frozen droplets then fall below the cloud forming the 'fallstreak clouds' we see.
A video from University of Manchester demonstrating nucleation of supercooled water can be found here. In fact, you can do similar experiments at home and there are many videos on YouTube demonstrating this.
Occasionally - as seen in Perth, Australia in October 2014 - an iridescent rainbow can be seen, similar to what is often seen in soap bubbles. This type of iridescence occurs in high level clouds due to diffraction of light by the small cloud droplets.