Fall Streak: A Hole in the Clouds. What causes this strange phenomenon?

A fallstreak hole forms when part of the cloud layer forms ice crystals which are large enough to fall as a 'fallstreak'.

It is a little known fact that water can exist in liquid form to temperatures as low as MINUS 40°C, especially in cloud. These are known as super-cooled water droplets. Fall streak holes form in clouds of supercooled water droplets, i.e. water below 0 °C but not yet frozen. These water droplets need a tiny particle, a nucleus, to freeze or to be cooled below -40 °C.

Once an ice crystal has been introduced to the cloud, the ice crystals grow, at the expense of the super-cooled water droplets, and fall from the cloud producing the 'hole' in the cloud

Aircraft punching through this cloud layer can cause air to expand and cool as it passes over the aircraft wings or propeller. This change in temperature can be enough to encourage the supercooled droplets to freeze and fall from the cloud layer in this distinctive pattern.