Hole-Punch Clouds, aka Fallstreak Holes

Sources: EarthSky and various internet sites

High to mid-level clouds, such as the altocumulus clouds, are often composed of tiny water droplets that are much colder than the freezing temperature of 32 degrees, but have yet to actually freeze into ice. These "supercooled" water droplets need a "reason" to freeze, which usually comes in the form of ice crystals.

Airplanes passing through the cloud layer are oftentimes the disturbance to bring these ice crystals. There are zones of locally low pressure along the wing and propeller tips that allow the air to expand and cool well below the original temperature of the cloud layer, forming ice crystals. Once the ice crystals are introduced, the water droplets quickly freeze, grow and start to fall. Once those ice crystals fall, the void/hole is left behind, which will start to expand outward as neighboring droplets start to freeze. Hole-Punch clouds expand for hours after being created.