Fallstreak Hole

An amazing cloud formation, the ABC reports, in the skies over east Victoria, in the Gippsland area. This type of cloud called a Fallstreak Hole.

Image © Peter Fell via ABC News

A fallstreak hole, also known as a hole punch cloud, punch hole cloud, skypunch, canal cloud or cloud hole, is a large circular or elliptical gap that can appear in cirrocumulus or altocumulus clouds. Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation particles (see supercooled water). When ice crystals do form it will set off a domino effect, due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud.

via io9

source wikipedia