The fascinating cloud formation seen above is known as a hole punch cloud or ‘fallstreak hole‘. The phenomenon occurs when part of a cloud’s water droplets freeze into ice crystals and fall below the cloud layer.

A large circular or elliptical gap can appear in cirrocumulus or altocumulus clouds. You can read more about these on the Cloud Appreciation Society’s page and Wikipedia.

This particular hole punch cloud was spotted on August 17, 2008 about 20 km south of Linz, Austria.