Cavum (so called hole punch cloud) is formed when the ice crystals from above fall into a large area of altocumulus or cirrocumulus contains super cool water and trigger Bergeron-Findeisen process. The water droplets around the ice crystals evaporate to form huge voids, also known as cloud holes. The cloud hole is usually circular, and there is often a cirrus cloud in the middle, and sometimes this cirrus cloud will drift towards the ground to form a streamer cloud.