Sometimes a large hole may appear in a thin widespread cloud layer, usually Altocumulus but sometimes also Cirrocumulus: this is called a hole punch cloud. The shape may vary from a perfect circle to an oval.  The hole is the result of the freezing of supercooled water droplets which spread out side wards, fall down (virga) and finally evaporates completely. The initial process of this freezing is usually the result of falling ice crystals (virga) of a contrail which was formed just above the thin cloud layer. So airplane paths and hole punch clouds go together. 


The picture shows in the middle and upper left part the begin of two large holes in a thin Altocumulus layer. The shape is more or less oval with within falling crystals (virga).