Fallstreak Holes

Fallstreak Holes, or their much cooler name: hole-punch clouds, are large gaps that appear in cirrocumulus or altocumulus clouds. Often reported as UFOs, these gaps occur when sections of supercooled water droplets in a cloud freeze into ice crystals and fall below the cloud, leaving a hole. Many times, this sudden freezing is caused by a passing plane overhead.

These holes can get up to 50km across within an hour of the first droplets falling.

http://www.weather.gov/arx/why_fallstreaks