A skypunch is a large circular or elliptical void, which can appear in cirrocumulus or Altocumulus clouds. Such holes are formed when the temperature of the water in the clouds is below zero, but the water has not yet frozen because of the lack of ice nucleation particles (see overfusion). When the ice crystals are formed a domino effect will start up, due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large hole in the cloud, often circular.1
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From @rohitshori