This strange cloud formation in the sky of Stockton is a so-called fallstreak hole, also known as a hole punch cloud.
Such holes are formed when the water temperature in the clouds is below freezing but the water has not frozen yet due to the lack of ice nucleation particles. When ice crystals do form it will set off a domino effect, due to the Bergeron process, causing the water droplets around the crystals to evaporate: this leaves a large, often circular, hole in the cloud.
Because of their rarity and unusual appearance, fallstreak holes are often mistaken for or attributed to unidentified flying objects.
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