The phenomenon was seen mainly in Bay Area cities such as San Bruno, San Mateo, and Burlingame. Chief Meteorologist from KRON4 Lawrence Karnow says the formation is known as a 'fallstreak hole' or 'hole punch cloud.' They form in clouds of supercooled water droplets - water below 0°C but not yet frozen. These water droplets need a tiny particle to freeze or to be cooled below -40 °C. Aircraft punching through this cloud layer can cause air to expand and cool as it passes over the aircraft wings or propeller.

Source: Daily Mail September 15, 2018 17:47 UTC