Images of clouds with unusual bright blue holes appeared on social networking sites such as Twitter and Facebook starting on May 9 and spreading rapidly in recent days.

The photo made curious people in California and many other places guess. Many people think this is a “bad omen” or comes from some strange cause.

According to many netizens, the cloud with a bright hole is a UFO (unidentified flying object) or caused by aliens. Many more realistic people hypothesize that the cloud is just a warning sign of bad weather, while others with rich imaginations guess this is a “wormhole” like in the horror movie Donnie Darko.

However, most opinions identify this as a Fallstreak, which is a large circular void that can appear in cirrocumulus or altocumulus clouds. And CBS13 network said that images like this are not rare in the world, especially in warm places like Stockton City, California.

Cloud holes form when the water temperature in the cloud is lower than the freezing point but the water has not yet frozen due to a lack of ice seeds.

When part of the cloud begins to freeze, it causes a knock-on effect that causes surrounding water vapor to also freeze and fall. This phenomenon causes a hole, usually circular, in the cloud.

It is believed that disturbances in the cloud layer, such as from aircraft, can trigger a chain evaporation effect and create cloud holes. Specifically, the air passing through the propeller or two airplane wings will expand and cool quickly. Ice crystals begin to form and then after the plane passes, the crystals remain, falling out of the cloud and creating round, sometimes very large holes.