What is that 'strange circle' hovering in the sky over Ho Chi Minh City?

The dark circle that appeared in the sky over Ho Chi Minh City on the morning of May 25th is most likely a physical phenomenon called a swirling smoke ring.

On May 25th, social media was abuzz with a video showing a black circle appearing in the sky, allegedly at a garment factory in Ho Chi Minh City. The black circle hovered, maintaining its shape in mid-air, surprising many onlookers.

Below the posts, many people left curious comments. Quite a few suggested that this phenomenon was related to unidentified flying objects (UFOs), while others suspected it was the smoke effect from an explosion or industrial activity.

Ms. Phuong Ngoc, a resident of Phu Tan ward, said that at around 6:30 a.m. that same day, she also saw this black circle while on her way to work.

"At first I thought it was just normal smoke, but the more I looked, the stranger it seemed because it was perfectly round and slowly growing larger," she recounted.

Speaking to Tri Thức - Znews , a representative of an astronomy club in Ho Chi Minh City stated that the circle appearing in the sky is most likely a "smoke ring vortex" - a type of swirling air current that forms when a stream of hot or compressed air is forcefully pushed into the environment and then curls up in the air.

According to this person, this physical phenomenon can occur after small explosions, industrial activities, or when a large amount of gas escapes through a round opening such as a chimney or pressure vessel.

When a mass of air is released, the center moves faster than the edges, which experience friction with the surrounding air. This difference in speed causes the airflow to curl, forming a closed, ring-like vortex. The visible smoke is actually just a trace that allows the naked eye to observe the movement of the air.

Experts say that the swirling smoke ring can maintain its shape for a few seconds to a few minutes if weather conditions are stable and there is little wind. However, when encountering air turbulence or temperature changes, this structure will quickly deform and gradually dissipate.

Source: https://znews.vn/vong-tron-la-lo-lung-tren-bau-troi-tphcm-la-gi-post1654019.html