GAINESVILLE — A mysterious brown cloud over northeast Gainesville on Wednesday afternoon and Thursday morning interrupted otherwise bluebird skies.

John O’Sullivan, a professor in the University of North Georgia’s Institute for Environmental & Spatial Analysis, spotted the cloud and snapped photos of it hovering above parts of Lake Lanier.

“It appeared to be almost sitting still,” he said.

There were additional sightings along Browns Bridge Road on Thursday.

O’Sullivan said it resembled an atmospheric brown cloud, though he couldn’t be sure.

According to Encyclopedia Britannica, an atmospheric brown cloud is “a layer of pollution containing aerosols such as soot or dust that absorb as well as scatter incoming solar radiation, leading to regional and global climatic effects and posing risks to human health and food security.”

These sources of air pollution are caused by fossil fuel emissions and have grown along with industrial development around the world. They can cause cardiovascular and respiratory problems.

Other causes for the cloud include fires in the area.

“If we can't find the source, it's sort of like a UFO reporting,” O’Sullivan said.

A prescribed burn was held Wednesday off Glade Farm Road in northern Hall County.

In addition, online U.S. Forest Service maps showed several fires on the northeastern end of Lanier between Wednesday and Thursday.

Recent wildfires in northern Alabama showed up on satellite images this week, and the smoke could have drifted east.

“This could be particulate matter or nitrogen dioxides from those, combined with the fact that we have not had rain in a while to wash out pollutants from our regional traffic and power plant/factory emissions,” said Jamie Mitchem, also a professor in the UNG Institute for Environmental & Spatial Analysis.

“In addition, even large swarms of bugs can look like brown clouds, but they usually move around.”

Mitchem said he could not conclusively identify the source.

“Based on the shadows, the sun appears to be shining in the direction of the cloud so the color does not appear to be merely a shadow, but more likely a cloud of moisture mixed with some air pollution,” Mitchem said.

The state Department of Natural Resources Air Quality Index showed rising levels of air pollution throughout the day in the north Georgia mountains, with levels nearing an “unhealthy” baseline.

The pollution levels there were higher than reported in metropolitan Atlanta.