ATLANTA (AP) — Authorities say a routine training exercise near the world's busiest airport produced a plume of heavy, dark smoke rising over Atlanta and visible for miles.

Atlanta Fire Rescue Sgt. Cortez Stafford says the smoke late Thursday morning appeared darker than in previous training drills, and said one possibility for that could be the cold weather the city is experiencing.

Airport officials said the planned drill was part of "live burn training" exercises at the Atlanta Fire Training Center near the airport.

Drills conducted several times each year produce noticeably lighter smoke. The darker smoke Thursday prompted some to wonder on social media whether the fire was an actual emergency.

Hartsfield-Jackson Atlanta International Airport is the world's busiest, serving more than 104 million passengers annually.