Passengers aboard a United Airlines flight from Houston to Denver were met with a dramatic and unusual sight as thick, fog-like vapor poured from the aircraft’s overhead air vents while the plane prepared to land.

A viral video posted Monday by extreme weather and disaster scientist “Hurricane Hal” showed what appeared to be dense smoke spreading through the passenger cabin.

“Check out the ‘fog machine’ on my United flight from Houston to Denver today!” the post read, explaining that the cabin was so humid that cool air from the air conditioning system immediately condensed.

The footage quickly gained attention online, with some social media users questioning whether smoke had entered the cabin or if the aircraft was experiencing an emergency. In similar incidents, flight attendants have even had to reassure concerned passengers that the strange-looking vapor was not smoke.

United Airlines says there is a simple scientific explanation.

“As we descend into warm, humid destinations like Houston, passengers may occasionally notice a visible mist or fog-like vapor emitting from the cabin air vents,” a United spokesperson told Chron.

The airline also made one thing clear: “There are no ‘fog machines’ on any United Airlines aircraft.”

The cloud-like effect is caused by condensation. When cold air from the aircraft’s air-conditioning system mixes with warm, moisture-rich air, the sudden temperature change causes water vapor to condense into tiny droplets.

The result? A temporary cloud forming inside the aircraft cabin.

United said the phenomenon is purely visual, does not affect cabin air quality and quickly disappears as air circulates through the aircraft.

Aviation experts say cabin fog is not unusual, particularly in warm and humid climates where moisture-filled air rapidly mixes with cold conditioned cabin air.

So, despite looking like smoke from a possible onboard emergency, passengers were simply watching physics in action — a tiny cloud temporarily forming inside their United Airlines cabin.