What Causes That Weird Fog in Airplane Aisles?

One of the many mysteries of air travel is the white mist that occasionally fills the airplane cabin right before takeoff. AFAR explains the science involved.

Aircraft cabin with smoke-like air vapor condensation due to differences of temperature between cabin and outside.

Condensation is caused by the difference between temperatures outside and inside the plane.

Photo by ThamKC / Shutterstock

The door to your airplane closes, and it’s time to leave the gate, but before the aircraft can pull away, a white mist appears out of thin air, swirling around the overhead bins. No, it’s not a disco-themed cabin crew introduction (cue the lasers!) or a new method to attract your attention to the safety video.

The phenomenon of in-cabin fog has been the subject of Reddit and Quora threads over the years and occasionally makes the evening news via a viral video, but what exactly is it? We have the answers.

Why does fog appear inside the airplane cabin?

What you’re experiencing is the basic physics of cloud formation. As the pilots prepare to take off, they let outside air into the air-conditioned cabin. You can usually see the vent slats by the overhead bins, but some may also be near the floor.

If the incoming fresh air is at a particular level of heat and humidity, the water vapor in the air (aka humidity) hits the dew point when it meets the cooler environment in the cabin.

Mark Miller, a professor of atmospheric science at Rutgers University in New Jersey, explains, “This cool air, which is mixing with warm, moist air in the cabin, settles to the bottom of the cabin. And if the air in the cabin contains enough water vapor, as it may on certain summer days, a cloud could form. Air-conditioning being activated at the beginning of the flight is the likely cause.”

Like your air conditioner at home, a plane’s cooling system is designed to reduce humidity in the air because excess moisture can harm its components. As the foggy air cycles through the plane, it quickly cools and dries, which is why cabin fog generally only occurs for a minute or so, and water droplets don’t have enough time to form and fall on you in your seat.

For those interested in more details on the science, a post on the Aviation Stack Exchange features a very detailed explanation with charts and graphs.

Why doesn’t cabin fog appear on every flight?

Because the outside air needs to be very hot and humid to cause the reaction, cabin fog occurs most often in tropical airports and during hot summers.

“The ‘cloud’ or ‘fog’ that customers may encounter on an aircraft is simply the formation of water droplets resulting from air of two differing temperatures meeting,” a spokesperson for Delta Air Lines told AFAR via email. “For example, these clouds of water droplets can be observed during summer operations when cool air from an aircraft’s A/C system encounters a hot and humid cabin environment.”

Should you be concerned?

Not at all. Consider the sensation to be the same as if you were at a high elevation in the mountains where the cloud cover was so low you could move a hand through it. And there’s no need to hold your breath during the experience. There’s nothing harmful about inhaling foggy cabin air. It’s like being in a steam room or breathing in an aerosol from an inhaler.

Should I report fog to the cabin crew?

Flight crews have likely experienced cabin fog many times in their careers, and again, this isn’t usually a safety concern. That said, cabin crews have been known to benefit from passengers identifying emergency issues, so if the fog seems abnormal—say, if the mist is gray or black or has a strange smell—or you notice any other incident, it’s perfectly fine to ring your call button to point out the irregularity and ensure everything is normal.

Adam Wisniewski is an Atlanta-based writer and editor who covers travel, technology, and food.
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