What Creates That Strange Fog in Airplane Aisles?

As the airplane door seals shut and the journey is about to begin, a mysterious white mist suddenly fills the cabin, twisting around the overhead compartments. It's not a funky cabin crew introduction (no disco lights here!) or a flashy way to get your attention for the safety briefing.

The appearance of fog inside the cabin has sparked countless discussions on Reddit and Quora over the years, and it occasionally makes headlines through viral videos. But what is this phenomenon, really? We have the explanations.

What causes fog to form inside the airplane cabin?

What you’re witnessing is simply the physics behind cloud formation. As pilots get ready for takeoff, they allow outside air to mix with the conditioned air in the cabin. The vent slats near the overhead bins are usually visible, and some might also be located close to the floor.

When fresh air enters the cabin at a specific temperature and humidity level, the water vapor (humidity) reaches its dew point upon encountering the cooler cabin environment.

Mark Miller, an atmospheric science professor at Rutgers University in New Jersey, notes, “This cool air, blending with the warm, moist air inside the cabin, sinks to the bottom. If there’s enough water vapor in the cabin air, especially on humid summer days, clouds can form. The activation of the air-conditioning at the start of the flight is usually the trigger.”

Similar to your home air conditioner, an airplane’s cooling system aims to lower humidity levels because excess moisture can damage its parts. As the foggy air circulates through the aircraft, it rapidly cools and dries, which is why cabin fog typically lasts only a minute or so, preventing water droplets from accumulating and falling onto you in your seat.

For those wanting to dive deeper into the science, a post on the Aviation Stack Exchange provides an extensive explanation complete with charts and graphs.

Why does cabin fog not occur on every flight?

Cabin fog tends to occur most frequently at tropical airports and during hot summer months, as it requires outside air to be quite warm and humid to trigger the reaction.

A Delta Air Lines spokesperson explained to Dinogo via email, “The ‘cloud’ or ‘fog’ that passengers might see on a plane is merely water droplets forming when air at two different temperatures converges. For instance, these droplets are often visible during summer flights when cool air from the aircraft's A/C system meets a warm and humid cabin atmosphere.”

Is there any reason for concern?

Not at all. You can think of the experience as similar to being at a high-altitude mountain location where the cloud cover is so low you can wave your hand through it. Plus, there’s no need to hold your breath—breathing in foggy cabin air is harmless. It’s akin to being in a steam room or inhaling from an inhaler.

Should I notify the cabin crew about the fog?

Flight crews have likely encountered cabin fog numerous times throughout their careers, and typically, it’s not a safety issue. However, cabin crews appreciate passengers alerting them to any potential emergencies. So, if the fog appears unusual—such as gray or black mist, or if there’s an odd smell—or if you notice anything else out of the ordinary, feel free to use your call button to notify the crew and ensure everything is alright.