What Causes the Peculiar Mist in Airplane Aisles?

One of the enigmatic phenomena during air travel is the sudden appearance of a white mist inside the airplane cabin just before takeoff. AFAR delves into the scientific explanation behind it. As the door to your airplane is sealed, and it's time to depart from the gate, a white mist materializes seemingly out of nowhere, swirling around the overhead bins. No, it's not a disco-themed introduction by the cabin crew (no lasers involved!), nor is it a novel method to captivate your attention for the safety video.

This in-cabin fog occurrence has been a topic of discussion on platforms like Reddit and Quora over the years, occasionally making its way to the evening news through viral videos. But what precisely is this phenomenon? We've got the answers.

Why does fog manifest inside the airplane cabin? What you're witnessing is a manifestation of basic cloud formation physics. As the pilots prepare for takeoff, they introduce outside air into the air-conditioned cabin. You can typically see the vent slats near the overhead bins, and there might be some near the floor as well.

When the incoming fresh air has a specific temperature and humidity level, the water vapor in the air, also known as humidity, reaches its dew point when it encounters the cooler cabin environment.

Mark Miller, an atmospheric science professor at Rutgers University in New Jersey, elucidates, "This cool air, which is mingling with the warm, moist air inside the cabin, settles at the cabin's bottom. And if the cabin air contains sufficient water vapor, as it often does on certain summer days, a cloud might form. The activation of the air-conditioning system at the start of the flight is the likely trigger."

Similar to the air conditioning in your home, an airplane's cooling system is engineered to reduce air humidity since excess moisture can be detrimental to its components. As the misty air circulates throughout the plane, it swiftly cools and dries, which is why in-cabin fog typically only lasts for a minute or so, and water droplets do not have enough time to develop and descend onto passengers in their seats.

For those interested in delving into the scientific details, a post on the Aviation Stack Exchange offers an in-depth explanation complete with charts and graphs.

Why doesn't in-cabin fog occur on every flight? In-cabin fog is not a common occurrence because it necessitates that the outside air be both hot and humid to trigger the reaction. It is most prevalent at tropical airports and during hot summer months.

A spokesperson for Delta Air Lines conveyed to AFAR via email, "The 'cloud' or 'fog' that passengers may encounter on an aircraft is simply the formation of water droplets resulting from the meeting of air with two different temperatures. For instance, these clouds of water droplets are observable during summer operations when the cool air from the aircraft's air conditioning system encounters a hot and humid cabin environment."

Is there cause for concern? Absolutely not. Think of the experience as akin to being at a high elevation in the mountains where the cloud cover is so low that you can pass your hand through it. There's no need to be apprehensive about inhaling misty cabin air. It's similar to being in a steam room or breathing in an aerosol from an inhaler.

Should I report the fog to the cabin crew? Flight crews have likely encountered in-cabin fog many times during their careers, and generally, it doesn't pose a safety risk. However, cabin crews do appreciate passengers' vigilance in identifying potential emergency situations. Therefore, if the fog appears unusual—such as being gray or black or emitting a strange odor—or if you observe any other irregularity, feel free to press your call button to bring it to their attention and ensure everything is in order.