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

It is quite common to see what looks like "smoke" or "fog" coming out of the vents in an airplane cabin before takeoff, but it is almost always condensation, not fire. This occurs when the cold air from the plane's Air Conditioning (AC) system meets the warm, humid air inside the cabin. As the AC cools the moist air, the water vapor condenses into tiny visible droplets, creating a mist effect very similar to seeing your breath on a cold day. This is particularly prevalent in tropical climates or during hot, humid summer days. Pilots and flight attendants refer to this as "fogging," and it usually disappears once the plane’s internal temperature and humidity levels have stabilized. If there were an actual fire, the "smoke" would typically have a distinct acrid smell, be darker in color, and trigger the aircraft’s smoke detectors. If you see white, odorless mist coming from the ceiling vents while sitting at the gate, it is simply the plane's cooling system working hard to keep you comfortable.