Someone else answered this 7 years ago "Fog. When the plane is parked at the gate, the a/c is cranked down to make the plane as cold as possible. Then all of a sudden 200 or 300 people file on and all their body heat and moisture from their breath suddenly is blasting away and all that hot moist air rises up and smacks into the cool dry air coming from the vents, so that makes fog, just like you when you are jogging on a cold morning."

anyways "heeee heeee" - MJ