Airlines will sooner throw sacks of hundred dollar overboard before they spit away for no good reason. Most larger jets do have fuel-jettison capability, but that’s not what you’re seeing. What you’re seeing is water vapor.
At the wings’ outermost extremities, the higher pressure air beneath is drawn toward the lower pressure air on top, resulting in a tight, circular flow that trails behind the aircraft like a pronged pair of sideways tornadoes (see also “wake turbulence,” in the earlier turbulence FAQ). Under the right combination of temperature, pressure and humidity, moisture in the cores of these vortices condense and become visible, writhing behind the plane like gray vaporous snakes. The vortices are most pronounced when the wing is working hardest to produce lift. Thus, prime time for noticing these trails is during approach or departure.
Moisture will condense around other spots too, such as the flap fairings and engine attachment pylons. You’ll witness what appears to be a stream of white smoke pouring from the top of an engine during takeoff. This is water vapor caused by invisible currents around the pylon. Other times the area just above the surface of the wing will suddenly flash into a white puff of localized cloud. Again, this is condensation brought on by the right combo of moisture, temperature and pressure.
Not only can you sometimes see wingtip vortices, but even cooler, you can often hear them from the ground:
You need to be very close to a runway — preferably within a half-mile of the end. The strongest vortices are produced on takeoff, but ideally you want to be on the landing side, as the plane will be nearer (i.e. lower) at an equivalent position from the threshold. A calm day is ideal, as wind will dissipate a vortex before it reaches the ground. About 30 seconds after the jet passes overhead you’ll begin to hear a whooshing, crackling and thundering. It’s a menacing sound unlike anything you’ve heard before. See — or hear — for yourself in this footage captured on my iPhone.
It was taken at the Belle Isle Marsh Reservation, a popular birdwatching spot about a half-mile north of runway 22R at Boston’s Logan International Airport. The plane is a 757. Excuse the atrocious video quality, but the sound is acceptable and that’s the important thing. You begin to hear the vortices at time 0:45, and they continue pretty much to the end. Note the incredible gunshot-like noises at 0:58.