Vortex

This stunning video shows off perfectly wake vortices on an Emirates Boeing 777-300ER. Patrick Smith from Ask The Pilot explains;

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.  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 [as per the video below].  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.