Some Folks Calls It A Vapour Trail...

I call it a contrail. As a wing generates lift, it causes a vortex to form at each wingtip, and sometimes also at the tip of each wing flap. These wingtip vortices persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible. This effect is more common on humid days. Wingtip vortices can sometimes be seen behind the wing flaps of airliners during takeoff and landing, and during landing of the Space shuttle. Any persisting rumors of contrails forming behind a certain, local, 6 foot 5 inch, single-speed rider on downhill sections, is merely conjecture.