Originally Posted by BigLar
What about the contrails formed at the wingtips? I've seen these very often.
These are very different. They're caused by local changes in temperature and pressure because of the aerodynamic effects of the relevant bits of the wing passing through the air at exactly that point. As soon as the disturbance has passed, the contrail will dissipate.
Those created by engines aren't the same, because they're largely made up of the huge amounts of water dumped into the air by the engine exhaust.