You may have seen twirling tendrils of white vapour on the rear edges of wings of a jet fighter, jet airplane or even on the propellers of some air craft.
The visible vapour is actually the result of vortices that are sometimes partly visible depending on the atmospheric conditions, primarily the temperature and humidity, due to condensation in the cores of the vortices. Each vortex is a mass of spinning air and the air pressure at the centre of the vortex is very low.
These wingtip vortices are not the same as contrails, which is a condensation of water vapour within the exhaust of a jet engine.
The science behind the visible vapour:
Water vapour is present in air and is normally invisible. For a given amount of pressure, that water vapour will remain unseen. Clouds form when a moist air mass encounters an area of low enough pressure to cause the water to condense. An example is when moist air rises. Atmospheric pressure declines as altitude goes up. As an air mass rises, due to terrain or other reasons, the pressure drops, water condenses and clouds form.
A propeller tip, rotor tip, wing tip and even just the upper surface of the wing can produce enough of a pressure drop to cause the available moisture in the air to condense. If the aircraft is in very cold air, the condensate will freeze. If not, it will only last a few seconds or until the pressure stabilizes.
And now you know…