Vortex formation
Air beneath an aerofoil is at a higher pressure than that above it. At an edge the high pressure air spills from the lower region and curls upwards into the region above the surface.
The swirling air eventually leaves the aerofoil as a tip vortex that forms a long backwards trail.
Vortices are strongest under the high lift demands of landing and take-off. Ends of leading edge slats or rear flaps are sources. Winglets reduce tip vortices from the main wing.
The trails always exist. High humidity made them visible here. Rapid expansion of the humid air caused moisture condensation into small droplets visible as a mist. Note the mist also around upper wing areas. Rapidly condensed droplets like these sometimes show strong iridescence.
