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Vortices ~ Dusk at Arlanda Airport, Stockholm, Sweden. Very humid with fog appearing and then dissipating. A landing aircraft leaves streams of what appear to be rings. They are actually long helical trails – vortices - of condensed moisture.

Images and video by Peter Rosén

All images ©Peter Rosén, shown with permission

See Peter Rosén's video
of the landing.

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.

Helical structure
in the trail