.. a minor correction to CoastRider26's explanation.
The local aircraft condensate occurs due to a significant pressure drop either
(a) inside vortex flow
(b) where there is a sharp change in flow direction
(c) transitioning to subsonic flow through a normal shock in supersonic flow.
Associated with the pressure drop is a temperature drop (adiabatic flow cooling). As the temperature drops, the relative humidity increases. If the RH reaches 100%, then the water vapour condenses and we "see" the centre of the vortex.
Examples are seen in the inflow to a propeller disc at takeoff in rain, shed vortices from the end of flaps, in the wingtip vortex, shed vortices from leading edge extensions (eg Hornet), the leading edge vortex on a delta (Concorde etc), supersonic aircraft "flying" through a shock wave cloud, etc....