A vortex trail belongs to the group of contrails and thus special clouds. It becomes visible just behind propellers, wingtips or flaps of an airplane which crosses humid air layers. In contrary to contrails no extra vapor is added to the surrounding air: the effect is only due to the sudden lowering of pressure which leads to the condensation of the (once) invisible water vapor. The more saturated the air, the longer the trails exist, but all these low level features are non persistent.

The picture was taken just after the passage of a frontal system. In the lower levels a lot of humidity was left giving way to the formation of two long trails just behind the tip of both flaps.