Rich Martin

In answer to your question, air molecules don't flow straight over a wing linearly from fore to aft. Instead, the air passing under the wing tends to curve outboard, whereas the air flowing over the wing tends to curve inboard. Combined with the fact that the pressure on top of the wing is lower than the pressure under the wing, the air under the wing is going to be sucked up and over the wing at certain points, typically at wing tips. This interaction between the two air flows causes a spiral motion. As the angle of attack of the wing is increased this interaction becomes more pronounced. Extending trailing edge flaps increases the effective angle of attack and some aircraft will experience vortices from their flaps, whereas by design others don't. Aircraft with winglets are less susceptible to wingtip vortices and indeed the purpose of the winglet is to reduce the vortex effect as it causes induced drag which costs fuel. Vortices are more visual on aircraft approaching to land in moist conditions. ie, conditions where the air temperature and dew point temperature are close. The accelerating air within the vortex causes the pressure and therefore temperature to drop to that of the dewpoint temperature and the vortex becomes visible. (You can test the theory of pressure and temperature dropping in accelerated air by clenching your teeth and sucking in quickly-your teeth go cold). I hope this answers your question. For the record, I fly the real thing professionally.