What's that visible airflow over the airplane wing and what causes it?
Specifically, what you are seeing here is a vortex which is basically a sideways tornado. Vortices occur when moving air forms a twisted stream when flowing over an object with a complicated shape. The air pressure in the core of the vortex is lower than ambient and if the air is near the dew point temperature, the moisture condenses out at the core and "decorates" the vortex so it becomes visible.
Vortices commonly form at the sharp corner of an extended flap or aileron and at the trailing edge corner of a wingtip. They slightly reduce the lift generated by the wing in the vicinity of the tip. Tip winglets are designed to minimize this effect by blocking air under the wing from spiraling up and around the tip of the wing and cancelling the low pressure zone on the top of the wingtip.