I have noticed several times when taking off or landing (and, oddly, only at those times, never in flight or above a few thousand feet), the appearance of a perfect "wind tunnel" appearing condensed vapor stream over and under the wing of the plane I've been on (mostly small to medium jets i.e. 737s). Kind of hard to explain but it's almost as if a smoke wand was being held just in front of the wing the way they do to show where the air is flowing when something is in a wind tunnel. It's a really neat effect - you can see the way the air curves up and over the wing, and then whirls in vortices (?) a few feet behind as it comes together behind the trailing edge. It's oddly perfect looking and well-formed; the vapor is uniformly maybe 9 to 12 inches "thick" and appears to be only over a very limited area of the wing, and it perfectly follows what I know from reading about flight is the airflow over and under the surfaces.
Any idea what causes this? My guess (and only a guess) is that the wing rapidly compressing the air in front of and around it causes air on the verge of being saturated with moisture to "lose" the moisture, condense and form water vapor, because the "smoke" looks exactly like cloud substance...
I know this explanation is a bit hard to follow, but it's hard to explain. Anyway, if anyone has any ideas or has seen similiar, I'd be curious, because it's an awesome effect and I would love to know what's going on.
Thanks in advance