Same thing that causes the smoke coming off the wings in this video. http://www.alexisparkinn.com/photogallery/...th_me_steve.wmv (the passing out is from the g-forces, and has nothing to do with this.) If you didn't get it from the video, the air is becoming compressed to a point where it will no longer hold moisture, and the pressure forces the moisture from the air. But that's just my theory.