[aerohead]

My hunch is that the roof is producing counter-rotating vortices at each side.
The mixing and shear of the highly energetic vortices is enough to evaporate the dew where they contact the body and backlight.
Down the center,where the vortices are helping to hold the boundary layer against the body,the straight-line flow does not contain enough energy to evaporate the moisture,leaving the pattern which you see.
Hucho depicts such a scenario in some of the schematic representations from his,and others research.
VGs would alter the situation.It's exactly what their special abilities are crafted for.