On a dirty car that's been travelling at a reasonably fast speed (I'm guessing at 50mph+) there is always dirt accumulation on the rear door rear edge, adjacent to the gap between the door and the bodywork. The accumulation is always on the upstream side of the gap i.e. on the door side of the gap. It's always on the sloping (saloon car) part of the gap, never on the more vertical parts of the gap. It's never on the equivalent gap on the front doors which are always nearly vertical so I'm thinking it's something to do with the gap being at an angle, but I can't understand why the dirt is always on the upstream side of the gap. Any ideas?