Dirt Streaking on Cars: Causes & Solutions

[Roller]

On a dirty car that's been traveling 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?

 

[LightHero]

It's likely due to the Bernoulli effect. Air flow over a curved surface creates lower pressure on the side of the curve closest to the direction of the airflow (the upstream side). This lower pressure can cause dirt particles and other debris to be drawn toward the gap, creating the buildup of dirt you're seeing. The Bernoulli effect is also why aircraft wings produce lift.