Who knew that a small amount of dust stuck to the roof could cause turbulence? But here is a photo of mist on the roof of my car after a short drive on a very foggy morning. The sun has just broken through:
The "stain" is in the center left, just above a crescent-like semi-circle created by the coffee cup I had momentarily laid on the roof before I noticed this interesting mist effect. The "stain" is rough, made by dust that suck to some dried coffee I had spilled there the day before as I got into the car. Extending from left to right, as the airflow, you can see how the rough spot seems to have generated turbulence that dried a widening area of the roof aft of it in the airflow.
Just an interesting pic, I thought I'd share.