I bought my first car, a '46 Ford, in 1955. I washed it's fadded paint and drove it out to the college and parked it in front of the administration building, then went to class. During class, I noticed that it was raining and I remember thinking, "Why not. I washed the car."
A couple of classes later I went outside and there was my car splatted with mud. Not just mine; everybody's car was muddy. There was a thin film of mud on everything. I got the explanation from a science professor who said that there had been an unusually strong duststorm out west. The wind blew 40 to 70 miles per hour for 36 hours. The resulting dustcloud reached almost to the stratosphere and drifted to the east. Over Middle Tennessee, it collided with cold air and it rained. Each droplet picked up a few dust particles and some of those landed on my car.
Most people washed it off, but I left mine that way for a week. I thought it was pretty cool.
Howard