Rain will spot your windows or car because there will probably be a fine layer of dust on there, very rarely is the rain so dirty it will spot the windows.
Occasionally we get dust laden clouds blow in from the desert, but they only happen a few times a year.
If rain is blowing around then if it soaks the walls above the windows, then the rain will pick up contaminants off the wall and leave them on the windows.
Rain by itself isn't dirty.
Ian