With the cooler temperatures and higher humidity, the system can experience condensation inside the exhaust. When the engine is started, it can sometimes spit out a black soot mixed with water. My cars often emit steam at first startup, even on a summer morning.
I remember the Lil Duke Coupe would often spit out a stream of water from the tiny weep holes in the lower ends of the muffler. I never could determine how water might get inside the muffler so it had to be condensation.
Black soot might be an indication that a tuneup is needed, coolant temperature sensor or O-2 sensor needs replaced or there may be other factors causing the car to run rich.
