Why does your car AC hiss and blow white smoke from vents?

Answer 1

The hissing is part of the operation of the air conditioner. Remember back to your basic physics and chemistry classes; we were all taught about "Boyles law", where a gas will raise in temperature as it is compressed.

Your AC compresses the refrigerant, and it gets hot. The compressed refrigerant is routed through a "condenser coil" (basically just a specialized radiator) and cooled to the outside temperature.

Then the still compressed but now considerably less hot refrigerant is allowed to expand; that's when it gets cold. As the refrigerant expands it "hisses", just like air leaking out of an air compressor hose; but in this case it's all controlled and the refrigerant is still contained in the refrigeration system. The now cold refrigerant passes through ANOTHER specialized radiator called an expansion coil and air is blown across the outside of the coil.

That answers the hissing part of your question.

The "white smoke", isn't actually smoke; it's water vapor.

Your expansion coils get quite cold, and any moisture in the air will start to condense as the air crosses the cold expansion coils. Some of the condensate (water) drips harmlessly out the bottom housing of the expansion coil, through a tube that's designed to route the dripping water to the outside of your vehicle.

SOMETIMES the water will start to form visible water vapor, much like clouds in the sky. That should answer the other part of your question.