RAY: Here's the answer:
When you start your car up, one of the products of this is water in the form of vapor, because of high combustion heat. However, by the time the vapor hits the exhaust system, often times, it has begun to condense, and it is visible. However, on very, very cold days the exhaust system may never get hot enough to keep this from happening. The chimney/smokestack which sometimes had this white stuff coming out burns two different fuels: oil, which is a hydrocarbon or coal, which is a carbon - no hydro.
[ Car Talk Puzzler ]