Originally Posted by
tjspiel
Bridges do ice up easier (most of them not even over open water in the winter) but I'm not talking about bridges. Yes hot air rises, eventually - if there is no force moving it any other way. Exhaust is often directed down.
The water only has to be partially open over water to have air saturated with water vapor. Rivers are usually warmer than the surrounding air as well...they have to be because water doesn't flow when it is frozen.
As to the exhaust, I would put the number of tailpipes that direct car exhaust downward in the minority to the number that direct it sideways or on a flat trajectory, not that it matters much. The exhaust is going to rise up out of the pipe very rapidly...much faster than the small amount of water in a given volume of exhaust could freeze on the ground.
Originally Posted by
tjspiel
Look. This is known phenomena. Not an urban legend or something we are making up.
Show me a credible source that's not "some guy on the internet"...I can find one...and I might believe you. Until then, yes, this is an urban legend. It has all the hallmarks..."everyone knows this", "there are lots of sources that say this", etc.
As it stands now, there just isn't enough water in car exhaust nor a high enough concentration of cars in any given area for me to believe it. There are also alternative explanations that make far more sense You might as well say that human breath is responsible for ice on sidewalks.