OK, as I understand it, hydrogen powered cars produce water as exhaust. How are they going to vent it? How will that impact our roads? In cold states will we have ice every where?
I’ve been wondering this myself for the past few months. More for a mold or moss issue as well. Will such (hopefully) massive amounts of new water vapor make the country into one huge fog bank or even raise the humidity, which is already a concern in NJ?
Does the phrase “water vapor” mean anything to you?
Is that the stuff that the hydrogen in hydrocarbons turns into when you burn them?
I’m not trying to be dense here, but “water vapor” exhausted into 10o air is pretty likely to turn into something else pretty quickly.
If there are enough cars in a given place to create smog, why wouldn’t the equivalent number of hydrogen-powered cars be similarly able to affect the local climate by generating all this water vapor? I much prefer fog to smog, of course, but to say that there will be no effect at all seems myopic to me.
I’ve been planning on buying stock in rock salt companies if hydrogen ever looks like it’s going to take off. 
There used to be loads of deaths in London 50 years ago before we introduced a clean air act.
In large Japanese cities now, you can see people wearing masks.
Water’s fine - the current exhaust fumes can kill.
Sure, but there is easily more to this. It wasn’t a stupid question as your overly pat answer seems to imply.
From Water Cycle
Water vapor as it cools turns to water. What happens in cool climates? Does that then turn to water on the roads and then ice on the roads?
That depends. As Squink cleverly notes, we don’t have that problem now, even though a significant fraction of automobile exhaust is water vapor. In order for water vapor to condense on a surface, such as a road, that surface must be cooler than the dew point. In low-traffic conditions, a road surface can easily get cold enough but there aren’t enough cars to emit enough vapor to cause problems. In high-traffic areas, there are likely enough cars to produce sufficient vapor to cause local icing but the road surface will be relatively warm from all the frictional heating. I suppose under certain unusual conditions, this could be an issue, however, those same problems exist naturally, and we already have an infrastructure in place to deal with it in those areas with a cold winter climate.
I don’t think it’s a problem, for the reason Squink noted.
However I don’t know what form the “exhaust” is from a fuel-cell car. Is it really vapor, and if so at what temperature? If it’s significantly cooler than the exhaust from internal combustion engines, there may be slight differences (e.g. cars dripping some liquid water?).
Even gasoline-powered cars drip water from their tailpipes - haven’t you ever seen this in cold weather, or when the engine is cold?
All existing hydrocarbon combustion creates water vapor as an output. I do not know how water vapor emissions per mile compare to internal combustion gasoline per mile, though conceivably someone could do the energy balance equations and figure it out in a few minutes.
Regardless, I do know that I feel better about water vapor “pollution” in any quantity than I do about carbon monoxide, carbon dioxide, airborne particles, and god only knows what else comes out the tailpipe of a gas-burner. If we can handle rain on roads, then we can handle water vapor coming out of tailpipes.
::SHRUG:: There was no judgment in the request for information. Wanting to know something doesn’t imply criticism.
In round numbers you create a gallon of water for every gallon of gasoline you burn. I will leave the hydrogen calculation to someone else as an exercise.
Let me try some high school physics ( from here ):
Burning 1 Megajoule worth of gas produces 1.6 moles of water. Burning 1 Megajoule worth of hydrogen produces 4 moles of water. So burning hydrogen does produce a bit more of water for a given amount of energy.
1 Megajoule drives you for maybe about half a mile in a small efficient car. Thus driving for one mile using hydrogen produces the equivalent of about 1 coffee cup full of water. Seems insignificant in comparison to natural evaporation from the environment (plants, ponds etc).
The amount of water vapor all the cars in a city would emit is a trivially small amount compared to the water vapor already being cycled through the atmosphere.
A single background swimming pool can lose thousands of gallons of water to evaporation in a year. A good rainstorm can dump inches of water on every bit of land within a city. Millions of gallons of water. A single puffy white cumulus cloud can hold hundreds of tons of water.
Here’s Cecil on the amount of water in clouds: Can a cloud weigh as much as a 747?
The amount of water vapor coming out of cars is utterly irrelevant. And if it weren’t, all it would mean is slightly more rain, since the air would just saturate more quickly.
I was not responding to criticism. I was responding to your implicit question of “Is this anything to worry about?” If it’s water vapor you’re worried about, I make a WAG that existing fossil fuels put far more water vapor into the environment. But more to the point, the amount of water we’re talking about is negligible compared to the amounts of other natural and manmade sources of water vapor in the water cycle.
Aha, but something like 80% of energy lost from a gasoline internal combustion engine is lost to heat. That brings the amounts to a fairly even level, which doesn’t even matter since they’re both trivial amounts in the grand scheme of things.
In many parts of the world, actual liquid water naturally falls from the sky with some regularity, but fails in most cases to cause massive disruption to traffic or infrastructure.
The amount of water being expelled from the exhausts of hydrogen powered cars cannot possibly exceed the sort of amounts that fall as rain during a gentle shower - and I suspect they fall short of that amount by orders of magnitude. Of course it might have some effect, but I don’t think it’s going to be a gross measurable one.
Why not collect it on-board as liquid water, and then ‘empty’ your car when you get home? Water your flowers.
Because water weighs nine times more than the hydrogen that was burned to produce it. That would make a not-insignificant dent in your fuel economy.