I was under the impression that when using hydrogen for fuel, the by-product was water (at least in automobiles). The article just triggered the question, I was

I was under the impression that when using hydrogen for fuel, the by-product was water (at least in automobiles). The article just triggered the question, I wasn't necessarily commenting on the technology. Oh, sorry! Yes, burnt hydrogen is water. My question was on whether or not large amounts of water vapor being released from cars will cause local ecosystem changes. (i.e. If every vehicle in NYC or Los Angeles ran on hydrogen, would it cause average rainfall in the immediate or surrounding areas to increase? Well, large urban areas already have significant impacts on the local climate--e.g., the heat island effect: http://en.wikipedia.org/wiki/Urban_heat_island http://en.wikipedia.org/wiki/Urban_heat_island The significant factor, I'd think, would be the amount of water being introduced, which in relative terms would be trivial amounts for most climates. New York is already plenty humid, for instance. I'd guess that any impact from increased ambient humidity would be swamped by the heat island and other such effects, except perhaps in an extremely arid climate.