I did some back of the envelope calculation on the amount of water vapor that would be geneerated in North Texas if all of the cars were burning hydrogen in internal combustion engines. I assumed DFW was an area measuring 30 miles by 50 miles containing six million people and four million cars. Each car drives about 30 miles per day, and hydrogen contains about three times as much energy by weight as gasoline.

The people would generate about 0.2 lb per hour of water vapor, or a total of about 30 million lbs of water per day. (Some of this water is taken out of the air with air conditioners, but I'm ignoring that.) The cars would produce about 100 to 110 million lbs of water vapor per day. Now consider how much water vapor is in the air during the Summer. If I consider the air above the DFW region up to 5000 ft, and assume that the air contains an average of 50 grains per lb of air, then the air contains about 150 to 200 billion lbs of water vapor. So, in the Summer the hydrogen fueled fleet of cars would only produce about three or four times as much water vapor as the people do, and about 0.07% of the naturally occuring water vapor. So the water vapor generated by the hydrogen fueled cars wouldn't appear to have much effect. Rainfall would increase slightly, all other things being equal. But, it could be that the water vapor builds up in the atmosphere, whcih would cause global warming. This would be difficult to figure out, even with complex computer models. Determining what happens to water vapor is the weak point of the GCM's that predict global warming. THe reason Hansen, Trenberth and others project run-away temperature increases due to increased CO2 concentration in the atmosphere is due to the assumption that water vapor in the atmosphere goes up because of the slight temperature increase due to slightly increased CO2 level. Even though the amount of water vapor generated by hydrogen fueled cars per day is small compared to the amount of water vapor naturally in the air, it is not clear what the long term effect on climate would be. This is a complex issue, and it will be difficult to accurately determine what the effect of fueling the entire fleet of cars in the world with hydrogen.