All Vehicles What Would Be The Effect On The Atmosphere If All Vehicles Were Hydrogen Fueled?

What would be the effect on the atmosphere if all vehicles were hydrogen fueled? - all vehicles

The reason I ask is that I have not seen any studies on the effect of hundreds of thousands of vehicles in constant pumping of water vapor in the atmosphere. as a city shrouded in a perpetual fog and pouring rain? If all vehicles in the world have been fed with hydrogen, the world would eventualy be under a blanket of clouds? If so, the clouds block the light, but can UV rays and heat out in the country. The cloud cover and warm as it melts in causeing polar ice and floods contribute world.Could this possible?