(Sigh) Vapour, gas, just different names for the same thing. Do I really need to go through this? Water doesn't heat evenly, if you start to boil water in a large pot on the stove, you will start to see bubbles of steam forming on the edges of the pot. At this point you can probably still put your finger in the water in the cooler centre. The water in the centre isn't at 100 c, probably around 60-70c but at the edges it is at 100c and changing state into steam. In your water heater, the water is standing in a pressure vessel at something like 500kpa under pressure. The water in contact with the heating element will be heated above 100c causing steam bubbles, which under pressure are kept pretty much in solution. The vast majority of the water is still only at around 70c. When you turn on your tap, the pressure is reduced and the water, containing the bubbles in solution flows down the pipe. Because the pressure is reduced, the steam starts to come out of solution and will react like normal steam and will rise to the top and your 70c water will have steam coming off it. Simply fill a glass with hot water and you can see the bubbles.
With the water on the road, it is a similar thing, tiny parts of the water, usually at its thinnest point, will absorb enough energy from the sun and the warm road to get to 100c. We are talking at a molecular level, individual molecules will get to 100 but the vast majority will still be at a much lower temp. It doesn't only happen on the road but everywhere, off leaves, grass, dirt and as the steam rises it becomes what we know as humidity or the amount of water vapour in the air. It continues to rise and eventually forms clouds as the temp and pressure reduces and starts to go back into a liquid form, water drops. When the drops are big enough and heavy enough they start to drop as rain.
This is the same as sweat on your body. Even though your body is not at 100c, sweat still evaporates(boils) off your body, as each molecule absorbs heat energy from your body.
I have tried to keep this as simple as possible without talking about the absolute temp scale, measured in Kelvin, absolute zero or explaining that there is no such thing as cold, cool etc, only lack of heat energy, as this will only confuse further.
I am a refrigeration and air conditioning technician with over 29 yrs of dealing with pressures, temperatures, thermodynamics etc and could probably teach at a university level.
Hope this is helpful.
Sorry if I am hijacking this away from IR Guns.