|>>|| No. 66175
The boiling point of water is where water will instantaneously turn to steam.
But air has a capacity to hold a certain amount of water below the boiling point. And the warmer it is the more water it can hold.
At -20C a cubic metre of air can hold about 1ml of water. At 0C it can hold nearly 5ml of water. At 30C it can hold 30ml.
Relative humidity is how close the air is to holding the maximum amount of water at the current temperature.
If you leave a glass of water out, the water will slowly evaporate into the air if the RH is below 100%, but the lower the humidity the faster it will evaporate.
Your piss steams because it heats up the surrounding air and evaporates into it, but then cools down very quickly and condenses back out as tiny droplets of water.
The same with fog, as the air cools down overnight, it gets too cold to hold onto the amount of water inside it, so the water condenses out as fog.