We had a summer rain shower yesterday. Afterwards I could see steam rising off the street as the water evaporated. I could walk barefoot on that street before the shower and while the water was evaporating. If the water was boiling away, surely I wouldn’t be able to walk across it without scalding my foot.
What’s going on here?
In: Planetary Science
Water consists of small particles called molecules. To leave the liquid a water molecule needs a certain energy, with which it can become airborne and become water vapor.
The average energy of the water molecules in a liquid depends on the temperature of the liquid. If you bring water to 100°C almost all molecules have enough energy to leave the liquid and form steam. Therefore at this high temperatures water boils and make steam very rapidly
However it is not like all molecules have the same energy. Some have more (are faster) and some have less energy (are slower) than the average molecules. And the fast, high energetic molecules have enough energy to leave the liquid and become vapor, even if the others do not have enough energy. Therefore small amounts of water can evaporate even if you are below the boiling point (the air must not be saturated with vapor already however). It is just that this process happens faster if the temperature is higher, as more molecules have then enough energy.
A side effect is, that these fast molecules leave the liquid, this causes the liquid to lose energy and cool down. Meaning that the evaporation is causing some cooling effect. That is why we are sweating, because it helps to cool down the body (and even below the temperature of the air, which would not be possible otherwise).
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