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
You’ve probably been taught what boiling actually is incorrectly. Most people are taught a sort of lie in grade school, and never really get told the truth. What is actually special about boiling? Water turning into a gas? No. Water always evaporates, and higher temperature just makes it faster. It isn’t some magical barrier where suddenly all the water molecules have the energy to escape, contrary to some answers. Water bubbling? Yes, that is unique to boiling and does not occur in normal evaporation.
Boiling is not the temperature at which water turns into a gas. Boiling is the temperature at which the evaporation becomes so fast its pressure (vapour pressure) equals atmospheric pressure, and hence water can evaporate not just at the surface, but throughout the water. That’s why it start bubbling, and evaporating much faster. That is, boiling. Bubbles can’t form below boiling point because atmosphere crushes them before they can form, so only the surface can evaporate.
Water and water vapour at 90C aren’t boiling, but they sure will burn you. Water evaporating at room temperature is not hot, because it’s, well, room temperature. In fact, if you took water into a vacuum chamber, it would boil at room temperature and still not be hot. Dropping atmospheric pressure drops boiling point, because it takes less rapid evaporation, less vapour pressure, to push back against the atmospheric pressure and cause the boiling effect. Inversely, put the water in a pressure chamber and you could get it to 150C without boiling, but it would be evaporating quickly and definitely not fun to touch. This is how we cook things fast via a pressure cooker.
In fact, in the mountains you need to boil water longer to disinfect it for safe drinking, as the boiling water isn’t as hot, but still hot enough to eventually kill pathogens given more time. Basically the same as if you tried to kill pathogens with water that isn’t boiling at 90C at sea level. And in a pressure cooker, you can kill pathogens even faster than with boiling water, without boiling ever occurring. Your foot is like the pathogens, you don’t care about boiling, that’s just bubbles, nor do you care about water vapour, you care about the temperature. If it’s room temperature, it’s room temperature, it’s not going to hurt you. The hotter it gets from there, the more it’s going to hurt you.
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