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 always evaporates, even when it’s not boiling. Steam is usually invisible in the air, but becomes visible when air is over-saturated with water or suddenly cooled. Evaporating from warmer pavement and then going into cooler air makes it visible.
Water can evaporate at any temperature. Even ice can sublimate directly to vapor.
What you are talking about re: boiling is water becoming steam. Steam has more energy in it than water vapor below the boiling point.
Think of the air as a sponge. The warmer the air, the bigger the sponge (warm air can hold more water vapor than cold air). If the sponge is big and dry, more water can evaporate into it. But if that same air cools down, it can’t hold that vapor anymore and it becomes liquid again (like when you have a cold drink and water forms on the container).
The temperature of a puddle of water is really just an average of the energy of all the water molecules that make it up.
While the whole puddle may not be warmer than room temperature, individual water molecules in the puddle can have significantly more energy due to random collisions with other water molecules. If that high-energy molecule is at the surface of the puddle, it can escape into the air.
If enough of these molecules escape the puddle, there won’t be a puddle anymore, it will have evaporated away.
Water molecules in liquid phase do not have to reach the boiling point of water to change to water vapor (gas phase). As you approach the boiling point, more and more liquid water molecules have enough energy to change to gas. After the rain shower, the ground still had residual heat to cause some of the liquid water to change to gas. This residual heat is nowhere near high enough to cause physical damage to your skin.
What you are seeing as “steam” is actually tiny liquid water droplets that have condensed from the gaseous water vapor. This happened because the air temperature very close to the ground was cold enough to change the just evaporated (from the ground’s heat) gas water molecules back to liquid water by decreasing the energy of the gaseous water molecules. What you are seeing is the same as fog, or a cloud; both composed of tiny liquid water drops.
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).
All molecules are constantly vibrating. When there is a body of water (like a puddle) the air molecules that are touching the water molceules bounce against them and cause the water molecules to pull away from the rest of the water molecules, and they bounce up into the air, which has more space between the molecules. That’s the process of evaporation. Sometimes this process is happening so quickly that we can see a ton of water molecules getting lifted into the air – that’s what steam is.
This isn’t exactly what’s happening, but for the sake of understanding this think of the liquid water as a bunch of bouncy balls jostling around in a container. If you just shake the container a little, the balls will mostly sit there hitting each other, but occasionally a ball will hit other balls just right to bounce high enough to fall out of the container. This is the water evaporating: sometimes an atom happens to have enough energy to just break free.
Boiling the water would be like shaking the container violently enough that the balls have to leave (you’re just shaking them out of the container now). If evaporating is one of the balls happening to bounce against the others hard enough to escape, boiling is when the balls are bouncing so much collectively that they can all escape.
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.
Temperature is the *average* speed of particles in a material. There’s a whole distribution of speeds and at any time *some* of them are going fast enough to fly out from the surface (aka evaporate) even if the *average* speed of the whole group is not fast enough to do this.
Note that even ice in the freezer shrinks over time – proving that even below 0C, at least some small fraction of water molecules are still bouncing fast enough to escape. Even when the average speed of the molecules in the whole cube is below 0, way below boiling point!
Air molecules in the air are constantly colliding with the water molecules on the street. This gives enough energy to the water molecules to break free from the other water molecules and become vapor. This happens until all the water has evaporated.
Now if it was humid… there is more science… that don’t want to type up right now.
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