Why does it feel cooler if you blow air on your skin vs. still air?

Gaseous water (i.e. water vapor) can exist at temperatures well below boiling.

And boiling water can certainly be in liquid form. Indeed, it takes a lot of energy to convert boiling water to steam.

The air around is is full of water in its gaseous state. That is what we measure when we measure humidity. The hotter the air, the more water vapour it can hold, but the air need not be at liquid water’s boiling point in order to hold water vapour. Indeed it can hold some water at quite low temperatures (eventually it holds very little as the temperature drops, the place on Earth with the lowest humidity is not some desert, it is Antarctica.)

A body of water evaporates if you leave it exposed to the air. You don’t need to heat and hold your laundry at boiling point in order for it to dry. Nor does your kitchen floor require the application of a flame thrower to dry it off. Rain is condensing water vapour that falls from clouds. Clouds form without the oceans boiling.

In order to understand the issue with energy - try this experiment. Put some ice cold water in a kettle. Turn the kettle on. Time how long it takes to come to the boil. Now leave the kettle boiling, time how long it takes the kettle to boil dry. It will take roughly five times as long to boil dry as it did to come to the boil if you started with iced water.

Nitpick: clouds are not gaseous, they are made up of droplets of liquid water. Water vapor (gas) is invisible.

Otherwise I agree.

Yes, of course (now he says that).

“Gaseous water” is not a phase change from liquid. Without me looking it up (:slight_smile: what’s an aerosol then? You’ve got H2O “in” atmospheric gas.

What’s the word for that state of materials? And some states of H2O in atmosphere, scr4 reminds us, is not “gaseous” either.

ETA: vapor, and that’s it?

“Gas” and “vapor” are synonymous. “Gaseous water” is not itself a phase change from liquid, but it is typically the result of a phase change from liquid.

We often speak of temperature as being a measure of the energy of the individual molecules, but this is misleading. Actually, temperature is a measure of the average energy per molecule, but at any given moment, some will have more and some will have less, according to a very specific distribution. When water is in its liquid state, there are weak bonds holding all of the molecules next to each other. But a few of those molecules will have energies high enough to break away from those bonds, and if those molecules are at the surface, they’ll escape the liquid and become gaseous. Likewise, in the air, there are some molecules of water that have a low enough energy that, if they hit any other water molecules, they will get caught by those bonds. Thus, given an open container of water, there will always be a few molecules transferring from the liquid state to the gas state, and vice-versa. But the more molecules there are in the gaseous state, the more will hit the water, and the the more will condense out, so there’s some amount of gaseous water molecules such that the rate of evaporation will equal the rate of condensation. This amount is called the “vapor pressure” of water, and depends on the temperature (the higher the temperature, the more water molecules will have enough energy to break their bonds, and so the more gas you’ll have). This is also what people are comparing to when they speak of “relative humidity”: 100% humidity means that the amount of water in the air is equal to the vapor pressure for that temperature.

So what is boiling? That’s when the temperature is so high that the vapor pressure of the water is a match for atmospheric pressure. That means that evaporation no longer just occurs at the surface: Water molecules anywhere in the body of the liquid can break free of their bonds, and form bubbles that rise through the water. It still occurs more easily at some sort of surface or another, but it doesn’t have to be the top surface where the water meets the air: It can also be where the water meets the sides of the container, or at the edge of an already-formed bubble.

In addition to what Chronos delightfully laid out above, that the saturated water vapor pressure changes with temperature … it also changes with pressure. Not only will lowering the temperature of a saturated parcel of air cause water vapor to condense, also lowering pressure with cause condensation.

This is critical to most cloud formation, we have to have air being uplifted in the air column and the pressure dropping. Once the air hits it’s saturation point, any further uplift will cause water to condense and form the cloud. Here’s a photo of a lenticular cloud. As the mountain forces the air flow up, a cloud forms due to lower pressure; then as the air flow starts back down the other side, pressure increases forcing the liquid water back into it’s vapor state.

[I’m assuming “with cause” here is intended to be “will cause”]

Yes, but that’s closely tied to the fact that lowering pressure also lowers temperature: it’s the old ** PV = nRT** story (ideal gas law).

An interesting thing to note is that those clouds are probably not over that mountain - they are downwind of it. That’s where lenticular clouds most commonly are found.

But there definitely are cap clouds that match your description.


It’s funny–I just posted an OP on oxygen saturation of H2O. Coincidence, probably.

bold added.

Something here why boiling blobs in microgravity are so dramatic, and to a lesser extent, water in microwaves, right? Can’t sort out why, at the moment.

I’ll go find some NASA YouTubes and you explain it–deal?

Not as closely tied as you would think. Although the ideal gas law is excellent for adiabatic processes (where there’s no energy exchange), once we start condensing water things go haywire. As pointed out above, water takes 2,300 kJ/kg of energy to evaporate. When water condenses this same energy is released. Some of this energy is radiated out into space, some is used to increase temperature and if available some is used to accelerate the cyclone.

If we then bring the air parcel down to it’s original pressure altitude, it’s temperature will be lower now. To evaporate the water back into the air we have to add the energy back in, but we’ve lost too much of the original energy to radiation, so we have to draw this energy out from temperature. It wouldn’t be very much in this example, but enough to garf up any explanation using just the ideal gas law. If we have rain drops heavy enough to drop out, we start messing with the mass term as well. So we have pressure, temperature, mass and energy all changing at the same time in an irreversible process. Kinda gives you a sense of why weather forecasters are wrong so often.

NOAA gives the definition as “a very smooth, round or oval, lens-shaped cloud that is often seen, singly or stacked in groups, near or in the lee of a mountain ridge.” So you are correct that the cloud may well be downwind in the photo I linked to. Strictly speaking, the term “lenticular” addresses it’s shape being like a lens, much like in your link mammatus is in the shape of mammaries (giggle).

If the surface of a container is very smooth, and the water is very undisturbed, then it’s possible for it to reach or exceed boiling temperature without actually boiling (this is called “superheating”), because there needs to be some place for the bubbles to start forming (“nucleation sites”). This can sometimes happen with a mug in a microwave. Once you take the mug out and put something in (like a teabag, or instant coffee crystals, or sugar), the something you put in can serve as nucleation sites, which results in the water very suddenly coming to a boil, often splashing out a lot of it (and you don’t want boiling water splashing out of something you’re holding in your hands).

I’ve never seen video of free-floating water boiling in zero-g, but it makes sense that the same thing would occur, since you wouldn’t even have any surface of container at all, there.

A deal’s a deal. Note that I am a shifty deal maker, in that I could not find/provide a film of truly free-floating mass of water heated externally to boiling.

Good detail (visual and commentary), soldering iron on ISS: Nucleate Boiling in Microgravity - YouTube

More detail/research, interacting with electric field and heat exchange devices, for long-term space flight, narrated: Boiling at Zero-Gravity - YouTube