Lessons from Teacher Ciruela
Invisible clouds

The kitchen is the place where people get mostly misled about the nature of clouds. While everybody thinks they are actually composed of water vapor, meteorologists come out to tell us it's not vapor but just liquid water... very small drops of liquid water floating in the sky, that's to say, water held suspended in the atmosphere.

How come liquid water is held suspended in the air? What about the weight of water? Isn't liquid water expected to fall? Again, meteorologists are good at enlightening us about that: although water droplets forming clouds come in different sizes, their individual weight will always be low enough for the wind force to drag them from one place to another, either upwards, downwards, sidewards... blowing the clouds any way or even making them keep still when it happens that the wind takes them up as much as the Earth attracts them down. But when their size and weight grow so large as to come down, then we have to put up our umbrellas.

Most physics teachers tend to wholeheartedly endorse their argument saying that clouds could not be seen if they were just made up of water vapor, as it is invisible. In fact, water vapor is actually a gas, a dispersion of molecules of water (H₂O) that run individually, independently of each other and the rest, and totally incapable of interacting with light rays. And yep, this combination as a whole is transparent, as invisible as air. So those teachers are right.

But the fact remains that their reasoning doesn't match the idea that suddenly pops into our heads and makes us helplessly doubt their gassy remarks: water vapor coming out of the kettle spout is visible! We're all keenly aware that a stream of that vapor would surely burn our hands. The point is that teachers are not telling us that what we see coming out of the spout is not vapor! They're not individual water molecules but microdroplets of liquid water each consisting of hundreds of millions of water molecules. Oh, really? Ciruela, you're driving me nuts! Everyone knows full well that when water gets to its boiling point it changes from a liquid to a gaseous state. Huh! I'm gonna quit reading this one, dude.

Let me explain myself a little more: it's true that at 100 ºC (212 ºF) liquid water turns into the gaseous form. It's the time when each individual water molecule transits to a less associated state by gaining an enormous amount of energy. But all of this happens inside the kettle. Just after reaching the boiling point molecules start spurting out of the kettle and immediately come up against the surrounding cool air (cooler than the one in the pot). Right at that very moment they condense into tiny water droplets (about 0.005 millimeters in diameter) which lead to a visible mist. Mist is not gas, it's a phenomenon caused by small droplets of liquid water suspended in air, just as it happens in cloud formation. But as the stream flowing from the spout keeps holding a very high temperature, then droplets evaporate once again (just like they would at room temperature) and end up becoming invisible at about one meter or so above the kettle spout.

Only the first stage of the stream is water vapor (about 5 centimeters from the spout, at most; see image). What follows -and what we all bear in mind- is just water suspended in air.

Got it? Being so kettle-blind will not help you understand such a tricky universe.