Water vapor, pressure, temperature and seeding are what it takes to make a cloud. Use a two-liter soda bottle, some warm water, a match, and some hand strength to become a rainmaker. It's all in the wrist.
This is a demonstration that is meant to be safe for groups of children in grade school classes. Frankly, I still don't recommend it be used by Io9 readers. I trust you with matches far less than I do a bunch of unsupervised fifth graders, and I'm pretty sure that at least some of you will do this at home with furry pets nearby. But I suppose it can't be helped.
Empty a two-liter bottle of soda. Not by drinking it. The last thing we need is you doing this while over-caffeinated. Fill the bottom with hot water. Just half an inch should do. Light a match and hold the head of the match inside the tip of the bottle until the bottle fills up with smoke. Cap the bottle tightly.
Wait until the air inside the bottle has cleared and then squeeze the bottle a few times. After a few squeezes, when you release, a cloud should form inside the bottle. Congratulations, you're a Wizard, Harry.
Or maybe not.
Our little bottle demonstration shows the conditions under which clouds are formed. The hot water, of course, evaporates and fills the bottle with water vapor.
The match is also necessary. Believe me, if it weren't I would have left it out. Apparently, clouds are yet another beautiful and useful consequence of every part of the world we live in being covered in filth. It turns out that perfectly clean air can get supersaturated, holding water vapor and attaining a relative humidity of over one hundred percent.
Which sounds like, by definition, complete crap, but remember, the key word is ‘relative.' Relative humidity is the percentage of water vapor in the air, divided by the highest possible amount of water vapor in air of that temperature. Absolutely clean air will carry more water than our usual, repulsive, muck-filled air, and so the absolute humidity can be at more than one hundred percent. That is, until a dust particle crashes the party.
Dust particles, or other types of grit, allow for nucleation – they are the sites on which other water particles condense, and they make clouds, and rain.
Many countries have taken advantage of this knowledge. Cloud seeding, usually with particles of silver iodide, which mimics the structure of ice, can make clouds form and rain. It's used to tamp down particles that cause air pollution, bring on some much-needed rain, or just clear the way for sunny skies, since clouds tend to break up after rainfall. The match, and the dust particles provided by the smoke, seeds our bottled cloud.
But why does the bottle need squeezing? A bad day? A longing for companionship? Does it have a piece of food stuck in its neck?
Actually, the squeeze warms things up inside the bottle. And releasing it cools things down pretty suddenly. Gas molecules whiz around, bouncing off each other and off the sides of the bottle. The temperature is basically a measure of the average speed of the gas molecules. When you let go of the bottle, the sides are released, and the bottle expands. Suddenly the gas molecules that were smacking off the sides have to travel a lot farther before they're getting a kick back. This slows them down and cools the bottle.
At cooler temperatures, the molecules of water slow down and attract each other, condensing around the dust particles from the match. And voila. Seriously, Harry, you're a Wizard.
Feel free to send me your tuition for Hogwarts.
And one of those chocolate frogs.