Where Do Clouds Come From?

Why is only one of two identical glasses covered in drops? On the table stand two identical glasses. The left one holds ice water, the right one holds the same water that simply sat in the room. Nobody touched them, nobody breathed on them, nobody splashed them — and within minutes the left one was covered in drops on the outside. The right one stayed dry. The first guess is usually this: the water spilled over the edge when the ice went in. It is an honest guess, and we test it the simplest way — we wipe the glass dry. Spilled water does not come back; these drops do. A minute later they are on the glass again. And the water level inside has not gone down by a single drop. So where does it come from? From the air. There is always water in the air, you just cannot see it — that invisible water is called water vapour. Warm air can hold more of it, cold air less. The glass is cold, the air beside it cools, all that water no longer fits — and the extra gathers into droplets right on the glass. In our episode about the puddle, the water went INTO the air and became invisible. Here it comes back and becomes visible. It is the same circle, seen from the other side. Up in the sky there is no glass, and height does the cold glass's job: up high the air is cold on its own. Warm damp air rises right above us, cools down — and each droplet grows on a tiny speck of dust hanging in the air. Millions of those droplets together are a cloud. They are so tiny that they hang there and do not fall. The main thing to remember: a cloud is not vapour, not smoke and not cotton wool. It is tiny water droplets. We left a second and a half of silence for your answer: if we wipe the glass dry, will the drops come back? Try it at home, it takes a minute. And if no drops appear at all, that means the air in your room is dry — that is an answer too, not a failure (conditions below). A bonus for grown-ups: in adult words this is condensation, droplets in the sky grow on condensation nuclei (dust, salt, pollen), and air cools with height because it expands. Three notes that would not fit: fog is the same cloud, only at ground level; the white over a kettle is already droplets, the vapour itself is invisible; dew on the grass is this very experiment without a glass. A science cartoon about physics for children aged 7–10, with Droplet and Skimmy. #physicsforkids #scienceexperiments #cloudsforkids #cartoonsforkids #scienceforkids #littlewhys In this episode: 0:00 Two matching glasses — the left one is covered in drops 0:26 The answer: that water came from the air 0:53 The test: wipe it dry and the drops come back 1:13 That invisible water is vapour — and the cold did not make it 1:37 The puddle went into the air — now it comes back 1:53 In the sky there is no glass — up high the air is cold 2:10 Each droplet grows on a speck of dust 2:41 Safety and try it yourself You will need: • two IDENTICAL glasses, same shape and size. Otherwise there is nothing to compare • ice. Cubes from the freezer, and the smaller the better: they chill the water faster • in the right glass, water that has already SAT in the room, not straight from the tap. In winter tap water is cold and the right glass will bead up too. Pour from a jug that has been standing out, or let the water sit a couple of hours • a warm room. The warmer and damper it is, the sooner the drops appear • a plastic cup works too: drops gather on any cold side. Glass is just easier to see through To get it right the first time: • pour the same amount into BOTH glasses and stand them side by side • put the ice in the left one and STEP BACK. Do not touch it, do not breathe on the glass: it all has to happen by itself • wait two or three minutes. The drops do not appear instantly • the test takes a minute: wipe it dry and watch them come back • no drops at all? Then the air in your room is dry — that happens in winter with the heating on. That is not a failure but an answer: there is little invisible water in the air, so there is nothing to gather • if your room is very dry, there is another experiment: a small mirror from the freezer that you breathe on. But that is a DIFFERENT experiment — there your breath adds the water, and nobody breathes on the sky Safety: • do not lift the wet glass: wet glass slips out of your hands • a grown-up gets the ice out of the freezer • if it breaks, do not clear it up yourself, call a grown-up • wipe water off the table at once: on the floor it gets slippery We have a separate episode about where wind comes from and why cold air moves in down low — two matching balloons by an open door —    • Where Does Wind Come From?   The same water from the other side — "Who Drank the Puddle? How to See Invisible Water":    • Who Drank the Puddle? How to See Invisible...   And next time we find out why rain falls out of a cloud. All episodes in English (playlist) —    • Science for Kids — Cartoons in English