Turn on a tap and out comes water. Look up at a rain cloud, or out across the sea — that is water too. But here is a puzzle. Rain falls from the sky day after day, runs into the rivers, and pours into the sea… yet the sea never overflows, and the sky never runs out of rain. Where does it all come from, and where does it all go?
The answer is one of the great secrets of our planet: it is the same water, going round and round in an enormous, never-ending journey. Up into the sky, across in the clouds, down as rain, back to the sea, and up again — forever. That grand loop is called the water cycle, and the whole thing is driven by one gigantic engine hanging in the sky: the Sun.
The journey begins wherever the Sun shines on water — the sea, a lake, a river, even a puddle on
the playground after rain. Sunlight pours its warmth into the water. Some of the water gains so
much energy that it stops being a liquid altogether and turns into a gas. This is
exactly the
The gas that floats up is called water vapour, and here is the surprising part: it is completely invisible. You cannot see the sea steaming away — but it is, every sunny day, silently and everywhere. You have watched it happen yourself: a wet playground dries in the sunshine, a puddle shrinks to nothing by lunchtime. The water did not leak into the ground — the Sun lifted it up into the air as vapour.
The higher you go, the colder the air gets — that is why the tops of tall mountains are capped with snow even in summer. As the invisible water vapour rises, it cools down. When it cools enough, it can no longer stay a gas, so it changes state back into a liquid. This is condensation: gas turning into tiny droplets of liquid water.
Millions upon millions of these microscopic droplets drift together, and — there it is — a cloud. So a cloud is not the vapour going up; it is the liquid that appeared once the vapour got cold. You have seen the very same thing on a cold day when you breathe out and see a little "cloud" of your own: the invisible water in your warm breath meets the cold air and condenses into droplets you can suddenly see.
Here is a fact that surprises most grown-ups. A cloud is not made of gas. Water vapour, the gas, is invisible — if a cloud were gas, you would not be able to see it at all! A cloud is actually made of countless tiny drops of liquid water (and, higher up where it is freezing, tiny specks of ice). Each droplet is far too small and light to fall, so they hang in the air like the finest mist, and there are so many of them that they turn the sky white. That is the whole reason you can see a cloud but you cannot see the vapour that made it. The cloud is a liquid; the vapour was a gas.
Inside a cloud the tiny droplets keep bumping into each other and joining up. Two make a bigger drop, that drop swallows more, and slowly they grow. For a while the air can hold them up — but a drop can only get so big before it is simply too heavy to float. Then gravity wins, and down it tumbles.
Water falling from a cloud has a grand name: precipitation. When it falls as liquid, we call it rain. When the air is cold enough it can freeze on the way and fall as soft snow, or as hard little balls of hail — but it is all the same water, just arriving in a different state. Rain, snow, hail: three costumes worn by the very same water on its way back down to the ground.
The water has landed, but its journey is not finished. Rain that falls on the hills trickles into little streams; streams tumble together into rivers; and rivers wind their way, always downhill, all the way back to the sea. Some water soaks into the ground first and creeps along underground before it too reaches a river or the sea. Either way, it arrives back where a drop's journey could begin all over again.
And that is exactly what happens. The Sun is still shining, so the sea water evaporates once more, rises, cools into cloud, falls as rain, and flows back to the sea — round and round with no beginning and no end. Nothing is ever thrown away and nothing new is ever needed. The same water just keeps taking the same trip, powered forever by the Sun.
Now walk one drop of water around the whole journey. Pick a stage below and watch that part of the picture light up: water rising invisibly from the sea, gathering high up into a cloud, then raining back down. Remember — it is the same water every time, just changing its state: up as an invisible gas, cooling into a liquid cloud, and falling as rain.
Three things trip up almost everyone. Keep them straight and you truly understand the cycle:
Because the water cycle never stops and never loses a single drop, the water on Earth has been going round and round for billions of years — far longer than there have been people, or even dinosaurs. That means the mouthful of water in your glass right now has already been sea, cloud, rain, river and sea again more times than anyone could ever count.
The very same water molecules could once have been a puddle a dinosaur splashed through, or the rain that fell on the first forests, or a snowflake on an ice-age mountain. Water is never really used up — it is only ever borrowed. Drink up: you are sharing a drink with the whole history of the planet.