The Water Cycle

One of the most popular projects in primary schools is the one where you learn about how the sun makes water evaporate from the sea as steam, how the steam goes high up in the sky, cools down and turns into clouds of water vapour, how the tiny water drops of vapour come together to make bigger drops which eventually become heavy enough to fall out of the cloud as rain, and finally how this rain falls down onto the land and runs into rivers which carry it back to the sea again. In school you might have to discuss this simple story for many hours and make a picture of it big enough to cover an entire wall of a classroom, but the truth is that there are far better things you could be doing with your time: save all that paper and just draw the picture in your mind instead.
Let's now draw a picture of this in our minds, this time going into more detail than you normally would at school. Imagine the blue ocean with the sun beating down on it. Does the sea get hot like the water in a kettle? Well, no. It does get a lot warmer as you go nearer to the equator, but it certainly isn't hot. When water boils, it rapidly turns from a liquid to an invisible gas called steam, but even at lower temperatures it is still able to evaporate away as steam - it just does so a lot more slowly. A kettle will boil dry within a few hours if it goes on boiling continually, but a pot of cold water will also go dry if it's left for a few months, or much faster if you leave it outside so that the wind can blow over the surface of the water (so long as you stop rain falling into it). The stuff you see rising out of a boiling kettle is not actually steam, although there is certainly lots of steam there too: what you are really seeing is the water vapour which forms as some of the steam cools down and turns back into liquid water again. This vapour is made up of thousands of miniscule droplets of liquid water which are small and light enough to be carried upwards with the steam. Anyway, the important thing is that you don't need to boil the sea to make steam - it evaporates off the surface of the sea even when the sun isn't shining on it, and it may surprise you to hear that it can even evaporate off snow and ice at temperatures well below freezing, though of course it only does this at a very slow rate.
Once steam is in the air, it drifts about as a gas, mixing in with the other gases of the air, but because it's much lighter than the oxygen and nitrogen which make up most of the air (it's only heavier than air as a liquid because it clumps tightly together), it gradually floats upwards into the sky. So, if it's lighter, how does it ever come back down? Well, the temperature falls as you go higher, which is why the tops of really high mountains can be covered in snow even in very hot countries, and this fall in temperature encourages the steam to try to turn back into liquid water. Oxygen and nitrogen never get cold enough to turn into liquids in the sky, so you won't see clouds of oxygen or nitrogen vapour up there and you'll never find oxygen or nitrogen rain falling on you, but steam is absolutely desperate to turn back into a liquid, and the higher up it goes and the colder it gets, the more keen it gets. Eventually the molecules of steam in the air clump together to form tiny droplets of water, condensing out of the air to become cloud: this is exactly the same stuff as the vapour that comes out of a kettle.
Now, steam is lighter than air, while drops of water are much heavier than air, so you would expect water vapour to fall down out of the sky, and yet clouds still float about in the sky rather than falling to the ground. How can this be? Well, when you see clouds, they are usually being held up by rising air, and that air will be rising because it contains a lot of invisible steam which makes that air lighter than the air around it. Sometimes if you watch a cloud you may see bits of it rotating: some parts of the cloud can disappear as they evaporate back into invisible steam, while other bits of cloud can appear elsewhere as the steam rises and condenses into visible vapour. It can go round and round, evaporating and condensing repeatedly. On average, the weight of a visible cloud and the weight of the invisible steam-rich air that accompanies it is much the same as the weight of the air elsewhere where there is no cloud, so it's actually very easy for clouds to stay up in the sky. You may have heard that all the water in a cloud may weigh thousands of tons, so it may sound amazing that all that heavy stuff can stay up in the sky, but you have to realise that the same amount of ordinary air weighs thousands of tons too.
If the water vapour in the cloud is thick enough, the tiny droplets will join together to make bigger ones, and the bigger they are, the harder it is for the rising steam-rich air to hold them up, so there will come a point where they fall to the ground as rain. The size of the rain drops which fall from a cloud will depend on how fast the steam-rich air is rising, so an energetic cloud like the ones in thunderstorms can hold up much bigger drops of water than a normal rain cloud, and it can even hold up clumps of hailstones the size of melons in extreme circumstances - it's all about how quickly the steam-rich air rises within the cloud, becasuse the faster it goes up, the longer the rain has to form larger drops, the higher they go into the sky (which makes them more likely to freeze into hail), and the longer they'll have to clump together into the kind of giant hailstones that can smash windows and dent the roofs of cars. Anyway, the long and the short of it is that all this water can be carried for thousands of miles across land before being dropped.
What happens to that rain next is very obvious: it finds its way into streams and rivers, and it is taken back to the sea, though a tiny amount of it will actually evaporate as steam straight out of a stream or river without waiting until it reaches the sea. Of course, most rain probably falls straight back into the sea, but we tend to be more interested in the rain that falls on land because it gives us water to drink and to grow crops with, as well as putting our lives at risk through floods. There is one more important thing to think about in our story, and that is salt. We can't drink sea water because there's a lot of salt in it, but when water evaporates from the sea it leaves all the salt behind, and that means that when it rains, the rainwater is free of salt - that is why the water cycle is important, because it makes fresh water for us which we can drink.
You might have wondered why there's salt in the sea and not in rivers or lakes. There are actually lakes with a lot of salt in them, but only in places where the water can't run out of them into the sea, and the reason they are salty is exactly the same as the reason the sea is salty. Often rain water runs into the ground before it finds its way into streams, and it can run through cracks in rocks deep underground for many years, or even for thousands of years in some cases before it gets back into the sea. On the way through all this rock it can pick up tiny amounts of salt, and this salt is carried by the water into rivers and on into the sea, so there is actually some salt in rivers, but there's so little of it that it's nearly impossible to detect. Once it's in the sea, the salt is stuck there forever: water can evaporate and get up to the clouds and back onto the land, but the salt is trapped behind in the sea, and so, little by little, the sea becomes saltier and saltier. There was a time, billions of years ago, when the sea was no more salty than any river, but rainwater has gradually taken tiny amounts of salt from the land into the sea, and it has stayed there ever since. Some lakes are much saltier than the sea, and this is because streams sometimes run into small lakes in really hot places where the water evaporates much more quickly than it can from the whole of the sea. In a few cases if you swim in these lakes you'll float so high in the water that you're really floating on it rather than in it - this is because salt water is much heavier than normal water.
Now, I've actually packed quite a lot of information into this description of the water cycle and you'll be hard pushed to remember it all, but there's no need to worry about trying to memorise any of it: just come back to this page from time to time and read it again, and there will come a time soon enough when you find that you have remembered the whole thing.

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