The water cycle: solid, liquid and gas
What would it be like to travel with a particle of water around the world? Imagine that the particle of water is in the sea. It is sliding round other water particles near the sea's surface. All the particles are pulling on each other too, so that they all hold together. Eventually the particle reaches the surface and no longer feels such a strong pull. This is because there are no water particles above it. There are only pulls from those on its sides and below it. At the sea's surface the particle receives extra energy from the Sun's heat. This makes the particle move faster. Eventually it moves so fast that it escapes from the pull of the other particles and lifts into the air. It has become a particle of water vapour.

Water vapour is just one of the gases in the air. The particle mixes with the particles of other gases but still keeps bouncing into other water particles. It is in warm air over the sea and as the air is lighter in weight than the air above, it rises. As the air rises it moves away from the warm surface of the sea and cools down. Eventually the water particle bounces against a grain of dust. There are other particles of water vapour hitting the dust grain but the surface of the dust is so cold that it takes heat from the particles. Heat is a form of energy and when the particles lose this energy they can no longer move freely. Condensation takes place and the particles form a drop of liquid water around the dust grain.

Billions of other water vapour particles have suffered the same fate on other dust grains and a huge cloud forms in the sky. A wind blows the cloud along high above the surface of the sea. Eventually the cloud moves over land and towards a range of hills. The droplet in which our water particle is trapped has moved up to the top of the cloud. Here it is so cold that the droplets freeze. When this happens the water particles stop sliding over themselves in the droplet and become locked together to make an ice crystal. This crystal joins with other crystals to make a small snowflake.

While the particle has formed part of a snowflake, the cloud has moved closer to the range of hills. The wind pushes the cloud higher and it becomes even colder at the top of the cloud. This makes the small snowflakes turn into larger ones. The large snowflakes are too heavy to be held up by air currents and they fall through the cloud. As they fall they reach warmer air and melt. The particle then forms part of a large raindrop. It leaves the base of the cloud and hurtles towards the ground. When the raindrop smacks onto a rock it breaks up then joins with others to form a stream. The particle slides around other particles in the stream water as the water flows along the streambed and into a river. In a few weeks the particle is at the mouth of the river. It mixes with water particles in the sea and is carried offshore. When it reaches the sea's surface and receives heat from the Sun it can begin its journey, called the water cycle, once more.

Some water goes into the soil when it hits the ground. What happens to that?
It seeps through the soil until it reaches rock. If the rock is porous the water can pass through the rock too. Eventually the water will meet a rock which is not porous that stops it moving downwards. The water then moves along the top of the non-porous rock underground. At some place the non-porous rock will form part of a hillside. When the water reaches this place it will gush out of the hillside. This place where underground water reaches the surface of the ground is called a spring.

What happens to the water in a spring?
It flows away into a stream and river and eventually reaches the sea again.

What happens to water that falls in a pond?
A pond is not attached to a stream or river so its water cannot flow back to the sea. The water can still escape from it. It does so by evaporation. The water vapour produced when water evaporates from the surface of a pond rises into the air. It can rise so high that it cools, condenses and forms a cloud. The water in puddles escapes from puddles in the same way.

What happens to clouds that go over very cold lands?
The snowflakes that form in the tops of clouds do not melt when they fall. They reach the ground and may lie there for several months. If the snow forms on very high mountains or at the North and South Poles it may turn to ice and become part of a glacier. It may stay in the glacier for thousands of years as the glacier moves downwards. When glaciers move down mountainsides they become warmer and eventually melt. The water flows away into streams and rivers and eventually reaches the sea. The glaciers at the Poles move towards the sea. When they reach it they break up and form icebergs. These float away and eventually reach warmer climates where they melt. The water from the melted iceberg mixes freely with the seawater.

We contain a large amount of water. Is this part of the water cycle too?
Yes, it is. About 70% of your body is made of water but it does not stay with you all your life. You lose water in sweat and when you go to the toilet. The water lost in sweat forms water vapour and can move high in the sky and become part of a cloud. The water lost at the toilet eventually finds its way back to the sea. The water you take in as drinks has been collected from lakes and rivers so you are just one stop on the way round the water cycle.

Is it possible that the same water particle visits your body twice?
As there are so many water particles and they can move all round the world it is probably unlikely but it is not impossible. There is nothing to stop you taking in the same water particle twice or even more times in your life.

What happens to the water that plants take in?
They lose most of it through their leaves. This occurs
owing to a special type of evaporation called transpiration. By losing water through their leaves, the plants can take more water with their roots and make a current of water run up their shoots. This carries water and minerals from the soil to all parts of the plant. When the water has left the plant owing to transpiration it is in the form of water vapour and can rise in the air and form a cloud.