The Land and Its Water
The water cycle
Name the four stages of the water cycle, put them in order, and explain where rain comes from and why the earth never runs out of water.
Water does not stay where you last saw it. The puddle outside dries up and is gone by the afternoon; a week later rain falls on the same spot. It is not new water. It is the same water, going round a journey it has been making for longer than there have been people to watch it, and that journey is called the water cycle.
The four stages
Evaporation — heat from the sun turns liquid water in seas, lakes and wet ground into water vapor, which rises into the air. Condensation — high up the air is colder, so the vapor cools and turns back into tiny droplets, and those droplets gathered together are a cloud. Precipitation — the droplets join, grow heavy and fall as rain, snow or hail. Collection — the fallen water gathers in seas and lakes, soaks into the ground, or runs down rivers back to the sea. Then it begins again.
The sun is what drives all of it. Nothing in the cycle can start until heat has lifted water into the air as vapor, and that is also why the cycle never stops. Plants help too: a tree draws water up through its roots and gives it off from its leaves as vapor, which is called transpiration. Water vapor itself is invisible — the white you can see is already a cloud, which is to say water that has cooled and turned back into droplets.
Two things worth noticing. The sea is salty and rain is not, because only the water rises as vapor and the salt is left behind. And no water is lost or made anywhere on the journey: the earth has the same water today that it had long ago, and the drop of rain on your hand has been round the cycle more times than anyone could count.
Worked example
A wet road dries in the morning sun. By evening it is raining again. Follow the water from the road to the rain.
The sun warms the water on the road, and it turns into vapor and rises. That is evaporation.
The water has not disappeared. It has changed from a liquid you can see into a gas you cannot.
Try it together
Follow one drop of water on a whole trip round the cycle, starting in the sea on a hot afternoon.
Answer each step with the name of one stage.
1.The sun warms the sea, and the drop leaves it as vapor and rises. Which stage is that?
Have a go
Have a go on your own. Water vapor high in the sky cools and turns back into the tiny droplets that make a cloud. Which stage of the water cycle is that?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.