E. Earth and Space Systems · Air and Water in the Environment

The Water Cycle

E2.3 E2.4 A2.1 A1.5About 40 minutes

Learning goals

I can name the four stages of the water cycle: evaporation, condensation, precipitation and collection.
I can explain how warming and cooling move water around the water cycle.
I can tell the story of a water drop's journey.
I can put coding steps in order, and use a loop, to show the water cycle.

Words to know

water cycle
The never-ending trip water takes from Earth's surface up into the sky and back down again.

evaporation
When liquid water warms up and turns into water vapour that rises into the air.

condensation
When water vapour cools down and turns into tiny drops of water. Clouds form this way.

precipitation
Water falling from clouds: rain, snow, sleet or hail.

collection
When water gathers in oceans, lakes, rivers, the ground or snow and ice, after it falls.

cycle
Something that happens again and again in the same order.

loop
In coding, an instruction that repeats steps.

A very old drop of water

The water in your glass today is not new. It might have fallen as snow in Nunavut, floated in Lake Superior, or been part of a cloud over the ocean. A long, long time ago, it could even have been a drink for a dinosaur!

That's because Earth keeps using the same water over and over. Water travels on a never-ending trip called the water cycle.

Remember

A cycle happens again and again in the same order, like the seasons. In the last lesson you learned that warming makes water evaporate and cooling makes it condense. The water cycle uses both!

The four stages

1. Evaporation. The Sun warms the water in oceans, lakes, rivers and puddles. Some of it turns into invisible water vapour and rises up into the air. Plants also give off water vapour from their leaves.

2. Condensation. High in the sky, the air is colder. The water vapour cools down and condenses into tiny drops of water (or tiny bits of ice when it is very cold). Billions of these tiny drops together make a cloud.

3. Precipitation. In the cloud, tiny drops bump together and join into bigger, heavier drops. When they get too heavy to float, they fall to the ground as precipitation: rain, snow, sleet or hail.

4. Collection. The water that falls collects in oceans, lakes, rivers and ponds. Some soaks into the ground. Some flows downhill into streams. In winter, it can stay on the ground as snow until it melts in spring.

Then the Sun warms the water again, and the cycle repeats!

Dark grey clouds over an Ontario lake with streaks of rain falling onto the water. — This is precipitation. Water that evaporated and condensed into clouds is falling back into the lake as rain. AI-generated image

The water cycle: 1 water evaporates from the lake, 2 it condenses into a cloud, 3 it falls as precipitation, 4 it collects and flows back to the lake. Then it starts again.

Warming and cooling make it go

Warming (from the Sun) makes water evaporate and go up.
Cooling (high in the sky) makes water vapour condense into clouds.
Temperature decides what kind of precipitation falls. If the air is above 0°C, we usually get rain. If it is below 0°C, the water freezes and we get snow.
In spring, warming melts the snow, and the water runs into rivers and lakes.

The same cloud can drop rain or snow. When the air is warmer than 0°C we usually get rain; when it is colder than 0°C, we get snow.

Worked example

Let's follow a water drop

Tell the story of a water drop that starts in Lake Ontario on a warm summer day.

Evaporation: The Sun warms the lake. Our drop turns into water vapour and floats up into the sky.
Condensation: High up, the air is cold. The vapour cools and condenses into a tiny drop in a cloud.
Precipitation: The tiny drop joins other drops. It gets heavy and falls as rain onto a farm field.
Collection: The rain soaks into the soil and flows into a stream. The stream runs into a river, and the river flows back into Lake Ontario.
Now the cycle can start again!

The drop goes up (evaporation), makes a cloud (condensation), falls (precipitation) and gathers in a stream, river and lake (collection).

Code the water cycle

In coding, we give a computer (or a robot, or a person) steps to follow in order. A loop tells it to repeat steps.

The water cycle repeats forever, so it is a perfect job for a loop!

start: water drop is in the lake

repeat forever:

evaporate (rise up as water vapour)

condense (become a tiny drop in a cloud)

precipitate (fall as rain or snow)

collect (land in a lake, river or the ground)

A water cycle program. The steps inside the loop happen again and again in this order.

Worked example

Let's debug the code

Sam's water cycle code has a bug (a mistake). Find it and fix it.

repeat forever: precipitate → evaporate → condense → collect

Start with the drop in the lake. What has to happen first? It has to go up: evaporate.
Sam has precipitate first. A drop in a lake can't fall from a cloud before it has gone up!
Next comes condense (make a cloud), then precipitate (fall), then collect.
Test it: lake → up as vapour → cloud → falls → back in the lake. It works!

Fixed code: evaporate → condense → precipitate → collect, inside the loop.

Just for fun

What did one raindrop say to the other raindrop?

"Two's company, but three's a cloud!"

Hands-on activity

Water cycle in a bag

You need: a zip-top sandwich bag, about 4 big spoonfuls of water, tape, a marker and a sunny window. (Optional: one drop of blue food colouring.)

With the marker, draw a sun, a cloud, some raindrops and a lake with waves on the bag.
Pour the water into the bag and seal it tightly.
Tape the bag to a sunny window, with the water at the bottom.
Check it after 1 hour, after 3 hours, and the next morning.
Look for: drops forming near the top of the bag (condensation), drops running down the sides (precipitation) and water at the bottom (collection). The Sun's warmth causes the evaporation you can't see.

Talk about it: Where is the "sky" in your bag? Why do the drops form near the top?

Coding bonus: Act out the water cycle program! A grown-up reads the loop and you move: crouch low (lake), rise up on tiptoes (evaporate), hug yourself into a ball (condense), wiggle your fingers down (precipitate), and crouch again (collect). Repeat 3 times.

Your turn

Name the four stages of the water cycle in order, starting with evaporation.

What makes the water in a lake evaporate?

How do clouds form?

Name two kinds of precipitation.

After rain falls, where can the water collect? Name two places.

Why do we call it a cycle?

In a code, what does a loop do?

More practice: warm-up

What stage is it when rain falls from a cloud?

What stage is it when water vapour turns into a cloud?

What stage is it when water gathers in a lake?

What stage is it when water turns into vapour and rises?

Is the water on Earth new every day, or is it used again and again?

More practice: on your own

Why does it snow instead of rain in January?

In the water cycle bag, drops form near the top. Which stage is that? Why does it happen there?

Snow sits on the ground all winter. What happens to it in spring, and where does the water go?

Which stages need warming? Which need cooling?

Put the steps of this code in order: collect, evaporate, precipitate, condense.

Challenge

Kai's code says: repeat 3 times: evaporate, condense, precipitate, collect. How many times does the drop fall as precipitation?

Clouds look grey and dark just before a big rainstorm. Use the water cycle to explain why rain falls.

Tell the water cycle story of a snowflake that falls on a hill in February.

Check your understanding

Quiz

Which stage comes right after evaporation?

What is precipitation?

What gives the water cycle the energy to start evaporation?

Clouds are made of...

Water gathering in oceans, lakes and rivers is called...

Which code shows the water cycle in the right order?

For parents and teachers

Big idea (E2.3, E2.4): Water moves in a continuous cycle: evaporation, condensation, precipitation and collection. Warming drives evaporation and melting; cooling drives condensation and freezing, which decides whether precipitation is rain or snow.

How to help: Do the bag activity and check it together. When it rains or snows, ask "Where was this water before? Where will it go next?" Grade 2 students are expected to use the four stage names correctly (A1.5).

Coding (A2.1): The loop activity teaches sequencing and repetition, a key Grade 2 coding idea. Children don't need a device: acting out or arranging cards works.

Common difficulties:

Mixing up evaporation and condensation. Link them: evaporate = goes away into the air (warming); condense = comes back as drops (cooling).
Thinking clouds are water vapour. Clouds are visible, so they are tiny drops (or ice). Vapour is invisible.
Thinking the cycle starts and ends. Stress that there is no real starting point; any stage can be first.