# Water Cycle Simulation

> A free water cycle simulation for class: change the Sun, wind and season, and watch drops evaporate, form clouds, fall as rain or snow, run off and soak in.

Source: https://olyclass.com/simulations/water-cycle/ · Updated 2026-11-03

This simulation is the water cycle as a living picture for the whole class on one screen. A coast is cut open from the sky down to the rock: the sea, a beach town, a lake, a forest and a snowy mountain. Two hundred tiny water drops travel through it. The Sun lifts them out of the sea as invisible vapour, rising air cools until the vapour turns into clouds, the wind blows the clouds over the land, and they rain (or snow, on the cold mountain). The water then runs off into the river, soaks into the ground, waits in the snow or seeps back to the sea underground, and the trip starts again. A panel counts where the water is right now: the numbers move, the total never does.

## How to use it

- **Watch it run:** press play (or Space). Vapour puffs rise from the sea and the lake, clouds grow and turn grey as they fill, raindrops fall, drops trickle down the river and through the soil.
- **Sun heat:** a hotter sun warms the sea, so more water evaporates and more clouds form. The temperatures at sea level and on the mountain top are shown at the top.
- **Wind:** with no wind, clouds stay over the sea and rain straight back into it, and the land slowly dries out. Turn the wind up to blow the clouds inland and onto the mountain.
- **Season:** in winter the air is colder, the trees lose their leaves and the mountain is below freezing, so the rain turns to snow there and piles up. Move back to summer and watch the snow melt into the river.
- **Labels and stages:** Labels names every stage on the picture with an arrow. Tap a label, or any part of the picture (the sea, a cloud, the river, the soil), to read what happens there and how many drops are doing it right now. The arrow keys and the number keys 1 to 9 go through the stages too.
- **Follow a drop:** one drop gets a yellow ring and a trail, and the panel tells its story at each step: "Up at 1.3 km the air has cooled to its dew point…". Press Follow another drop to start a new journey.
- **Where is the water now?** The bar chart counts the drops in the sea, the air, the clouds, snow and ice, rivers and the lake, and the ground. The total is always 200: water is never made or lost, it only moves.
- **On a projector or touch board:** use the full screen button so the labels, panel and sliders grow for the back row.

## The stages of the water cycle

| Stage | What happens | Where to look |
|---|---|---|
| Evaporation | Liquid water at the surface turns into water vapour | Puffs rising from the sea and the lake |
| Transpiration | Plants pull water from the soil and give it off through their leaves | Puffs rising from the forest |
| Condensation | Rising air cools, and vapour turns into tiny droplets: clouds | Clouds forming and growing |
| Precipitation | Droplets join into drops heavy enough to fall as rain or snow | Under the big grey clouds |
| Snow and ice | Snow on frozen ground stays, then melts when it warms | The mountain top in winter |
| Runoff | Water flows over the ground into rivers, lakes and the sea | The river, the town |
| Infiltration | Water soaks into the soil | Drops going into the ground |
| Groundwater | Water moves slowly underground and seeps out again | The blue zone under the land |
| Collection | Water gathers in the sea (and lakes) | Drops arriving in the sea |

## The science behind the model

**Evaporation grows with temperature.** How much water the air can take depends on the saturation vapour pressure, which rises steeply with temperature (about 6–7% more for each degree). The model uses the Tetens formula:

e_s = 0.611 × exp(17.27 T / (T + 237.3)) kPa

so water at 30 °C can evaporate almost twice as fast as at 20 °C. Wind helps too, by carrying the moist air away from the surface (Dalton's law).

**Air cools as it rises.** On average the air is 6.5 °C colder for every kilometre up. With 22 °C at sea level, the 2.5 km mountain top is about 22 − 6.5 × 2.5 ≈ 6 °C. Rising vapour cools even faster (about 9.8 °C per km), while its dew point drops only about 1.8 °C per km, so the two meet after the air has risen about 125 m for every degree between them. In the model that is 1.25 km: clouds never form lower down.

**Rain or snow.** A cloud that is below 0 °C makes snowflakes. Falling into warmer air, they melt into rain; landing where the ground is frozen, they stay as snow. That is why, in winter, the town gets rain while the mountain turns white, and why mountains facing the wind get the most rain and snow.

**On the ground.** Rain on soil and forest mostly soaks in; rain on roofs, roads and steep rock mostly runs off. The river collects the runoff and the meltwater, fills the lake and flows to the sea. Groundwater creeps along much more slowly and seeps out into the lake and the sea.

## The real numbers

The model has 200 drops so the class can see every store. The real Earth is far more lopsided:

| Where the water is | Share of all water on Earth |
|---|---|
| Oceans | 96.5% |
| Ice caps, glaciers and permanent snow | 1.74% |
| Groundwater | 1.69% |
| Lakes | 0.013% |
| Soil moisture | 0.001% |
| Atmosphere | 0.001% |
| Rivers | 0.0002% |

A drop also waits very different times in each place: about 9 days in the air, weeks to months in rivers, years to centuries in lakes, up to thousands of years deep underground, and thousands of years in the ocean and the great ice sheets. The simulation runs these trips in seconds, and in Follow mode the chosen drop waits less, so the class sees a whole loop.

## Try this

1. Before pressing play, ask: "Where does rain come from?" Then switch on Follow a drop and listen to the journey.
2. Set the wind to calm. Where does the rain fall now? What happens to the river and the groundwater after a while?
3. Turn the sun up from weak to hot and read the evaporation rate (tap Evaporation) at each setting.
4. Set winter and wait. Which bar grows? Where did that water come from? (During the last ice age so much water was locked in ice that the sea was about 120 m lower than today.)
5. Back to summer: follow the meltwater from the mountain to the sea.

## Teaching ideas

The simulation fits lessons on states of water and the water cycle (Grades 3–7; NGSS 2-ESS2-3, 5-ESS2-1 on how the spheres interact and MS-ESS2-4 on the cycling of water driven by the Sun and gravity; KS2 science, Year 4 states of matter).

1. **Predict (5 minutes):** "If the Sun got hotter, would there be more or less rain?" Vote, then try it.
2. **Name the stages (10 minutes):** hide the labels. Students point at the picture and name each stage; tap to check.
3. **Follow a drop (10 minutes):** each group follows a different drop and draws its journey as a chain of arrows. Compare: did any two drops take the same path?
4. **Count (5 minutes):** read the bar chart in summer and in winter. Which stores change, and why does the total stay the same?
5. **Review (10 minutes):** make a review game with the button above and play it as a class, in teams, with your phone as the remote.

## Questions teachers ask

### Is it really the same water going round and round?

Yes. The Earth has had about the same amount of water for billions of years; the water cycle only moves it between the sea, the air, the land and the ice. The counter panel shows this: drops move between the bars, but the total stays at 200.

### Why does the simulation not show the sea holding 96.5% of the water?

With 96.5% of 200 drops in the sea, only 7 would be left for the clouds, rivers, ice and ground together, and the class could not see the cycle. The model keeps every store visible, and the page gives the real shares in a table.

### Why do clouds only form high up?

Air cools as it rises. Vapour condenses only when the air has cooled to its dew point, which happens at a certain height (here 1.25 km above where the water evaporated). Below that, the vapour stays an invisible gas.

### Why does it snow on the mountain but rain in the town?

The air is about 6.5 °C colder for every kilometre up. In winter the mountain top is below 0 °C while the coast is not, so snow falls and stays up high and melts into rain lower down. The panel shows the height above which it snows.

### Do I need to install anything or sign up?

No. It runs in any modern browser on a computer, an interactive whiteboard, a tablet or a phone, and it's free.

[Make a review game](https://olyclass.com/create?subject=science&grade=4&topic=The%20water%20cycle:%20evaporation,%20transpiration,%20condensation,%20precipitation,%20runoff,%20infiltration%20and%20groundwater)

## Credits

Model and drawings: Oly. Where the Earth's water is: [USGS Water Science School](https://www.usgs.gov/special-topics/water-science-school/science/where-earths-water), after Gleick (1996). Air cools 6.5 °C per km: the U.S. Standard Atmosphere (1976). Saturation vapour pressure: the Tetens formula (Tetens 1930, Murray 1967). Rising air condenses about 125 m higher for every degree between its temperature and its dew point (the lifting condensation level). Snowmelt uses the degree-day method. Time runs far faster than in nature and the amounts are not to scale.