Watch how the Sun lifts ocean water into the sky, the wind blows it inland, mountains squeeze rain out of the clouds, rivers carry it back to the sea — and feel what happens when you turn the thermostat up.
The water cycle is a giant solar-powered conveyor belt. The Sun heats the ocean → water evaporates into the sky → wind carries the vapour inland → it condenses into clouds → falls back as rain or snow → rivers, glaciers, and groundwater move it back to the sea. Globally about half a million cubic kilometres of water make this round trip every year. The crucial fact: warm air holds about 7 % more water vapour per °C (Clausius–Clapeyron). So when you crank the temperature slider, the whole conveyor belt speeds up — more evaporation, more intense rain, longer droughts, and a faster cycle overall. That's the whole story.
| Slider | Real-world meaning |
|---|---|
| Global ΔT | Warmer atmosphere = more water vapour per cubic metre + faster evaporation. Cooler = ice ages, less rain everywhere. |
| Solar intensity | How much sunlight reaches the surface. Low values mimic large volcanic eruptions (Mt. Pinatubo dropped global temps for years). |
| Forest cover | Trees pump water from soil to sky via transpiration. Cutting forests kills that pump — the Amazon "flying river" is a real, measurable effect. |
| Soil moisture kS | How readily the land surface gives up moisture. Wetter soils evaporate faster; bare desert hardly evaporates at all. |
Start simple. Leave a wet towel on a sunny windowsill and come back later — it's dry. The water didn't disappear; the Sun quietly lifted it into the air. Now do that to an entire ocean, let the wind carry the moisture inland, let it fall as rain, and let rivers run it back to the sea. That round trip, powered entirely by sunlight, is the water cycle. Nothing is created or destroyed — the same water just keeps moving.
Now name the pieces. Water leaving a surface as vapour is evaporation $E$ (from plants we call it transpiration); water returning as rain or snow is precipitation $P$. Over a whole year the planet is in balance, so $E \approx P$. The surprising part is how little water the sky holds at once: only about $13{,}000\ \text{km}^3$. Yet roughly $486{,}000\ \text{km}^3$ rains down each year. Dividing the flow by the stock, $486{,}000 / 13{,}000 \approx 37$, tells you the air's entire water content is swapped out about 37 times a year — once every 9 to 10 days. A tiny bucket can move an ocean if you empty and refill it fast enough.
Go deeper. The atmospheric store changes at the rate vapour comes in minus the rate it falls out:
What sets the pace is temperature. The Clausius–Clapeyron relation says warmer air can hold about 7 % more water vapour for every $1\,^\circ\text{C}$, a factor of $1.07^{\Delta T}$. That single factor is the engine behind the sliders: Global ΔT multiplies every flux by $1.07^{\Delta T}$, Solar intensity scales how hard the Sun drives ocean evaporation, Forest cover turns the transpiration pump up or down, and Soil moisture $k_S$ sets how willingly the land gives water back. Warm the world and the whole conveyor belt speeds up: fiercer downpours, but longer dry spells in between.
1. Drag Global ΔT to +4 °C and press Run — watch the CC factor climb and precipitation rise. 2. Pull Forest cover down to 30 % and see land evaporation and runoff collapse while the ocean keeps evaporating. 3. Set Solar intensity to ~80 % (a "volcanic winter") and watch the whole cycle slow as the air cools and dries.
The Earth holds roughly 1.386 × 10⁹ km³ of water. Almost all of it (97 %) sits in the oceans; another 2 % is locked up in ice. The atmosphere holds only ~13 000 km³ at any moment — less than 0.001 % of the total — yet that tiny pool turns over in just ~9 days, because precipitation and evaporation flows are huge.
| Reservoir | Volume (km³) | Mean residence time |
|---|---|---|
| Oceans | 1 338 000 000 | ≈ 3 200 yr |
| Ice caps & glaciers | 24 000 000 | 1 000–100 000 yr |
| Groundwater | 23 000 000 | days–10 000 yr |
| Lakes & rivers | 178 000 | weeks–years |
| Atmosphere | ~ 13 000 | ≈ 9 days |
| Biosphere | ~ 1 100 | ~ 1 week |
Saturation vapour pressure of water roughly doubles for every 10 °C of warming. To good approximation:
So when ΔT = +2 °C, atmospheric water capacity rises by ~ 14 %. Heavy-rain events scale even faster (~ 2× C-C ≈ 14 %/°C) because storms concentrate the available moisture.
To keep the simulation responsive, we use first-order kinetics for each flux scaled by current temperature and surface state:
Set Forest cover to 30 % and run for 30 years. Land evaporation drops dramatically (less transpiration), the Amazon and Congo "flying-river" moisture flux fails, and the downwind rainfall (parts of southern Brazil, central Africa, southwest U.S.) collapses by 20–40 %. Total global precip falls ~ 3 % because the ocean still evaporates the same amount — but rainfall is now concentrated near coasts. Try it: set forest cover to 30, press Run, and watch land precipitation in the readouts.