What it does
The water on Earth goes round and round. The Sun warms the sea and water evaporates into the air as invisible vapour. Wind carries the moist air inland, and where it is pushed up over hills it cools. Cool air cannot hold as much vapour, so the extra condenses into tiny droplets: a cloud. When the droplets grow heavy they fall as rain. On the ground some rain runs off into streams and rivers and back to the sea; the rest soaks in, refilling the water in the ground, and plants pull some of it up and breathe it out again (transpiration).
This page puts all of that on one small coast. You set the sun, the air temperature, the wind and the kind of ground, press Play or Run 1 day, and watch the arrows, cloud, rain and river change. A water budget adds up every flow in millimetres of water over the land, and its last line checks that water is never made or lost, only moved.
It follows Class 6 to 8 Science (the water cycle, evaporation and condensation, why the Earth is a life-sustaining planet, conserving water) and Class 9 work on evaporation and humidity, with a Class 9 to 12 level that shows the numbers behind “warm air holds more water”. To see what evaporation and condensation mean for the particles, try states of matter; for the plants’ part, the transpiration lab; and for gases dissolving in water, the dissolving lab.
How to use it
- Set the four settings. Sun (0 to 100%) sets how hard the sea is heated. T is the air temperature at the coast (0 to 40 °C). Wind (0 to 100%) is how strongly air moves in from the sea. Ground is forest, fields or city: it decides what share of the rain runs straight off (15%, 35% or 80%).
- Press Run 1 day to move on 24 hours, or Play to watch it go (12 model hours a second) and Pause to stop. Start again resets the water, starting with the air half full of vapour for the temperature you set.
- Watch the picture: the evaporation arrows grow with the flow, the cloud grows with the water in it, and the river thickens after rain.
- Read the water budget bars and lines. Switch the Level to Class 9 to 12 for the vapour pressure and grams of water a cubic metre of air can hold.
Worked example
Leave the settings as the page opens (sun 70%, 25 °C, wind 50%, forest) and press Run 1 day. About 5.8 mm of water has evaporated from the sea and 0.2 mm has come up from the plants. At 25 °C the air over the land can hold up to 30 mm of vapour, but over the hills, 9 °C colder, only 17.2 mm, so the vapour beyond that has started to form cloud, and the first 0.4 mm of rain has fallen. Press it twice more: after 3 days 11.9 mm of rain has fallen, of which 1.8 mm ran off and 10.1 mm soaked into the forest floor. Do the same with City and about 9.1 of the 11.4 mm runs off, leaving only 2.3 mm to refill the ground: that is why cities flood and their wells run dry.
Check yourself
Five fresh questions: runoff from a rain shower, what soaked in, grams of water that condense when air cools, litres a pond loses to evaporation, and cubic metres of rain on a town. Nothing is saved.
Tips
- Millimetres of rain are litres per square metre: 1 mm on 1 m² is 1 litre.
- No wind means the moist air stays over the sea, so the land gets far less rain. Mountains facing a sea wind (like the Western Ghats in the monsoon) are among the wettest places on Earth.
- Forests act like a sponge: most rain soaks in and is released slowly, so rivers keep flowing in the dry season.
FAQ
Is this a weather forecast? No. It is a teaching model with a few round numbers chosen so the parts of the cycle are easy to see. Real weather has many more processes.
Why does the river carry water even before it rains? Water already in the ground seeps slowly into the river. Rain then adds runoff on top.
Is this from the NCERT book? It covers the same topics with fibub’s own pictures and model. fibub is not part of NCERT.
Where the numbers come from
- Evaporation, condensation in rising and cooling air, precipitation, runoff, infiltration and transpiration in the water cycle are standard school science. Wording, pictures and examples are fibub's own.
- Saturation vapour pressure from the Tetens formula, e = 0.6108 exp(17.27 T / (T + 237.3)) kPa, and the gas constant of water vapour, 461.5 J/(kg K), which gives the most vapour a cubic metre of air can hold (about 23 g at 25 °C). Air cools about 6.5 °C per km as it rises (standard atmosphere).
- The rates of evaporation, cloud, rain and seepage are made-up round numbers for a teaching model. Tests check that water is conserved and that the model behaves as the science says (warmer and windier gives more rain, cities shed more water).
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.