What is the water cycle?

In one line

The rule

The water cycle is the continuous process by which water circulates through the Earth's atmosphere, land, and bodies of water. It involves various stages including evaporation, condensation, precipitation, and collection.

Formula (for higher classes): P = E + R + ΔS Where:

• P = Precipitation (rain, snow, etc.)
• E = Evaporation (water turning to vapor)
• R = Runoff (water flowing over land)
• ΔS = Change in storage (water stored in lakes, soil, etc.)

Let's understand it simply

Imagine you are playing cricket on a hot day. You sweat, and the sweat dries up. Where did it go? It became vapor and mixed with the air. The same thing happens with lakes, rivers, and even puddles after rain. The sun heats water, it turns into vapor (gas), rises, cools, and forms clouds. Later, these clouds give us rain, filling lakes and wells again. It’s like water is doing a never-ending round trip—like a bus going from your home to school and back, again and again.

How it works

1. Evaporation: The sun heats water in rivers, lakes, and oceans. Water turns into vapor (gas) and rises into the air.
2. Transpiration: Plants also release water vapor from their leaves into the air. This is like plants 'sweating.'
3. Condensation: As water vapor rises, it cools down and turns back into tiny water droplets, forming clouds. This is like steam from a pressure cooker turning into water drops on the lid.
4. Precipitation: When clouds get heavy, water falls back to the ground as rain, hail, or snow.
5. Collection: Rainwater collects in lakes, rivers, and underground (in wells or soil). Some water flows back to the sea. The cycle starts again.
6. Infiltration: Some rainwater soaks into the ground, filling underground water storage (aquifers).

The story behind it

The idea of the water cycle has been known in some form for thousands of years. Ancient Indian, Greek, and Arab scholars wrote about rain and rivers. In the 1600s, scientists like Pierre Perrault and Edme Mariotte in France measured rainfall and river flow, proving that rainwater feeds rivers. Before that, people thought rivers came from underground oceans! The word 'hydrological cycle' comes from 'hydro' (water) and 'logos' (study).

Where you see it in real life

• Drying clothes outside: Wet clothes dry because water evaporates into the air.
• Foggy windows in winter: Warm air inside meets cold glass, water vapor condenses into droplets.
• Rainwater harvesting tanks: Rain (precipitation) is collected and stored for later use.
• Monsoon rains: Huge clouds form over the sea, then bring heavy rain to fields and cities.
• Village wells: Rainwater seeps into the ground (infiltration), filling wells.
• Morning dew on grass: Water vapor cools at night and condenses on plants.

Why it matters

• Drinking water: The water cycle refills rivers, lakes, and wells, giving us water to drink.
• Farming: Rain from the cycle waters crops—no water cycle, no food.
• Weather and climate: The cycle controls rainfall, droughts, and floods.
• Clean water: Evaporation leaves dirt and salt behind, so rainwater is fresh.

Worked example

Let's see how much rainwater collects in a pond after a shower:

Given: Rainfall (P) = 10 cm, Pond area = 100 m²
Step 1: Find volume of rainwater = Rainfall × Area
Step 2: Convert 10 cm to meters = 0.1 m
Step 3: Volume = 0.1 m × 100 m² = 10 m³
Step 4: 1 m³ = 1,000 litres, so 10 m³ = 10,000 litres
Step 5: Answer: The pond collects 10,000 litres of rainwater.

Common mistakes

• Thinking the cycle has a start or end: It is always going on, everywhere.
• Forgetting plants: Plants also add water vapor (transpiration), not just lakes and rivers.
• Mixing up condensation and evaporation: Evaporation is liquid to gas; condensation is gas to liquid.
• Ignoring infiltration: Some water goes underground, not just into rivers and lakes.

Remember it

Water goes up as vapor, forms clouds, comes down as rain, and starts again—like a never-ending merry-go-round.