Geography · Physical geography
The water cycle
Evaporation, condensation, precipitation, runoff.
What is the water cycle?
Water constantly moves between Earth's surface and the atmosphere. This endless journey is called the water cycle. The same water has been recycled for billions of years! Four main processes keep water moving: evaporation, condensation, precipitation and runoff. Together they deliver fresh water to rivers, lakes and underground stores.
Evaporation and condensation
Evaporation happens when liquid water heats up and turns into invisible water vapour (a gas). The Sun warms oceans, rivers and lakes, causing water molecules to escape into the air. High in the cooler atmosphere, water vapour cools and condenses back into tiny liquid droplets. Billions of these droplets cluster together to form clouds.
Precipitation
Precipitation is any water that falls from clouds to the ground. Rain is the most common type. When clouds rise higher where temperatures drop below freezing, water droplets turn to ice crystals and fall as snow or hail. Cloud droplets keep colliding and merging until they grow heavy enough to overcome air resistance and fall.
Runoff and collection
When precipitation hits the ground, some soaks into soil (infiltration) and some flows over the surface as runoff. Runoff travels downhill through streams and rivers, eventually returning water to oceans and lakes. Gravity pulls water to the lowest point. Underground water slowly seeps through rock layers and may resurface at springs.
Worked examples
1. A puddle disappears on a sunny day. Explain which part of the water cycle is happening.
- The Sun heats the water in the puddle.
- Liquid water gains energy and changes into water vapour (a gas).
- This process is evaporation.
- The invisible vapour rises into the atmosphere.
2. Describe the journey of a water droplet from the ocean to a mountain stream.
- Evaporation: The Sun heats ocean water, turning it into water vapour that rises.
- Condensation: High in the cool atmosphere, vapour condenses into cloud droplets.
- Precipitation: Droplets merge and fall as rain or snow over mountains.
- Runoff: The water flows downhill through streams back towards the ocean.
3. Why do clouds form high in the sky rather than near the ground?
- Water vapour rises because warm air is less dense than cold air.
- As vapour climbs, the surrounding air gets cooler with altitude.
- Cold air cannot hold as much water vapour as warm air.
- Vapour condenses into liquid droplets, forming clouds where the air is cool enough.
Try it yourself
A glass of cold water left outside develops drops on its surface. Which water cycle process causes this?
Ready to test yourself?
Take a short topic test — every question maps to a syllabus point so you’ll see exactly which parts you’ve mastered.