The Water Cycle: KS3 Science Guide

The water cycle is one of Earth's great recycling systems — the same molecules of water that fell as rain on ancient forests are falling on your roof today. Driven by solar energy, water moves continuously between oceans, atmosphere, land, and living organisms, distributing freshwater across the planet and shaping every climate and ecosystem it passes through.

What is the water cycle?

The water cycle (also called the hydrological cycle) describes the continuous movement of water between Earth's surface, the atmosphere, and living organisms. No water is ever truly lost — the same molecules are recycled endlessly. The cycle is powered primarily by solar energy, which drives evaporation, and by gravity, which pulls precipitation back to Earth.

Understanding the water cycle connects three school subjects: the chemistry of state changes, the biology of transpiration, and the geography of weather and rivers.

What are the main stages of the water cycle?

Stage | Process | Energy change
Evaporation | Liquid water from oceans, lakes, and rivers converts to water vapour | Energy absorbed (endothermic)
Transpiration | Plants release water vapour through leaf pores (stomata) | Energy absorbed
Condensation | Water vapour cools and forms liquid droplets (clouds and mist) | Energy released (exothermic)
Precipitation | Water falls from clouds as rain, snow, sleet, or hail | Gravitational potential energy converted to kinetic energy
Surface run-off | Water flows across the land surface into rivers and back to the sea | Gravity-driven
Percolation | Water soaks into the ground and replenishes aquifers | Gravity-driven

What is evaporation and what drives it?

Evaporation is the conversion of liquid water into water vapour at the surface of a body of water. At the particle level, the fastest-moving water molecules at the surface gain enough kinetic energy to overcome intermolecular forces and escape into the air as gas.

Factors that increase evaporation rate:

Higher temperature — more particles have sufficient energy to escape

Greater surface area — more molecules are exposed at the surface

Wind — removes water vapour from above the surface, maintaining a steep concentration gradient

Low humidity — when the air already holds less water vapour, more can evaporate

The oceans supply approximately 86% of the water vapour that enters the atmosphere — making them the engine of the entire water cycle.

What is condensation and how do clouds form?

Condensation is the reverse of evaporation: water vapour cools and converts back into liquid water. As warm, moist air rises, it expands and cools. When it reaches the dew point temperature — the temperature at which the air becomes saturated — water vapour condenses onto tiny airborne particles called condensation nuclei (dust, pollen, sea-salt crystals).

Billions of tiny water droplets form around these nuclei, creating a cloud. Individual droplets are typically 10–100 micrometres across — small enough to remain suspended. When droplets collide and merge into larger drops (a process called coalescence), they become heavy enough to fall as precipitation.

Fog is simply a cloud that has formed at ground level, where warm humid air meets a cool surface.

What is precipitation and what forms can it take?

Precipitation is any water that falls from the atmosphere to Earth's surface. Its form depends on temperature:

Rain — liquid droplets; air temperature is above 0 °C throughout

Snow — water vapour freezes into ice crystals that aggregate into snowflakes; temperature below 0 °C throughout most of the atmosphere

Sleet — snow that partially melts before reaching the surface, or a rain-snow mix

Hail — ice pellets formed when raindrops are repeatedly lifted by strong updraughts in thunderstorms and refreeze in layers

The UK receives around 1,220 mm of precipitation per year on average, ranging from under 600 mm in the dry south-east to over 3,000 mm in parts of Wales and the Lake District.

What is the role of transpiration in the water cycle?

Transpiration is the loss of water vapour from plant leaves through tiny pores called stomata. Water is drawn up from roots through xylem vessels to leaves, where it evaporates from the spongy mesophyll cells and diffuses out through open stomata.

Transpiration is biologically significant at scale:

A single large oak tree can transpire over 450 litres of water on a warm summer day.

Globally, transpiration accounts for roughly 10% of all atmospheric moisture.

Tropical rainforests transpire so much water that they effectively generate their own rainfall — deforestation in the Amazon has demonstrably reduced regional precipitation.

The combined loss from evaporation and transpiration is termed evapotranspiration in environmental science.

How does the water cycle affect weather patterns?

The water cycle is inseparable from weather. Key links:

Latent heat release: When water vapour condenses into cloud droplets, it releases latent heat into the atmosphere. This warms the surrounding air, causing it to rise further and driving the powerful updraughts inside cumulonimbus (thunderstorm) clouds.

Ocean currents: Evaporation from tropical oceans concentrates salt, increasing seawater density and driving deep ocean circulation, which redistributes heat around the globe.

Urban heat islands: Cities replace permeable soil with tarmac and concrete — reducing infiltration, increasing run-off, and decreasing evapotranspiration. Less evaporative cooling makes cities warmer than surrounding countryside.

Climate feedback: As global temperatures rise, evaporation rates increase, putting more water vapour into the atmosphere. Since water vapour is itself a greenhouse gas, this creates a positive feedback loop that amplifies warming.

Frequently asked questions

What is the water cycle in simple terms?

The water cycle is the continuous journey water takes from Earth's surface into the atmosphere and back again. The Sun heats water in oceans and lakes, causing evaporation into water vapour. The vapour rises, cools, and condenses to form clouds. Water then falls as precipitation (rain or snow), flows back to the sea via rivers or soaks into the ground — and the cycle repeats. The same water molecules are recycled indefinitely; none is permanently created or destroyed.

What is the difference between evaporation and transpiration?

Evaporation is the direct conversion of liquid water from open water surfaces (oceans, lakes, puddles, soil) into water vapour, driven by solar energy. Transpiration is water loss specifically from plant leaves through stomata, and it is biologically controlled — plants can close their stomata in dry or dark conditions to reduce water loss. Both processes add water vapour to the atmosphere, but transpiration involves an active biological component that responds to environmental signals.

Why does it rain more in some places than others?

Rainfall distribution depends on several factors. Proximity to the ocean provides a large source of moisture. Altitude forces moist air upwards, cooling it and triggering condensation (orographic rainfall) — this is why the upland west of Britain is wetter than the low-lying east. Prevailing winds (in the UK, south-westerlies) bring moisture from the Atlantic. Warm ocean currents also increase evaporation and moisture availability downwind.

How does deforestation affect the water cycle?

Forests contribute to the water cycle through transpiration and interception (leaves catching rainfall, much of which re-evaporates). Removing trees reduces the amount of water vapour released into the atmosphere, potentially decreasing local and regional rainfall. Deforestation also compacts soil, reducing infiltration and dramatically increasing surface run-off — raising flood risk. In the Amazon, large-scale deforestation has been linked to measurable reductions in regional precipitation, threatening agriculture and biodiversity across South America.

Explore the water cycle and interconnected Earth systems with Professor Darwin at aitutors.me — where every question helps you see the whole picture.