How the water cycle works

The main processes

The water cycle has no start or finish, but it is easiest to follow from the ocean. Each process below moves water from one place, or one state, to another:

• Evaporation: liquid water at the surface of oceans, lakes and soil turns into water vapour, an invisible gas.
• Transpiration: plants draw water up from their roots and release vapour through pores in their leaves. Together with evaporation it is often called evapotranspiration.
• Sublimation: ice and snow turn straight into vapour without melting first, common on dry, sunny, windy mountains.
• Condensation: rising air cools, and vapour condenses into tiny liquid droplets or ice crystals around specks of dust or salt, forming clouds and fog.
• Precipitation: when droplets or crystals grow heavy enough, they fall as rain, snow, sleet or hail.
• Runoff: water flows over the ground into streams, rivers, lakes and the sea.
• Infiltration: water soaks into the soil. Some is held there for plants; some percolates deeper to become groundwater, stored in layers of rock and sediment called aquifers.
• Storage: water spends time in oceans, ice sheets, glaciers, lakes, soil, aquifers and living things before moving on.

What powers the water cycle

Two sources of energy drive the cycle. The sun supplies the energy that evaporates water and melts ice; it takes a lot of energy to pull water molecules apart into a gas. Gravity pulls precipitation down from clouds and makes water run downhill, through rivers and through the ground.

Energy also moves with the water. When vapour condenses into a cloud, it releases the energy that evaporated it, warming the surrounding air. This is why the water cycle carries heat around the planet and helps drive weather, including thunderstorms and hurricanes, which are fuelled by heat released as warm, moist air condenses.

Why rising air makes clouds

Warm air can hold more water vapour than cold air. When air rises, the pressure around it drops and it expands and cools. Once it cools to its dew point, the temperature at which it is saturated, vapour starts condensing faster than it evaporates, and a cloud forms.

Air is pushed upward in three main ways: it is heated by the ground and rises, it is forced up the side of a mountain, or it is lifted along a weather front where warm and cold air meet. The same idea explains dew on grass on a cool morning and droplets on a cold can of drink: air touching a cold surface cools to its dew point.

Where Earth's water is

Nearly all of Earth's water, about 97 percent, is salty ocean water. Only about 2.5 to 3 percent is fresh, and most of that is locked in ice sheets and glaciers or stored underground as groundwater. Lakes, rivers, soil moisture and the atmosphere together hold a tiny fraction of a percent.

Water stays in each store for very different lengths of time. A molecule might spend around nine days in the atmosphere, weeks to months in a river, and thousands of years in the deep ocean, a large ice sheet or a deep aquifer. That is why pumping old groundwater faster than rain refills it can drain an aquifer for generations.

Worked example: a water budget

Hydrologists track water like money in a bank account: what comes in must equal what goes out plus any change in storage. Here is a simplified example for a made-up field over one year, with every amount measured as a depth of water spread over the field.

How people change the water cycle

Human activity changes how water moves. Roads, roofs and car parks stop water infiltrating, so more of it runs off quickly, raising flood risk and reducing groundwater recharge. Cutting down forests reduces transpiration, which can mean less rain downwind. Pumping groundwater for farms and cities can lower water tables faster than rain can refill them.

A warmer climate speeds up parts of the cycle: warmer air holds more vapour, so evaporation increases and heavy downpours can become heavier, while some dry regions dry out further. Snowpack that melts earlier in spring changes when rivers run high, which matters in places that depend on mountain snow for summer water.

Common misconceptions

Watch for these on tests and in everyday explanations:

• Clouds are made of water vapour. Vapour is invisible; clouds are tiny liquid droplets or ice crystals that have already condensed.
• Water has to boil to evaporate. Evaporation happens at any temperature, which is how a puddle dries on a cool day; heat just makes it faster.
• The water cycle is one neat circle. Real water takes many paths: a drop might evaporate, fall on land, soak in, sit in an aquifer for centuries, and reach the sea through a spring.
• Evaporation leaves salt behind in the air. Only water evaporates from the sea, which is why rain is fresh water.
• Condensation needs cold weather. It needs air cooled to its dew point, which can happen on a hot day high in the sky or on a cold glass.

How to practise and check your understanding

Draw the cycle from memory with a mountain, an ocean, a river, a tree and a cloud. Label every arrow with a process, and for each arrow write whether the sun or gravity drives it. Check that you included transpiration, infiltration and groundwater, which are the ones most often left out.

Then explain one real event using the cycle: a puddle disappearing, fog forming over a lake at dawn, or a river flooding after snowmelt. If you can say where the water came from, what changed its state, and what energy was involved, you understand the cycle rather than just the diagram.

For a simple at-home model, cover a bowl of warm water with cling film, place ice on top, and watch vapour condense under the cold film and drip back down.