The Water Cycle: An Overview

Introduction

The glass of water you drink today may once have fallen as snow on a mountain, flowed down a river, floated in a cloud over the ocean or been breathed out by a plant. Water is constantly on the move between the sea, the sky and the land. This never-ending journey is called the water cycle (or hydrological cycle). It is driven by energy from the Sun and gravity, and at its heart it is a series of physical changes of state that you already know: evaporating, condensing, freezing and melting.

Core explanation

The main stages. The water cycle has no true start or end, but it is easiest to follow from the oceans.

1. Evaporation. The Sun heats the surface of oceans, lakes and rivers. Some liquid water gains enough energy to become water vapour , an invisible gas that rises into the air. Salts and other dissolved substances are left behind, so evaporated water is fresh. 2. Transpiration. Plants take up water through their roots and release water vapour from tiny pores in their leaves. Together, evaporation and transpiration are sometimes called evapotranspiration . 3. Condensation. As warm, moist air rises it cools. Cool air can hold less water vapour, so the vapour condenses into tiny droplets of liquid water or ice crystals, forming clouds . 4. Precipitation. Cloud droplets join together until they are too heavy to stay in the air and fall as rain, snow, sleet or hail . 5. Collection and flow. On land, water either flows over the surface as run-off into streams and rivers, soaks into the soil ( infiltration ) to become groundwater, or is stored as snow and ice. Eventually, most of it returns to the sea, and the cycle continues.

A loop of physical changes. None of these stages changes water into a new substance. The molecules remain H₂O throughout; only their state and position change. That is why the water cycle is a cycle of physical changes . It can be summarised as:

liquid water → (evaporation) → water vapour → (condensation) → liquid droplets → (precipitation) → liquid or solid water on the ground

A closed system. Almost no water escapes from Earth into space, and very little is added. The total amount of water stays almost constant; it is simply moved from store to store. The water on Earth today is essentially the same water that has been cycling for billions of years.

Natural distillation. Evaporation followed by condensation is exactly how distillation purifies water in the laboratory. The water cycle is a giant natural still that turns salty seawater into fresh rain.

Step-by-step reasoning

To decide which stage of the water cycle a process belongs to:

1. Ask whether water is changing state or just moving. 2. Liquid to gas is evaporation (or transpiration from plants). 3. Gas to liquid is condensation. 4. Falling from clouds is precipitation; flowing over or through the ground is run-off or infiltration.

Visual explanation

Picture a diagram with the sea on the left, mountains on the right and the Sun above. Wavy arrows rise from the sea (evaporation) and from trees (transpiration), clouds form and drift over the mountains (condensation), rain falls (precipitation), and a river carries water back to the sea.

Real-world analogy

The water cycle is like a school bus route. The bus (water) visits different stops — ocean, sky, mountains, rivers — picking up and dropping off, but it never leaves the route, and the same bus keeps going round year after year.

Real-world example

Weather forecasts are largely forecasts of the water cycle. When forecasters describe moist air from the Atlantic being pushed over the hills of western Britain, they are describing water that evaporated from the ocean, cooling and condensing as it rises, then falling as rain.

Why?

Why is rain fresh even though most of it comes from salty oceans? During evaporation only water molecules escape into the air. The dissolved salts are not volatile, so they stay behind in the sea, just as they would in the flask during a distillation.

Common misconception

"Clouds are made of water vapour." Water vapour is invisible. Clouds are made of tiny liquid droplets or ice crystals that have already condensed from the vapour.

Worked example

Question: For each of the following, name the stage of the water cycle: (a) puddles drying up on a sunny day, (b) dew forming on grass at night, (c) snow falling on a mountain.

Reasoning: Match each to a change of state or a movement.

Answer: (a) evaporation, (b) condensation, (c) precipitation.

Quick check

1. Which process in the water cycle turns water vapour into cloud droplets? Answer: Condensation.

Exam focus

Learn to label a water cycle diagram with evaporation, transpiration, condensation, precipitation, run-off and infiltration. State clearly that these are physical changes and that the Sun supplies the energy for evaporation.

Advanced insight

About 500 000 km³ of water evaporates from Earth's surface each year, most of it from the oceans. More rain falls on the land than evaporates from it; the difference returns to the sea as rivers and groundwater flow, balancing the budget so that ocean levels stay roughly steady.

Summary

The water cycle is the continuous movement of water between oceans, air and land. Evaporation and transpiration put water vapour into the air; condensation forms clouds; precipitation returns water to the ground; run-off and infiltration carry it back towards the sea. All the stages are physical changes, and Earth's total water stays almost constant.

Practice questions

1. Name the two processes that add water vapour to the air. Answer: Evaporation and transpiration. 2. Explain why the water cycle is described as a series of physical changes. Answer: Water changes state and location, but it remains H₂O throughout; no new substance is formed. 3. What supplies the energy that drives evaporation? Answer: The Sun. 4. Explain how the water cycle resembles distillation. Answer: Water evaporates, leaving dissolved salts behind, then condenses and falls as fresh water, just as in distillation. 5. Describe two routes by which rain that falls on land can return to the sea. Answer: It can flow over the surface as run-off into rivers, or soak into the ground and move slowly as groundwater towards the sea.