The water filling a glass today may once have fallen as rain on a forest thousands of years ago, or drifted as vapour above an ocean on the other side of the world. This article explains how the water cycle moves the same finite supply of water endlessly through the planet, and where all that water actually sits.
Earth does not get new water. The planet has essentially the same amount of water today as it did billions of years ago. What keeps that water usable, and what fills rivers, fuels rain and refills lakes, is a continuous loop called the water cycle, powered by energy from the Sun. This article follows a single drop of water through that loop, then looks at where Earth's water actually sits and how people affect it.
The water cycle is the continuous movement of water between the oceans, the atmosphere and the land. It has no real starting point, because it is a loop, but it is usually described in four main stages: evaporation, condensation, precipitation and collection. The entire cycle is driven by heat from the Sun, which provides the energy needed to turn liquid water into vapour. Gravity then pulls water back down and makes it flow downhill.
A water molecule that falls as rain today might have evaporated from an ocean thousands of kilometres away just days earlier. The same molecule could be evaporating and falling again as rain somewhere else within weeks.
Evaporation happens when heat from the Sun causes liquid water at the surface, mostly the oceans, to turn into water vapour and rise into the atmosphere. Oceans account for the large majority of the water that evaporates, because they cover about 71% of Earth's surface. Evaporation can happen at any temperature, not only when water boils, although warm, dry and windy conditions make it faster.
Plants add to this process through transpiration: water absorbed by roots travels up through the plant and evaporates from tiny pores in its leaves. Because evaporation and transpiration happen together across a landscape, scientists often refer to the combined process as evapotranspiration. Ice and snow can also change directly into vapour without melting first, which is called sublimation.
As water vapour rises higher into the atmosphere, it cools. Cooler air can hold less water vapour, so the vapour condenses, changing back into tiny liquid droplets, around microscopic particles of dust, salt or pollen floating in the air. Billions of these tiny droplets clustered together form what we see as a cloud. Water vapour itself is invisible. A cloud is made of liquid droplets or ice crystals, not vapour.
Condensation is not limited to the sky. The same process explains why a cold glass of water "sweats" on a humid day. Warm, moist air touching the cold glass cools rapidly, and the water vapour in it condenses into visible droplets on the outside of the glass.
Inside a cloud, water droplets constantly collide and merge with each other, gradually growing larger. Once a droplet becomes too heavy for rising air currents to keep it suspended, it falls to Earth as precipitation: rain, snow, sleet or hail, depending on the temperature of the air it falls through. Geographers describe three main ways that air is lifted and cooled to make rain.
| Type of rainfall | How it forms |
|---|---|
| Convectional | The Sun heats the ground, warm air rises quickly, cools and condenses. It often brings heavy showers or thunderstorms in hot weather. |
| Relief (orographic) | Moist wind is forced up over hills or mountains, cools and condenses. The windward side gets more rain, and the far side, the "rain shadow", is drier. |
| Frontal | Warm air meets cold air, and the warmer, lighter air rises over the cold air, cools and condenses. |
To learn more about how the air behaves, see Weather and Climate: How the Atmosphere Works.
Once precipitation reaches the ground, it does not just sit still. It takes one of several paths, together known as collection or runoff:
An area of land where all the runoff drains toward the same river, lake or ocean outlet is called a watershed, or drainage basin. From here, most water eventually reaches the ocean, ready to evaporate and begin the cycle again. Rivers such as the Indus are fed by snow and glacier melt in the mountains and by monsoon rain, which shows how the water cycle and the land are tied together.
Water moves between stores at very different speeds. The times below are rough averages, and real values vary a lot from place to place.
| Where the water is | Roughly how long it stays |
|---|---|
| Atmosphere | About 9 days |
| Rivers | Weeks to months |
| Lakes | Years to decades |
| Groundwater | Weeks to many thousands of years, depending on depth |
| Oceans | Thousands of years |
| Ice sheets and glaciers | Thousands of years or more |
Despite Earth being called the "Blue Planet", most of that water is not available for drinking or farming. The vast majority is saltwater, and even the small freshwater share is mostly locked away in a form we cannot easily use.
| Water source | Share of Earth's total water |
|---|---|
| Oceans (saltwater) | About 97% |
| Freshwater, total | About 3% |
| ↳ Glaciers and ice caps | Roughly 69% of all freshwater |
| ↳ Groundwater | Roughly 30% of all freshwater |
| ↳ Rivers, lakes, soil moisture and atmosphere | Only about 1% of all freshwater |
That means the rivers and lakes people rely on most directly for daily water make up only a tiny sliver of Earth's total water supply. This is part of why freshwater conservation matters so much, even on a planet that looks mostly blue from space.
Imagine all of Earth's water as 1,000,000 litres. Let us share it out using the percentages above.
So 300 out of 1,000,000 litres, which is only 0.03%, is in the places people can reach most easily. Even this small share is shared by people, plants and animals.
Try it yourself: if Earth's water were 10,000 litres, how many litres would be locked in ice? Answer: 3% of 10,000 = 300 litres of freshwater, and 69% of 300 = 207 litres.
Human activity changes how water moves. Cutting down forests reduces transpiration and can increase runoff. Covering the ground with concrete and roads stops water soaking in, so rain runs quickly into drains and rivers and can cause floods. Pumping groundwater faster than rain refills it makes wells run dry. Pollution can make water unsafe. Farming is the biggest user of freshwater, taking about 70% of the freshwater people withdraw worldwide.
The climate also affects the cycle. Warmer air can hold about 7% more water vapour for each 1 °C of warming, and scientists expect this to make some rainstorms heavier while other places become drier. Glaciers in many mountain regions are shrinking, which changes the flow of the rivers they feed. You can read about related hazards in Natural Disasters, about the planet's structure in Earth's Structure, and about the Sun that powers the cycle in The Solar System.
Activity: make a mini water cycle. Put a little water in a clear sealed bag, tape it to a sunny window, and watch evaporation, condensation and "rain" over a few hours. Students can then label the stages on the diagram.
Please note: this article is general education. Figures are rounded estimates and may be updated as scientific measurements improve.
10 questions. Select an answer for each, then submit to see your score instantly.