The water in your glass may have once fallen as rain on a distant mountain, flowed down a river, rested for centuries deep underground or drifted as vapor high above the ocean. It may even have been part of a glacier during the last ice age. Earth's water is constantly moving, changing between liquid, vapor and ice, in a vast system called the water cycle or hydrologic cycle. Powered by the Sun and pulled by gravity, the water cycle shapes weather, carves landscapes and sustains every living thing. This explainer follows water on its journey and explains the science behind each step.
How much water does Earth have?
Earth holds about 332 million cubic miles (1.39 billion cubic kilometers) of water. Almost all of it is in places where we cannot easily use it.
| Location | Approximate share of Earth's water |
|---|---|
| Oceans | about 96.5 percent |
| Ice caps, glaciers and permanent snow | about 1.7 percent |
| Groundwater | about 1.7 percent, roughly half of it salty |
| Lakes, rivers, soil moisture, atmosphere and living things | less than 0.1 percent |
Only about 2.5 percent of Earth's water is fresh, and most of that is locked in ice or underground. The water in rivers and lakes that people depend on is a tiny fraction of the total.
The amount of water on Earth stays nearly constant over human timescales. The water cycle does not create or destroy water; it moves it from one place to another and changes its form.
The Sun and gravity: the engines of the cycle
Two forces drive the water cycle:
- The Sun provides the energy to evaporate water and lift it into the atmosphere. Evaporation requires a lot of energy, which is why evaporating sweat cools your skin.
- Gravity pulls water back down as precipitation and makes it flow downhill in rivers and underground.
Step by step around the cycle
Evaporation
When sunlight warms the surface of oceans, lakes, rivers and wet soil, water molecules gain energy. Some gain enough energy to escape from the liquid into the air as water vapor, an invisible gas. This is evaporation. About 86 percent of global evaporation comes from the oceans.
When water evaporates, the salt and other dissolved substances stay behind. That is why rain is fresh even though most of it originally evaporated from salty seawater.
Transpiration
Plants also release water vapor. They draw water from the soil through their roots and release it through tiny pores in their leaves called stomata. This process is called transpiration. A large tree can release hundreds of liters of water on a hot day. Together, evaporation from land surfaces and transpiration from plants are often called evapotranspiration. In rainforests, this recycled moisture produces much of the region's own rainfall.
Sublimation
Ice and snow can turn directly into water vapor without melting first, a process called sublimation. It happens on cold, dry, sunny and windy days, especially at high altitudes. The opposite process, vapor turning directly into ice, is called deposition; it is how frost forms.
Condensation and clouds
As warm, moist air rises, it expands and cools. Cooler air can hold less water vapor. When the air cools to its dew point, the water vapor begins to change back into tiny liquid droplets or ice crystals. This is condensation.
For condensation to happen, water vapor needs tiny particles to condense on, called condensation nuclei: specks of dust, sea salt, smoke or pollen. Billions of these tiny droplets and crystals together form clouds. Fog is simply a cloud at ground level.
Condensation releases the heat that was absorbed during evaporation. This released heat, called latent heat, powers storms, especially thunderstorms and hurricanes.
Precipitation
Cloud droplets are extremely small, about 0.02 millimeters across, and stay suspended in the air. To fall as rain, they must grow about a million times in volume. They do this by colliding and merging with other droplets, or, in cold clouds, by ice crystals growing at the expense of droplets and then falling and melting. When drops or crystals become too heavy for the air to support, they fall as precipitation: rain, snow, sleet or hail.
Precipitation is not evenly distributed. Some places receive more than 400 inches (10 meters) of rain a year, while the driest deserts may go years without any.
Runoff
When precipitation reaches the ground, some of it flows over the surface as runoff, collecting in streams and rivers that eventually reach lakes or the ocean. The area of land that drains into a particular river is called its watershed or drainage basin. Runoff also carries soil, nutrients and pollutants, shaping landscapes through erosion over long periods.
Infiltration and groundwater
Some precipitation soaks into the ground, a process called infiltration. It moves downward through soil and rock until it reaches a zone where all the spaces between particles are filled with water, called the saturated zone. The top of this zone is the water table. Underground layers of rock or sediment that hold and transmit water are called aquifers.
Groundwater moves slowly, sometimes only a few feet per year, and can stay underground for thousands of years. It feeds springs, wells and streams, keeping rivers flowing during dry periods.
Storage in ice
In cold regions, precipitation falls as snow and can build up over many years into glaciers and ice sheets. The ice sheets of Antarctica and Greenland store most of the world's fresh water. Melting snow and glaciers supply rivers with water during warmer months, which is vital for billions of people in regions fed by mountain snowpack, such as the Himalayas, the Andes and the Rocky Mountains.
How long does water stay in each place?
The average time a water molecule spends in one part of the cycle is called its residence time. It varies enormously:
| Reservoir | Approximate average residence time |
|---|---|
| Atmosphere | about 9 days |
| Rivers | weeks to months |
| Soil moisture | weeks to months |
| Lakes | years to decades |
| Shallow groundwater | years to centuries |
| Oceans | about 3,000 years |
| Deep groundwater | up to 10,000 years or more |
| Antarctic ice sheet | up to hundreds of thousands of years |
Because water spends only about nine days in the atmosphere on average, the atmosphere constantly recycles its moisture, which is why weather can change so quickly.
The water cycle and life
The water cycle is essential for life. It delivers fresh water to land, where plants, animals and people need it. It transports heat around the planet: evaporation in the tropics absorbs heat, and condensation releases it elsewhere, helping regulate Earth's climate. It shapes landscapes by eroding mountains and depositing sediments. It also cleans water, since evaporation leaves salts and many contaminants behind.
Human influence on the water cycle
People change the water cycle in many ways:
- Dams and reservoirs store water and change river flows.
- Irrigation moves huge amounts of water from rivers and aquifers to farmland, where much of it evaporates.
- Groundwater pumping in some regions removes water faster than it is replenished, lowering water tables.
- Urbanization replaces soil with pavement, increasing runoff and flooding while reducing infiltration.
- Deforestation reduces transpiration and can decrease rainfall in some regions.
Climate change and the water cycle
As Earth warms, the water cycle is intensifying. Warmer air can hold more water vapor, about 7 percent more for every degree Celsius of warming. This leads to:
- Heavier rainfall in many regions, increasing the risk of floods.
- More evaporation, which can intensify droughts in dry regions.
- Less snowpack and earlier melting, changing the timing of river flows.
- Melting glaciers and ice sheets, contributing to sea level rise.
Scientists sometimes describe this as "wet regions getting wetter and dry regions getting drier," although real patterns are more complicated.
A simple experiment
You can create a miniature water cycle at home. Put a little warm water in a large glass bowl, cover it tightly with plastic wrap and place a few ice cubes on top of the wrap. Set the bowl in sunlight. Water will evaporate, condense on the cold plastic like a cloud and drip back down like rain.
Questions readers ask
Why is rain fresh when the ocean is salty? When seawater evaporates, salt is left behind. Only pure water vapor rises into the air.
Does the amount of water on Earth change? It stays nearly constant. The water cycle moves water between oceans, air, land and ice without creating or destroying it.
How long does water stay in the atmosphere? On average, about nine days before falling as precipitation.
What is groundwater? Water that fills the spaces in soil and rock below the water table. It feeds wells, springs and rivers.
How is climate change affecting the water cycle? Warmer air holds more moisture, leading to heavier rainfall in some areas, more intense droughts in others and changes in snow and ice.
In short
The water cycle is the continuous movement of water between oceans, the atmosphere, land, ice and living things. Powered by the Sun, water evaporates, rises, cools and condenses into clouds, then falls as precipitation pulled by gravity. It runs off into rivers, soaks into groundwater or freezes into glaciers before eventually returning to the sea. Different parts of the cycle hold water for days or for thousands of years. This endless journey sustains life, shapes landscapes and regulates climate, and it is now being intensified by a warming world.