The water cycle around the world

Water is always moving. It goes from the sea to the sky. It goes from the sky to the land. Then it goes back to the sea.

This never ends. It goes round and round, like a wheel. We call it the water cycle.

The water you drank today may once have been in an ocean, a cloud or a glacier. That is because Earth's water is always moving from place to place. Scientists call this journey the water cycle.

Water is stored in many places: in the oceans, in the air, in ice and snow, in lakes and rivers, and under the ground. It also changes form. It can be a liquid (water), a solid (ice) or a gas (water vapor). Two things keep the cycle turning: the Sun's energy, which lifts water up into the air, and gravity, which pulls it back down.

The U.S. Geological Survey (USGS) describes the water cycle as where water is on Earth and how it moves. Water is stored in pools, the oceans, atmosphere, ice sheets and glaciers, snowpack, lakes, reservoirs, rivers, wetlands, soil moisture, permafrost and groundwater aquifers, and it moves between them by fluxes: evaporation, evapotranspiration, condensation, precipitation, snowmelt, runoff, streamflow, infiltration, groundwater flow and discharge from springs.

Two forces drive it. Energy from the Sun evaporates liquid water into vapor, and gravity pulls water down: it brings rain, snow and hail from clouds and makes water flow downhill on land and through aquifers. Water changes state along the way, between liquid, solid and gas. People are part of the cycle too: dams, reservoirs, irrigation, wells and drainage all change where water is stored, how it moves and how clean it is.

The Sun warms the sea. Some water floats up into the air. You cannot see it. It is called water vapor.

Plants let water out of their leaves too. High in the sky, the air is cold. The water vapor turns into tiny drops. The drops make clouds!

When the Sun warms the ocean, some water turns into an invisible gas called water vapor and rises into the air. This is evaporation. Plants add water vapor too: they pull water up from the soil and let it out through tiny holes in their leaves. This is transpiration. The USGS says about 90 percent of the water in the air comes from evaporation from oceans, seas, lakes and rivers, and about 10 percent from plants.

Rising air carries the vapor up. High up, the air is cooler, and the vapor condenses into tiny droplets that gather as clouds. Once water has evaporated, it stays in the air for about 10 days before it falls again.

Evaporation turns liquid water into vapor using the Sun's energy; transpiration releases water drawn up through plants; sublimation turns ice straight into vapor. The USGS estimates that about 90 percent of the moisture in the atmosphere came from evaporation from oceans, seas, lakes and rivers, with the rest from plant transpiration and a very small amount from sublimation. Because oceans cover over 70 percent of the surface, they supply most of it.

Rising air currents lift the vapor, and cooler temperatures aloft let it condense into cloud droplets. The atmosphere holds only about 12,900 cubic kilometers of water at any moment, about 0.001 percent of Earth's water, enough to cover the globe about 2.5 centimeters deep if it all fell at once. It moves quickly: once evaporated, a water molecule spends about 10 days in the air.

Wind blows the clouds along. They can travel a long way.

Most water from the sea falls back into the sea as rain. Some is blown over the land. That rain waters forests, farms and towns.

The USGS calls the air a superhighway in the sky. Winds carry clouds and water vapor around the world until the water falls as rain or snow, sometimes far from where it started.

Here is a surprise: most of the water that evaporates from the oceans falls straight back into the oceans as rain. Only about 10 percent of it is blown over land before it falls. Over the oceans, more water evaporates than falls as rain. Over the land, it is the other way around: more falls than evaporates. Rivers and groundwater carry the extra back to the sea, which keeps the oceans from drying up.

The atmosphere is a small store but a fast conveyor. Winds move vapor and clouds around the globe, so rain in one region may have evaporated thousands of kilometers away. The balance differs by surface: over the oceans, evaporation exceeds precipitation; over land, precipitation routinely exceeds evaporation. The USGS estimates that only about 10 percent of the water evaporated from the oceans is carried over land and falls there.

The imbalance is closed by the return flow. The extra water falling on land runs off in rivers and seeps back through aquifers to the sea. The USGS puts it plainly: evaporation is so great that without precipitation runoff and groundwater discharge, the oceans would become nearly empty. The global water cycle is therefore a loop of linked imbalances, with the atmosphere carrying water inland and rivers and groundwater returning it.

When cloud drops get big, they fall. They fall as rain, snow or hail.

Rain flows into streams and rivers. Some soaks into the ground. Snow can stay on mountains until it melts. In the end, the water flows back to the sea.

When the droplets in a cloud grow big and heavy, they fall as rain, snow, sleet or hail. This is precipitation. What happens next depends on where it lands.

Some water runs over the ground into streams and rivers, which carry it downhill toward the sea. Some soaks into the soil. Plants drink some of it, and some sinks deeper to become groundwater, which moves slowly through cracks and spaces in rock like water in a sponge. Groundwater can come back out at springs or seep into rivers. Snow and ice can store water for a long time before it melts and flows on.

Precipitation returns water to the surface, and gravity directs what follows. Water moves across the land through snowmelt, surface runoff and streamflow; it enters the ground through infiltration and recharges aquifers. The USGS stresses that groundwater does not flow like underground rivers; it moves like water in a sponge, driven downward and sideways by gravity and pressure through the spaces between rocks, and eventually discharges into rivers, the ocean or springs.

Stores hold water for very different times. Water may pass through the air in about 10 days, but snowpack, glaciers, ice sheets and aquifers can hold it far longer; the USGS notes that water in glaciers moves very slowly compared with water in rivers. Rivers keep flowing between rains because groundwater seeps into their beds. Every route ends in the same place: back in the ocean, ready to evaporate again.

Some places get lots of rain. Some places get very little.

Near the middle of the Earth, the Sun is very hot. Warm, wet air rises and makes big storms. So it rains a lot there. Some other places are dry, and many deserts are there.

Rain does not fall evenly around the world. Near the Equator, the Sun is strongest. It heats the air and ocean, so warm, wet air rises. Winds called trade winds blow in from the north and south and push even more air upward. As the air rises it cools, and its water forms towering clouds and thunderstorms. NASA satellites see this as a band of rain wrapped around Earth near the Equator, called the Intertropical Convergence Zone, or ITCZ.

This rain band moves north and south with the seasons. That is why many tropical places have a wet season and a dry season instead of summer and winter. Farther from the Equator, around 30 degrees north and south, the air pressure is usually high, the weather is often fair and dry, and many of the world's deserts lie there.

Where rain falls is set by where air rises. NASA explains that intense sun and warm water near the Equator heat the air and raise its humidity, making it buoyant; the converging northeast and southeast trade winds force it upward, and as it rises it expands, cools and releases its moisture in an almost continuous series of thunderstorms. This Intertropical Convergence Zone forms the rising branch of the Hadley circulation. NASA's GPM satellite data show it as a remarkably uniform rain band just north of the Equator when averaged over a year.

The ITCZ shifts with the seasons, producing the tropics' wet and dry seasons, and longer-term changes in its position can cause severe droughts or floods. NOAA notes that low-pressure bands lie at the Equator and at 50° to 60° latitude, and high-pressure bands near 30° north and south and at the poles; high pressure usually brings fair, dry weather, and many deserts lie along 30° latitude. Mountains add local contrasts, such as rain shadows, and seasonal wind reversals bring monsoons.

People use water from rivers, lakes and the ground. We drink it, wash with it and grow food with it.

When we keep water clean, it stays good for people, plants and animals.

People are part of the water cycle too. We build dams to store water in human-made lakes called reservoirs. We pump water from under the ground. We use water in our homes, on farms to water crops and for animals, and in factories and power plants.

What we do can also change how clean water is. Rain can wash fertilizers, chemicals and soil from farms and streets into rivers. Scientists measure where water is, how fast it moves and how clean it is, so that people can use water wisely. The USGS also says climate change is changing patterns of rain and snow.

The USGS lists the ways people reshape the cycle: redirecting rivers, building dams and reservoirs, draining wetlands, and withdrawing water from rivers, lakes and aquifers for homes, irrigation, livestock, power generation, mining and aquaculture. Water quality is affected too: runoff carries fertilizers, pesticides, sediment and sewage into rivers and groundwater, and power plants and factories can return heated or contaminated water.

The USGS also states that climate change is affecting the water cycle: precipitation patterns are changing, the frequency, intensity and length of floods and droughts are changing, and sea level is rising. It frames water availability in three parts, quantity (how much is in each pool), timing (when and how fast water moves) and quality (how clean it is), and says understanding human impacts on all three helps people use water sustainably.