Explainer: Earth’s water is all connected in one vast cycle | Science News for Students

Explainer: Earth’s water is all connected in one vast cycle

Water is constantly on the move in an endless loop known as the water cycle
Dec 6, 2018 — 6:30 am EST
a photo of heavy rain falling on a colorful umbrella outside

Rain is just one part of the planet’s water cycle.


It’s a summer day at the lake. Creeks spill into the clear water. Puffy clouds roll across the sky. On the horizon, a grey curtain of rain sweeps across distant snow-capped peaks.

This is Earth’s water cycle in action. Water, shape-shifting through three phases — liquid, vapor and ice — is on the move 24/7. As it moves, it connects every environment and living thing on the planet. Without the water cycle to replenish, clean and transport water, life on Earth could not exist.

The water cycle is driven by a series of linked processes in an endless loop.

a diagram showing Earth's water cycle
This diagram of Earth’s never-ending, connected water cycle illustrates major processes such as evaporation. It also includes more minor processes such as sublimation, in which ice evaporates directly into vapor.

Let’s start with evaporation. Heat from the sun causes liquid water from oceans, rivers and lakes to evaporate into an invisible vapor. Because vapor is lighter than air, it rises into the atmosphere.

Water vapor also enters the water cycle through transpiration. This is the process of water moving through plants and being released from plant leaves as vapor into the atmosphere. Transpiration accounts for about 10 percent of the water vapor in the atmosphere.

On average water vapor spends ~10 days in the sky. It takes at least a million cloud droplets to form one rain drop. During a heavy rainfall, rain drops fall about 6.7m (22 feet) per second. An inch of rain falling onto the roof of a medium sized house = ~2,120 liters (560 gallons). Evaporation is such an important process that Earth’s oceans would dry up if rain and rivers didn’t refill them. Only 10% of the water evaporated from oceans falls as precipitation over land.

Next up is condensation. As water vapor rises, it cools. Cooling causes the vapor to condense, or re-organize, into tiny droplets. We see those droplets as clouds. Condensation and evaporation constantly shape and reshape clouds. Watch a cloud, and you will see that even as some parts of it evaporate and disappear, other parts grow where condensation is occurring.

Transportation of water occurs as water vapor is moved from place to place with wind, stream currents and clouds.

Precipitation happens when cloud droplets merge into bigger drops. They may collect around particles such as ice, dust or smoke, or they may freeze into ice crystals. When the drops are heavy enough, down they come as rain, hail, sleet or snow. Not all precipitation reaches the ground. Some evaporates instead, or is transported back up by air currents, even as other drops fall.

When precipitation does reach the ground, several things can happen. Water may infiltrate, or soak into, the soil, and percolate deeper into the ground. It may run off right away, collecting in trickles and torrents as it flows downhill across the surface. Or, it may be intercepted by plants, collecting in leaves or taken up by their roots.

Then, there may be a lull in the action, called storage. Water may collect in lakes, ice, snow or underground (as groundwater). But eventually, snow melts, lakes drain or evaporate, and ice changes back to liquid or vapor. Even groundwater moves, ever so slowly, as it makes its way back to the surface.

Then the water cycle repeats, starting with evaporation once again.

The water cycle is as old as life itself. Yet scientists are still working out important details of the roles these different processes play, says Patrick Keys. Keys is a sustainability scientist at Colorado State University in Fort Collins. And, he adds, people can play a big role in that water cycle.

“What we do to the land around us — like cutting down a lot of trees or planting crops in dry places — can lead to enormous changes in evaporation and transpiration, the invisible parts of the water cycle,” he says. “These changes to the land can sometimes lead to big changes in the amount of rain a location downwind may receive.”

Water cycles through the planet, transitioning between ice, liquid and vapor.
National Science Foundation/YouTube

Power Words

(more about Power Words)

atmosphere     The envelope of gases surrounding Earth or another planet.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

condensation     The process of moisture in the air turning into a liquid as it comes into contact with a very cold surface. Or the term can refer to the liquid water that condenses out of the air.

condense     To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of things in the vicinity of an item of interest).

evaporate     To turn from liquid into vapor.

groundwater     Water that is held underground in the soil or in pores and crevices in rock.

infiltrate     To move into something, such as a liquid moving into and through the pores in soil.

particle     A minute amount of something.

percolate     The action of a liquid that gradually moves through a filter or porous material.

precipitation      (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.

smoke     Plumes of microscopic particles that float in the air. They can be comprised of anything very small. But the best known types are pollutants created by the incomplete burning of oil, wood and other carbon-based materials.

sublimation     (v. sublimate) A process whereby some solid turns into a gas without first melting into a liquid. One example: When ice cubes shrink in their tray in the freezer, it’s because they underwent sublimation.

sustainability     (adj: sustainable) To use resources in a way that they will continue to be available in the future.

transpiration     (v. transpire) A process in which plants carry moisture from their roots to their leaves, where it is then lost to the atmosphere as vapor.

water vapor     Water in its gaseous state, capable of being suspended in the air.


Website: USGS, The water cycle.

Website: USGS, How much water falls during a storm?

University announcement: World's largest cities depend on evaporated water from surrounding lands. Colorado State University, March 13, 2018.