Water is practically everywhere on Earth, and the water cycle is the path that all of it follows as it moves around the planet: an endless process that connects all the water on Earth, moving it through the atmosphere, into and out of the oceans, over the land and underground. Water is stored in the atmosphere, on the land surface and below the ground, and it moves between the places it is stored. The atmosphere is the superhighway in the sky that moves water everywhere over the Earth, although the air holds only about one thousandth of one percent of all Earth's water. About 96.5 percent of all the water on Earth is in the oceans, as of November 2026, and about two-thirds of Earth's fresh water is stored in ice caps, snow packs and glaciers. Following a single drop through these stores answers a simple question: where does rain come from?
Evaporation is the first step, and it needs energy: heat breaks the bonds that hold water molecules together, and the heat is carried off in the vapor as latent heat, which is released again when the vapor condenses into cloud droplets. About 86 percent of the world's precipitation comes from water that evaporated from the ocean, and nearly 90 percent of the moisture in the atmosphere comes from oceans, seas, lakes and rivers. Most of the rest of the moisture comes from plants, which take in water through their roots and release it through small pores on the underside of their leaves, and a cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day. Evapotranspiration, the evaporation from land plus plant transpiration, is the main way water moves into the atmosphere from the land surface and oceans.
Condensation turns the vapor back into liquid, and it happens when air is cooled to its dew point, the temperature at which air reaches saturation; a higher dew point indicates more moisture in the air, and warmer air can hold more water vapor than cooler air. As air rises it cools and the pressure drops, which makes some water vapor condense into small droplets and form a cloud. Dew and fog form the same way. Cloud droplets form around tiny aerosol particles of dust, salt and smoke, and without aerosols clouds could not exist, because it is easier for water vapor to condense when it has a particle to condense upon. The droplets and crystals are so small and light that they stay up in the air, and clouds float because the moist air in them is less dense than dry air.
A cloud is not yet rain, because cloud droplets start far too small to fall, though they are big enough to form a visible cloud, and it takes millions of them to make a single raindrop. Droplets grow as more water vapor condenses onto them, and rain falls when they get too heavy to stay in the cloud. In cold clouds there is a second route: if the air in a cloud is below freezing, ice crystals form, and ice crystals grow at the expense of nearby water drops, because the vapor pressure over supercooled liquid water is greater than over ice, a pathway known as the Bergeron-Findeisen process. Rain can start as water droplets or ice crystals in a cloud but always falls as liquid water. Many people picture raindrops as tears, which is a popular misconception: small drops under 1 mm radius are round like a ball, larger drops flatten into the shape of the top of a hamburger bun, and at about 4 to 5 mm the bottom caves in like a parachute and the drop breaks apart.
Not all precipitation reaches the ground as rain. Falling ice crystals may land as snow, or melt on the way down and land as rain, and snow falls when all the air between the cloud and the ground is below freezing. Sleet forms when a thin layer of warmer air comes between layers of cold air, so that ice crystals melt as they fall through the warm layer and then refreeze, while freezing rain falls like rain but freezes as soon as it touches the ground. Hail forms inside thunderstorm updrafts as ice grows when liquid drops freeze onto it; small hailstones fall at 9 to 25 mph, and very large ones can fall at over 100 mph. Most precipitation falls as rain, with most of it landing in the oceans.
On land, rain meets the ground, and what happens next depends on the ground. Snowmelt, runoff and streamflow move water across the land surface, while infiltration and groundwater recharge move it into the ground. Clays absorb less water at a slower rate than sandy soils, and soil already saturated like a wet sponge cannot absorb much more. Soils absorbing less water result in more runoff overland into streams, and water falling on steeply sloped land runs off quicker and infiltrates less than water falling on flat land. Vegetation can slow the movement of runoff, allowing more time for it to seep into the ground, and some infiltration stays near the surface where plants put down their roots.
Groundwater is a huge store: about 1.7 percent of all of Earth's water is groundwater, which is about 30 percent of Earth's freshwater, and it is always moving, though not like rivers flowing below ground. It eventually emerges back at the land surface, into rivers and into the oceans, but at depth it can take centuries to recharge, and in some places thousands or millions of years. If water flows through rock easily enough to be pumped and used by people, the formation is called an aquifer, and the top of the surface where groundwater occurs is the water table; excessive pumping can lower it so much that wells no longer supply water. GRACE, a pair of satellites about 220 kilometers apart, detects dips and bumps in Earth's gravity that are affected by the mass of water, and one GRACE study found that of Earth's 37 largest groundwater basins, 21 lose more water each year than they gain, as of November 2026. Yet GRACE cannot measure exactly how much water is left in an aquifer, and as of November 2026 scientists say they do not actually know how much is stored in each one.
The idea that rain feeds rivers had to be proved. Perrault experimentally showed that only one-sixth of the annual rainfall was necessary to sustain the river flow, so rain alone could feed the river, and Edmé Mariotte, a French physicist working independently, later used more sophisticated measuring techniques to confirm his findings. Halley found in 1687 that water exhales the thickness of 60 parts of an inch in two hours from its whole surface. Even after these experiments, Derham argued in 1720 that springs have their origin from the sea, and not from rains and vapors. Today the measuring uses simple and advanced tools alike: a rain gauge must stand in an open area away from trees and buildings, and a standard National Weather Service 4-inch gauge holds exactly 1 inch in its inner cylinder.
The numbers show how fast the cycle turns. On average a drop of water may spend over 3,000 years in the ocean before evaporating, but only about nine days in the atmosphere before falling back to Earth, and water spends thousands to hundreds of thousands of years in the large ice sheets that cover Antarctica and Greenland. About 3,100 cubic miles of water is in the atmosphere at any one time; if it all rained down at once, it would only cover the globe to a depth of 2.5 centimeters. Yet about 495,000 cubic kilometers cycle through the air each year, as if the air's water were replaced nearly 40 times a year. On a global basis evaporation approximately equals precipitation, but over the oceans evaporation exceeds precipitation, while less water evaporates over land than falls onto it, and streams and rivers return the difference to the ocean. Precipitation is concentrated in some parts of the world and scarce in others: in 2017 Colombia received about 3,240 mm while Egypt received only 18 mm.
The ocean's saltiness is itself a record of the cycle. Salt in the ocean comes from rocks on land, and the ocean collects all of the salt and minerals from all of the rivers that flow into it. Rivers carry an estimated four billion tons of dissolved salts to the ocean each year, but the ocean today probably has a balanced salt input and output, so it is no longer getting saltier. When seawater evaporates, the water goes into the air and the sodium chloride is left behind, because salt ions stick to liquid water much better than water molecules do, so rain is almost salt-free.
Human water use, land use and climate change all impact the water cycle, and some of the changes have been measured. A warmer atmosphere holds more water vapor: on average, for every 1 degree Celsius (1.8 degrees Fahrenheit) of warming, it can hold 7 percent more, as of November 2026, and that extra water vapor can make storms rain more heavily and lead to more flooding. Globally, daily extreme rainfall events will likely intensify by about 7 percent for every 1 degree Celsius of warming, according to the IPCC as summarized by PBS NewsHour. In the Northeast and Midwest United States, the precipitation falling on the heaviest 1% of days increased 60% and 45% from 1958 to 2021, according to the Fifth National Climate Assessment. The IPCC found the water cycle has been intensifying and will continue to intensify as the planet warms, and both wet and dry extremes will increase.
Other effects pull in different directions. Air pollution particles called aerosols have generally offset some of the effect of warming on the water cycle in recent decades, as of November 2026. Atmospheric rivers are long, narrow bands of concentrated water vapor that transport moisture from warm tropical oceans to higher latitudes, and when they reach land they account for roughly 30 to 50 percent of annual precipitation in the West Coast states, as of November 2026. For groundwater, as of November 2026, the total volume is likely far less than rudimentary estimates made decades ago.
Words to know
- evapotranspiration: land-plant evaporation
- latent: hidden
- aerosols: airborne specks
- aquifer: water-holding rock
- saturation: holding all vapor
