Water is practically everywhere on Earth. It is in the air, on the land, in the ocean and underground. All of it follows one path, and that path is called the water cycle. The cycle is endless, because it connects all the water on Earth. Energy from the sun helps power it: the sun gives water at the surface of the ocean, rivers and lakes the energy to become water vapor. Rain comes from that vapor, which went up into the air. Follow one drop along the way, and ask how people worked out the path.

Evaporation is when liquid water turns into water vapor, a gas. It needs energy, because heat breaks the bonds that hold water molecules together. Nearly 90 percent of the moisture in the atmosphere comes from water that evaporated from oceans, seas, lakes and rivers. Most of the rest comes from plants, which send water vapor into the air through transpiration. Scientists add the two together and call the total evapotranspiration, the water that evaporates from the land plus the water that plants release. It is the main way water moves into the atmosphere from the land surface and oceans.

Cooling turns the gas back into liquid. Air gets cooler as it rises higher into the sky. As air rises it cools and the pressure drops, which makes some water vapor condense into small droplets and form a cloud. The droplets form around tiny aerosol particles, and without aerosols clouds could not exist. A cloud is made of tiny drops of liquid water or tiny ice crystals, so small and light that they stay up in the air.

Cloud droplets are far too small to fall, though they are big enough to form a visible cloud. It takes millions of cloud droplets to make a single raindrop. Droplets grow bigger as more water vapor condenses onto them, and rain falls when they get too heavy to stay in the cloud. Many people picture raindrops as tears, but that is a popular misconception. Small drops are round like a ball. As drops grow they flatten, until they look like the top of a hamburger bun, round on top and flat on the bottom. At about 4 to 5 mm the bottom caves in like a parachute and the drop breaks apart.

Not all precipitation starts as a liquid. If the air in a cloud is below freezing, ice crystals form. Falling ice crystals may land as snow, or melt on the way down and land as rain. In cold clouds, ice crystals grow at the expense of nearby water drops, a pattern called the Bergeron-Findeisen process. Sleet forms when a thin layer of warmer air comes between layers of cold air, so ice crystals melt as they fall and then refreeze. Hail forms inside thunderstorm updrafts, where liquid drops freeze onto growing ice.

Rain that reaches the land has more than one way to go. Some of it seeps into the soil and becomes groundwater, and some runs off the land to keep rivers and lakes full. Clays absorb less water at a slower rate than sandy soils, and soil that is already saturated like a wet sponge cannot absorb much more. Soils that absorb less water send more runoff overland into streams. Vegetation can slow the movement of runoff, which gives the water more time to seep into the ground. Rivers, streams and groundwater return water to the ocean.

A drop of water spends very different amounts of time in different places. On average a drop may spend over 3,000 years in the ocean before evaporating. It spends only about nine days in the atmosphere before falling back to Earth. In the large ice sheets that cover Antarctica and Greenland, water spends thousands to hundreds of thousands of years. Only about a thousandth of 1 percent of Earth's water is water vapor in the atmosphere. 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.

Rain feeding rivers sounds obvious, but it had to be tested. 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. Edmé Mariotte, a French physicist who worked independently, later used more sophisticated measuring techniques to confirm Perrault's findings. Mariotte did concede that infiltration was not universal and uniform. Each of these men measured water to learn where it went.

As of November 2026, scientists say 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. That extra vapor can make storms rain more heavily and lead to more flooding. 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 that the water cycle has been intensifying and will continue to intensify as the planet warms, so both wet and dry extremes will increase.

Rain is not shared out evenly across the planet. In 2017 Colombia received about 3,240 mm of precipitation while Egypt received only 18 mm. Precipitation is concentrated in some parts of the world and scarce in others, and it can vary substantially from season to season and year to year. Whatever falls, the cycle carries it on, because rivers return the water to the ocean. The water cycle is an endless process that connects all the water on Earth.

Words to know

• evapotranspiration: land-plant evaporation
• aerosol: airborne speck
• updrafts: rising air
• sleet: refrozen rain
• precipitation: falling water