Hail is created in thunderstorm clouds, and it forms inside thunderstorm updrafts, where liquid water drops freeze onto ice and the stone grows. Small hailstones fall at 9 to 25 mph, and very large ones can fall at over 100 mph. A hailstone comes out clear if the water freezes slowly, because the air bubbles can escape. Rain always falls as liquid water, even when it started as ice crystals in a cloud, and freezing rain falls like rain but freezes as soon as it touches the ground. Each of these points describes a different kind of frozen or freezing water. It helps to keep them separate when you read about storms.

Clouds do two jobs at once. They help shield the Earth from the Sun, and they also trap heat from below. Water vapor is itself a powerful greenhouse gas. A cloud forms when air rises, cools and loses pressure, which makes some water vapor condense into small droplets. The droplets or crystals are so small and light that they stay up in the air. Clouds float on air because the moist air in them is less dense than dry air. Together these facts make clouds both a result of the water cycle and a part of how Earth holds its heat. Both roles matter, which is why scientists study clouds as part of the water cycle and as part of how the planet holds heat.

Every cloud droplet needs a start. Cloud droplets form around tiny aerosol particles, and without aerosols clouds could not exist. Tiny particles of dust, salt and smoke in the air give water vapor something to condense onto, and it is easier for vapor to condense into droplets when it has a particle to condense upon. Air pollution particles are aerosols too. As of November 2026, scientists say that air pollution particles called aerosols have generally offset some of the effect of warming on the water cycle in recent decades. In other words, water vapor alone is not enough to make a cloud, because the vapor also needs something small and solid to condense onto.

Evapotranspiration is the main way water moves into the atmosphere from the land surface and oceans, and it is the evaporation from land plus plant transpiration. Plants take in water through their roots and release it through small pores on the underside of their leaves. A cornfield 1 acre in size can transpire as much as 4,000 gallons of water every day. Satellites with thermal sensors can measure the temperature of the land, crops and cities. Scientists combine those readings with other data to calculate evapotranspiration. This makes evapotranspiration a combined figure for two processes, evaporation from the land and transpiration from plants.

One NASA mission weighs water from orbit. GRACE uses two satellites in tandem about 220 kilometers apart to measure dips and bumps in Earth's gravity, which are affected by the mass of water. A study using GRACE found that of Earth's 37 largest groundwater basins, 21 lose more water each year than they gain, as of November 2026. Some of the depleted aquifers have nearly no natural replenishment to offset usage. In the Colorado River Basin the groundwater reduction was about 34 cubic kilometers, which was 65% of the total water lost. GRACE cannot measure exactly how much water is left in an aquifer. These results all come from measuring gravity, which responds to the mass of water below the satellites.

Groundwater is always moving, though not like rivers flowing below ground. It eventually emerges back at the land surface, into rivers and into the oceans, which keeps the water cycle going. It can return to the surface through natural discharge into rivers, the ocean and springs. Groundwater at depth can take centuries to recharge. In some places it can take thousands or millions of years. The top of the surface where groundwater occurs is called the water table, and excessive pumping can lower it so much that wells no longer supply water. These facts explain why groundwater matters to the cycle and to people: it moves slowly, it returns to the surface in several ways, and pumping can lower the water table.

The history of the water cycle is a story of people measuring and arguing. Perrault showed experimentally that only one-sixth of the annual rainfall was necessary to sustain the river flow, so rain alone could feed the river. Edmé Mariotte, another French physicist, worked independently and later used more sophisticated measuring techniques to confirm Perrault's 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. Some water isotopes also act as natural tracers in the water cycle.

The ocean is salty, and the rain that feeds rivers is not. 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. Rain physically erodes rock on land, and acids in rainwater chemically break the rocks down. 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. 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.

The pace of the cycle is changing as the planet warms. A warmer atmosphere holds more water vapor. On average, for every 1 degree Celsius of warming it can hold 7 percent more, as of November 2026. 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. 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. Atmospheric rivers are long, narrow bands of concentrated water vapor that carry moisture from warm tropical oceans to higher latitudes. The water cycle is an endless process that connects all the water on Earth.

Words to know

• aerosols: airborne specks
• recharge: refill
• aquifer: water-holding rock
• greenhouse: heat-trapping
• isotopes: atom variants