Evaporation needs energy, because heat breaks the bonds that hold water molecules together. The heat does not vanish when the water rises. Water vapor carries latent heat: heat is taken up when water evaporates and released when it condenses into cloud droplets. Evaporation is when liquid water turns into water vapor, a gas, and water vapor turns back into liquid water when it cools. That is called condensation. So every cloud is a place where heat that was taken from the surface is given back to the air.

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 water that evaporated from oceans, seas, lakes and rivers. Sea surface temperature, surface winds and air temperature all influence evaporation at the ocean surface: warm tropical water supports high evaporation. Wind increases it. On a global basis evaporation approximately equals precipitation, but over the oceans evaporation exceeds precipitation. Less water evaporates over land than falls onto it. Streams and rivers return the difference to the ocean.

The air can hold only so much water vapor, and how much depends on its temperature. Warmer air can hold more water vapor than cooler air. The dew point is the temperature to which air must be cooled to reach saturation. A higher dew point indicates more moisture in the air. Condensation occurs when saturated air is cooled, such as on the outside of a glass of ice water. Dew is water vapor that has condensed back onto Earth's surface. Radiative fog develops at night when surface temperatures cool. Fog also develops when humid air meets a colder surface.

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. That extra water vapor can make storms rain more heavily and lead to more flooding. The IPCC also found that the water cycle has been intensifying and will continue to intensify as the planet warms. Both wet and dry extremes will increase.

The evidence is already in the record. 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, as of November 2026. As of November 2026, air pollution particles called aerosols have generally offset some of the effect of warming on the water cycle in recent decades. Atmospheric rivers are long, narrow bands of concentrated water vapor that transport moisture from warm tropical oceans to higher latitudes. When they reach land they account for roughly 30 to 50 percent of annual precipitation in the West Coast states, as of November 2026.

Where is all the water? About 96.5 percent of all the water on Earth is in the oceans, as of November 2026. About two-thirds of Earth's fresh water is stored in ice caps, snow packs and glaciers. About 1.7 percent of all of Earth's water is groundwater, which is about 30 percent of Earth's freshwater. Rivers hold only about 1/10,000th of one percent of Earth's water, and only a little more than 1.2% of freshwater is surface water, which serves most of life's needs. The water in the air is about one thousandth of one percent, which is more than the rivers hold but still very little.

If water flows through rock easily enough to be pumped and used by people, the formation is called an aquifer. Confined aquifers have layers of rock above and below that are not very permeable to water. People get groundwater by drilling wells deep enough to tap into an aquifer. 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. Groundwater is always moving, though not like rivers flowing below ground. At depth it can take centuries to recharge.

Scientists can now watch groundwater from space. 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. Of Earth's 37 largest groundwater basins, 21 lose more water each year than they gain, according to a GRACE satellite study, as of November 2026. Some of the depleted aquifers have nearly no natural replenishment to offset usage. GRACE cannot measure exactly how much water is left in an aquifer. Available measurements are insufficient to know total groundwater.

Estimates of groundwater are particularly difficult to make and vary widely. Estimates of remaining storage might vary from decades to millennia, as of November 2026. Scientists say they do not actually know how much is stored in each of these aquifers, and the total groundwater volume is likely far less than rudimentary estimates made decades ago. That honest gap matters. Aquifers are a huge storehouse that people around the world depend on. Human water use, land use and climate change all impact the water cycle, and the atmosphere is the superhighway in the sky that moves water everywhere over the Earth.

Words to know

• latent: hidden
• saturation: holding all possible
• permeable: letting water through
• aquifer: water-holding rock
• condenses: turns liquid