Picture one drop of water in the ocean. It will not stay there. The water cycle is the path that all water follows as it moves around Earth. Water is in the air, on the land, in the ocean and underground. The sun gives this drop its first push, because energy from the sun helps power the whole cycle.

Heat gives the drop the energy to evaporate, which turns it into water vapor. Water vapor is water as a gas in the air. How much is up there at one time? Very little. Only about 0.001 percent of Earth's water is water vapor. Even so, about 500,000 cubic kilometers of water move through the air and fall as precipitation every year.

Our drop might also come from a plant. Plants and trees send water vapor into the air too, and this is called transpiration. Air gets cooler as it rises, and warmer air can hold more water vapor than cooler air. When the air cools below the dew point, the vapor condenses into liquid water. It condenses more easily when it has a particle to condense upon.

Many tiny drops make a cloud. Clouds are made of tiny drops of liquid water or tiny ice crystals. Fog and dew form the same way, when water vapor condenses. Fog can form at night when the ground cools. Our drop grows as more vapor condenses onto it. At last it gets too heavy to stay in the cloud, and it falls as rain or snow.

Where does it land? It may hit the ocean, a lake, a stream or the soil. Some water seeps into the soil and becomes groundwater. Some runs off the land and keeps rivers and lakes full. Snow can wait as snowpack or ice until it melts. Sooner or later, rivers, streams and groundwater return water to the ocean. Most of Earth's water is there, about 96.5 percent as of November 2026. Then the sun lifts the drop again, and the trip never ends.

Words to know

• vapor: water as gas
• condenses: turns to liquid
• groundwater: water underground
• snowpack: stored snow