Water Cycle

The water cycle is the path that water takes as it moves between the oceans, the air, the land, and living things. Water is always on the move. It rises into the sky, falls back down, flows across the ground, and soaks into the soil. This loop never stops. Scientists also call it the hydrologic cycle.

The sun powers the whole cycle. Its heat warms the oceans, lakes, and rivers. When water gets warm enough, it turns into a gas called water vapor. This change from liquid to gas is called evaporation. Water vapor is invisible, and it rises into the air. Plants add to it too. Their leaves release water vapor in a process called transpiration.

Higher in the sky, the air is cold. The water vapor cools down and turns back into tiny drops of liquid water. This change is called condensation. Millions of these tiny drops clump together to make clouds. A single cloud can hold an amazing amount of water. A medium-sized puffy cloud weighs about a million pounds, as much as 100 elephants floating in the sky.

When the drops in a cloud grow big and heavy, they fall. This is called precipitation. It can come down as rain, snow, sleet, or hail, depending on how cold the air is. Some of the water lands in the ocean. Some lands on mountains, forests, farms, rooftops, and streets.

Once water reaches the ground, it takes different paths. Some of it flows downhill across the surface as runoff. Runoff fills streams, rivers, and lakes. Most rivers eventually carry their water back to the ocean. Some water soaks into the soil and sinks deep underground. This hidden water, called groundwater, fills tiny spaces between rocks and grains of sand. Wells pull it back up for people to drink.

A lot of water also gets locked away for a long time. Glaciers and the ice sheets of Antarctica and Greenland hold about 69 percent of all the fresh water on Earth. Some of that ice has been frozen for hundreds of thousands of years. When it finally melts, it joins the cycle again.

The amount of water on Earth stays almost exactly the same. The same water molecules have been recycled for billions of years. A raindrop falling on your head today might have once flowed in the Nile, frozen in a glacier, or floated above a forest where dinosaurs walked.