Academic Reading: 1

The Water Cycle

Water on Earth is always moving. This continuous movement is called the water cycle, and it has no clear beginning or end. The main stages of the cycle are evaporation, condensation, precipitation, and collection. Understanding these stages helps explain why rain falls, why rivers keep flowing, and why the total amount of water on Earth stays almost the same year after year.

The cycle starts, in a simple sense, with evaporation. When the sun heats the surface of oceans, lakes, and rivers, some of the water changes from a liquid into a gas called water vapor. This vapor rises into the atmosphere, where the air is much colder. As the vapor rises and cools, it changes back into tiny drops of liquid water. This process is called condensation, and it is the reason clouds form in the sky. A cloud is really just a large group of very small water droplets floating together in the air.

When the droplets inside a cloud become too heavy to stay in the air, they fall back to the ground as precipitation. Precipitation can take several forms, including rain, snow, sleet, and hail, depending mainly on the temperature of the air near the ground. Most precipitation falls directly into the ocean, since oceans cover roughly seventy percent of the planet's surface. The rest falls on land, where it may soak into the soil, flow into rivers and lakes, or, in cold regions, remain frozen as ice and snow for long periods of time.

Water that lands on the ground does not stay in one place forever. Some of it is absorbed by plant roots and later released back into the air through a process called transpiration, which works in a similar way to evaporation. Other water flows downhill, through streams and rivers, eventually returning to the sea. A smaller portion sinks deep underground and becomes part of what scientists call groundwater, which can remain hidden beneath the surface for many years before slowly reaching a river, a lake, or the ocean again. In this way, the same water is recycled endlessly, moving between the sky, the land, and the sea.