The Water Cycle

The Water Cycle

Water travels around the planet through the water cycle.
Credit: UCAR

Water is always on the move. Rain falling where you live today may have been water in a distant ocean days before. And the water you see in a river or stream may have been snow on a high mountaintop.

Water can be in the atmosphere, on the land, in the ocean, and even underground. It is recycled over and over through the water cycle.  As it moves through the water cycle, water often changes from a liquid, to a solid (ice), to a gas (water vapor).

Water that is at the surface of the ocean, rivers, and lakes can become water vapor in the air of our atmosphere with a little added energy from the Sun through a process called evaporation. Snow and ice can also become water vapor through a process called sublimation. And water vapor get into the atmosphere from plants by a process called transpiration.

As the water vapor rises in the atmosphere it cools, forming tiny water droplets high in the sky by a process called condensation. Those water droplets make up clouds. If they grow large enough they eventually become too heavy to stay in the air, falling to the ground as rain, snow, and other types of precipitation.

Most of the precipitation that falls becomes part of the ocean or part of the rivers, lakes and streams that eventually flow into the ocean. Some of the snow and ice that falls as precipitation stays at the Earth surface as a part of icy mountaintop glaciers or the ice sheets that cover places like Greenland and Antarctica. Some of the precipitation seeps into the ground and joins the groundwater that is often tapped by wells to provide water to farms, towns and cities.

Water stays in certain places longer than others. A drop of water may spend over 3,000 years in the ocean before moving on to another part of the water cycle while a drop of water spends an average of just eight days in the atmosphere before falling back to Earth.

© 2011 NESTA with modifications by UCAR