The Water Cycle: How the Same Water Travels Through Earth

The water in a glass may once have been ocean spray, mountain snow, groundwater, a cloud droplet, or part of a living organism. Earth continually moves water among oceans, atmosphere, land, ice, soil, and life. This circulation is called the water cycle.

Evaporation and transpiration

Solar energy allows liquid water to become vapor and enter the atmosphere. Most evaporation occurs from oceans, but lakes, rivers, soil, and wet surfaces also contribute. Plants release water vapor through their leaves in a process called transpiration.

Ice and snow can sometimes change directly into vapor without first becoming liquid. This is sublimation.

Condensation and clouds

As moist air rises and cools, water vapor condenses onto tiny particles such as dust or sea salt. Vast numbers of droplets or ice crystals form clouds. A cloud is not water vapor itself; it consists mainly of visible liquid droplets and ice particles suspended in air.

Precipitation

Cloud particles collide and grow. When they become large enough, gravity brings them down as rain, snow, sleet, or hail, depending on atmospheric conditions.

Not all precipitation reaches rivers immediately. Snow can remain stored for months or years. Water can collect on vegetation, return to the atmosphere, soak into soil, or flow over land.

Infiltration and groundwater

Some water enters pores and cracks in soil and rock. It may replenish aquifers—underground formations that store and transmit water. Groundwater moves slowly and can emerge through springs or feed rivers during dry periods.

Groundwater is not usually a huge empty underground lake. It more often occupies spaces between sediment grains and fractures in rock.

Runoff and return to the ocean

Water that does not infiltrate may move downhill as runoff, entering streams, rivers, lakes, and eventually oceans. Urban pavement reduces infiltration and can increase rapid runoff and flooding, which is why drainage and permeable surfaces matter.

The cycle is not a perfect circle

Water can remain in different reservoirs for very different periods—from days in the atmosphere to thousands of years in deep groundwater or ice. Human activity changes the routes through dams, irrigation, groundwater pumping, deforestation, pollution, and climate change.

The water cycle is driven mainly by solar energy and gravity. The Sun lifts water into the atmosphere through phase changes, and gravity brings it back and moves it across land. It is one connected system, but the water available at a particular place and time is limited—which is why careful water management remains essential.