What Is the Water Cycle?

Water can be found all over Earth in the ocean, on land and in the atmosphere. The water cycle is the path that all water follows as it moves around our planet.

A green box with icons and white text. Text reads: Where is Earth's Water? 96.5 percent is in the oceans; 1.7 percent is in lakes, rivers, streams, and soil; 1.7 percent is in polar ice caps, glaciers, and permanent snow; 0.001 percent is in water vapor in Earth's atmosphere

Credit: NASA/JPL-Caltech Data source: NASA's Earth Observatory

On Earth, you can find water in all three states of matter: solid, liquid and gas. Liquid water is found in Earth’s oceans, rivers, lakes, streams—and even in the soil and underground. Solid ice is found in glaciers, snow, and at the North and South Poles. Water vapor—a gas—is found in Earth’s atmosphere.

How does water travel from a glacier to the ocean to a cloud? That’s where the water cycle comes in.

The Water Cycle

An illustration of the water cycle showing how water travels from rivers and streams to clouds to snow and back again

Credit: NASA/JPL-Caltech

The Sun’s heat causes glaciers and snow to melt into liquid water. This water goes into oceans, lakes and streams. Water from melting snow and ice also goes into the soil. There, it supplies water for plants and the groundwater that we drink.

How does water get into the atmosphere? There are two main ways this happens:

  • Heat from the Sun causes water to evaporate from oceans, lakes and streams. Evaporation occurs when liquid water on Earth’s surface turns into water vapor in our atmosphere.
  • Water from plants and trees also enters the atmosphere. This is called transpiration.

Warm water vapor rises up through Earth’s atmosphere. As the water vapor rises higher and higher, the cool air of the atmosphere causes the water vapor to turn back into liquid water, creating clouds. This process is called condensation.

When a cloud becomes full of liquid water, it falls from the sky as rain or snow—also known as precipitation. Rain and snow then fill lakes and streams, and the process starts all over again.

A photograph of white clouds against a blue sky over the savannah in Kenya

Clouds, like these over the savannah in Nairobi, Kenya, form when water vapor in the atmosphere condenses back into liquid water. Credit: Department of State

Why Do We Care About the Water Cycle?

We care about the water cycle because water is necessary for all living things. NASA satellites orbiting Earth right now are helping us to understand what is happening with water on our planet.

an illustration of a green plant sprout growing out of the soil

Water in the Soil

Humans need water to drink, and to water the plants that grow our food. NASA has a satellite called SMAP—short for Soil Moisture Active Passive—that measures how much water is in the top 2 inches (5 cm) of Earth’s soil. This can help us understand the relationship between water in the soil and severe weather conditions, such as droughts.

an illustration of water vapor droplets floating in the atmosphere

Water in the Atmosphere

NASA’s CloudSat mission studies water in our atmosphere in the form of clouds. CloudSat gathers information about clouds and how they play a role in Earth’s climate. Also, the international satellite called the Global Precipitation Measurement Mission (GPM) observes when, where and how much it rains and snows on Earth.

an illustration of a dolphin jumping out of ocean waves

Water in the Oceans

As Earth’s climate becomes warmer, land ice at the North and South Poles starts melting. The water then flows into the ocean, causing sea level to rise. NASA’s Jason-3 mission—short for Joint Altimetry Satellite Oceanography Network-3—orbits Earth collecting information about sea level and ocean temperature. This helps track how the ocean responds to Earth’s changing climate.

NASA is also tracking how Earth’s water moves all around our planet. This is the work of the GRACE-FO—or Gravity Recovery and Climate Experiment-Follow On—mission. It tracks the movement of water from one month to the next, and can even measure changes in deep groundwater hundreds of feet below Earth’s surface.

NASA’s Aqua satellite also collects a large amount of information about Earth’s water cycle, including water in the oceans, clouds, sea ice, land ice and snow cover.