The water cycle is Earth's continuous process of moving water through the air, land, and oceans, constantly changing its form.

Key idea: The sun's energy powers the water cycle by turning liquid water and ice into water vapor, which then rises into the atmosphere.

The sun is the main engine of the water cycle. Its energy heats water in oceans, lakes, and rivers. This heat causes the water to turn into a gas called water vapor, a process called evaporation. You see this when a puddle dries up on a sunny day. Plants also release water vapor into the air through their leaves, a process called transpiration. Together, evaporation and transpiration are sometimes called evapotranspiration. Ice and snow can also turn directly into water vapor without melting first; this is called sublimation. All this water vapor rises into the atmosphere.

Key idea: Water vapor cools and condenses to form clouds, which then release water back to Earth as precipitation.

As water vapor rises higher in the atmosphere, the air gets colder. This cold air causes the water vapor to change back into tiny liquid water droplets or ice crystals. This process is called condensation. These tiny droplets or crystals clump together to form clouds. Think of how a cold drink glass gets water droplets on the outside; that is condensation. Clouds can travel long distances carried by winds, a process called advection. When enough water droplets or ice crystals gather in a cloud, they become too heavy and fall back to Earth as precipitation. This can be rain, snow, hail, or sleet. Most precipitation falls back into the oceans, but a lot also falls on land.

“As water vapor rises higher in the atmosphere, the air gets colder. This cold air causes the water vapor to change back into tiny liquid water droplets or ice crystals.”

Key idea: Water on land either flows over the surface into bodies of water or soaks into the ground, eventually returning to the oceans.

Once water falls on land, it can take several paths. Some water flows over the ground as surface runoff, eventually reaching rivers, lakes, and oceans. Some water soaks into the ground, a process called infiltration. This infiltrated water can become soil moisture, which plants use, or it can sink deeper to become groundwater. Groundwater can stay underground for a very long time, sometimes thousands of years, in underground storage areas called aquifers. Eventually, much of this water, whether it flowed over the surface or through the ground, makes its way back to the ocean, ready to start the cycle again.

Key idea: Water spends different amounts of time in various parts of the cycle, from days in the atmosphere to thousands of years in ice or underground.

The amount of time a water molecule spends in a particular part of the water cycle is called its residence time. For example, water vapor stays in the atmosphere for about 9 days before falling as precipitation. Water in soil stays there for a very short time. However, water in large ice sheets, like those in Antarctica, can stay frozen for hundreds of thousands of years. Groundwater can also remain underground for thousands of years. Scientists estimate residence times by dividing the total amount of water in a reservoir by how quickly water enters or leaves it.

“Groundwater can spend over 10,000 years beneath Earth's surface before leaving.”

Key idea: The water cycle is vital for life and Earth's geology, but human activities and climate change are causing significant and harmful disruptions.

The water cycle is incredibly important for life on Earth. It purifies water during evaporation, leaving behind salts and other particles. It brings fresh water to land through precipitation. It also shapes our planet's landscapes by moving minerals and causing weathering and erosion. Most importantly, it provides the water necessary for all living things and ecosystems to survive.

However, human actions are significantly changing the water cycle. Cutting down forests (deforestation), building cities (urbanization), and pumping too much water from underground (groundwater extraction) all affect how water moves and where it goes. Climate change, caused by human activities, is making the water cycle more intense. This means we are seeing changes in rainfall patterns, more extreme weather events like floods and droughts, and shifts in when and how much rain falls. These changes impact our water supply, farming, and the health of our planet.

“Human actions are greatly affecting the water cycle.”