Water cycle

The sun warms the water in our oceans.

Soon, the water falls back to the ground. It can fall as rain or snow. Much of this water flows into rivers. Some water soaks deep into the ground. This water stays in the soil for a long time. It can even stay under the ground for thousands of years. The water always moves in a big circle.

Water moves in a big circle around our planet. This is called the water cycle. The sun powers this whole way.

First, the sun heats up oceans and lakes. This causes evaporation. This is when liquid water turns into a gas called water vapor. The ocean is the main source for this. It provides 86% of the world's evaporation. Plants also help. They let out water through a way called transpiration.

Next, the water vapor rises high into the sky. As it goes up, the air gets colder. This causes condensation. This is when the gas turns back into liquid droplets. These droplets form clouds. When the drops get big, they fall as precipitation. This can be rain, snow, or hail.

Water then moves across the land. It can flow over the ground as runoff. It can also soak into the soil. This is called infiltration. Some water stays in deep underground pools called aquifers. This water can stay there for 10,000 years! Eventually, the water finds its way back to the ocean. The cycle starts again.

The water cycle is a huge, moving system on Earth. It is a way that water changes form and moves around our planet. Water can be a liquid, a solid like ice, or a gas called vapor. This cycle is very important for all living things. It helps keep life going by moving water to different places. Even though water moves constantly, the total amount of water on Earth stays mostly the same.

The sun powers the whole cycle with its heat. First, the sun heats oceans, lakes, and rivers. This causes evaporation, where liquid water turns into gas. The ocean is a huge part of this, providing 86% of all global evaporation. Plants also help by releasing water through transpiration. Sometimes, snow and ice turn directly into gas through sublimation. As this vapor rises, it cools down in the higher air. This cooling causes condensation, which turns the gas back into liquid droplets to form clouds.

Once clouds get heavy, water falls back to the ground as precipitation. This can be rain, snow, hail, or sleet. Most of this water lands in the ocean or on the land. On land, water moves in many ways. It can flow over the ground as surface runoff. Some water soaks into the earth through infiltration. This water can fill deep underground pools called aquifers. This water might stay hidden there for 10,000 years!

Water stays in different places for different amounts of time. Scientists call this residence time. For example, water stays in the atmosphere for only about 9 days. It moves much slower in other places. Water in a river might stay for only 2 to 6 months. However, water in the deep groundwater can stay for a very long time. Even ice in Antarctica can store water for 20,000 years. The oceans are the biggest storehouse, holding about 97% of all water.

Humans are changing how this cycle works. Activities like cutting down forests or building cities change the land. These changes can affect how much water soaks into the ground. Global warming is also making the water cycle more intense. This can change when and how much rain falls in different places. These shifts can make weather events more extreme. This matters because we rely on the water cycle for our food and our homes.

The water cycle, also known as the hydrologic cycle, is a continuous biogeochemical cycle. It involves the constant change in the form of water on, above, and below the Earth's surface. This cycle moves water between different reservoirs through various physical and chemical processes. While water moves constantly, the total mass of water on Earth remains fairly constant over time. However, how that water is partitioned into ice, fresh water, salt water, and atmospheric water can change based on climate.

The entire cycle is powered by energy from the sun. This energy drives several different processes that move water between states. First, solar radiation heats water in oceans, lakes, and rivers. This causes evaporation, where liquid water transforms into a gas called water vapor. The ocean is a massive driver of this process, providing 86% of all global evaporation. Another process is transpiration, which is the release of water vapor from plants and soil. When these two processes happen together, it is called evapotranspiration. Additionally, snow and ice can turn directly into gas through a process called sublimation.

As water vapor rises into the atmosphere, it undergoes a change due to physics. Water molecules have a smaller molecular mass than the main gases in our air, like nitrogen and oxygen. This lower density creates buoyancy, which helps drive the vapor higher into the atmosphere. As the vapor reaches higher altitudes, the air pressure decreases and the temperature drops. This cooling forces the water vapor to undergo condensation, turning it back into liquid droplets. These droplets are supported by updrafts to form clouds. If this condensation happens very close to the ground, it is known as fog.

Once cloud particles collide and grow, they fall to the Earth as precipitation. This can take many forms, including rain, snow, hail, sleet, or graupel. About 78% of all global precipitation occurs over the ocean. When precipitation hits land, it moves through several pathways. Much of it flows over the ground as surface runoff, eventually entering rivers and streams. Other water moves through infiltration, where it soaks into the ground. This water can replenish aquifers, which are deep underground layers that store freshwater. Some water stays near the surface as soil moisture before moving into larger bodies of water.

Water spends different amounts of time in these reservoirs, a concept known as residence time. This is a measure of the average age of the water in a specific area. For instance, water in the atmosphere has a very short residence time of about 9 days. Rivers hold water for only 2 to 6 months on average. In contrast, deep groundwater can remain in aquifers for 10,000 years, often called fossil water. The largest storehouse is the ocean, which holds approximately 97% of the planet's total water. The Earth's ice caps and glaciers also act as major reservoirs, storing about 1.7% of all water.

The water cycle does more than just move moisture; it shapes our entire planet. As liquid water flows, it transports minerals across the globe. It also reshapes geological features through weathering, erosion, and deposition. The cycle also acts as a natural purification system. During evaporation, water molecules separate from the salts and particles present in the liquid. Furthermore, the cycle helps regulate temperature. The ocean moves heat from the tropics toward the poles through ocean circulation. This movement of heat is a critical part of the Earth's climate system.

Human activities are now significantly altering these natural patterns. Changes in land use, such as deforestation and urbanization, affect how water moves across the landscape. Building cities creates impervious surfaces that change how much water can soak into the ground. Additionally, climate change is causing an intensification of the water cycle. Global warming is shifting precipitation patterns and increasing the frequency of extreme weather events. These changes impact everything from agriculture and water availability to entire ecosystems and human societies.