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The water cycle on Earth is a “closed system.” Water is very, very rarely lost to outer space. It never disappears.
That means the same water falls as rain over and over again. So, water that fell as rain on dinosaurs could also fall on you. Rain that just fell on you may fall on someone in Spain someday. How can the same water keep moving around in one big cycle? If we could follow a raindrop, we would see how this endless cycle works.
Follow the Dripping Raindrop
What happens to rain after it falls on you? Let’s follow one raindrop, and keep this rule in mind: Water always seeks the lowest point to which it can go. ▼
A raindrop bounces off your head and falls to the ground.
The raindrop seeps down through the surface soil. Below the surface, it meets the underground water (aquifer).
The raindrop joins an underground river. It flows downhill between the layers of rock beneath the ground.
The raindrop goes above ground. It gets carried upward by a spring of water that pops out of the ground in a valley.
The raindrop, within the spring waters, flows into a river. The river flows downhill into the ocean.
The raindrop in the ocean evaporates and becomes water vapor, a gas.
The water vapor rises high into the sky. As the air cools, the water vapor condenses into tiny drops of liquid water. The droplets cling to bits of dust in the air and form a cloud.
Wind blows the cloud toward Spain.
The tiny droplets join together to form larger drops. The drops get larger and heavier. If they become too large and heavy to remain in the air, they fall to Earth as rain.
In Spain, a raindrop bounces off someone’s head and falls to the ground.
Earth, the Water Planet
Rivers flow even in the driest deserts. However, the water is often deep under the ground. Oases are fertile areas in the desert. They reach deep below the sand surface to the underground water. Trees and other life-forms, including humans, can live in these low-lying islands of moisture. ▶
◀ The Earth is often called a “water planet” or “blue planet.” Hydrologists are people who study the Earth’s water. They put water into two types: surface water and groundwater. Surface water is found in rivers, lakes, ponds, glaciers, and oceans. But more water is below the ground than on the surface. Groundwater is water that soaks into the ground. It’s also rivers that disappear beneath the Earth’s surface. So, it’s any water that soaks into the ground and is trapped between layers of rock. This trapped water keeps filling underground aquifers (ponds, lakes, or rivers below the ground). Water also exists in the air as water vapor.
Why Does It Rain?
Some things float in water, and some things don’t. The same is true for air. The ice crystals and tiny droplets of water that make up clouds sometimes float in the sky. And sometimes they don’t. When the surrounding air is cold, the water droplets in a cloud join with the ice crystals. Eventually, the crystals become so heavy they fall to the ground. When the air is warm, water droplets collide with one another. Some merge together. The resulting drops grow larger and larger until they weigh so much they fall to Earth. And it rains. ▶
◀ How do rainbows form? Most of them form when sunlight passes through water droplets in the air after a storm.
A Waterlogged Nation
Bangladesh is a country on the Indian Ocean. It sits on the coast at a low level. That makes it a wet and often dangerous place to live. In May or June, heavy rains called monsoons flood the lowest places, even cities. Typhoons create big ocean waves that hit the coast. Typhoons are called “hurricanes” in the Western Hemisphere. In 1970, a huge tsunami (ocean wave) hit Bangladesh. It killed more than 200,000 people there. Millions of people lost their homes.
In the nineteenth century, some people said they were rainmakers. They told farmers they could actually make it rain. But they charged farmers a lot of money for that. Many of these traveling rainmakers were crooks. But today, farmers really can make it rain—sometimes. Airplanes drop small pieces of silver iodide into clouds. It is made up of tiny, pale yellow crystals. In the clouds, water particles stick to the silver iodide. Then they fall as rain. But, to make rain, there must be supercooled clouds—not blue skies. Dropping these crystals into clouds to make rain is called cloud-seeding. On the right is the “father of cloud-seeding,” Vincent J. Schaefer. He is shown doing cloud-seeding experiments in 1948. ▶
