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Rain comes and goes all over the Earth, never disappearing. In fact, the water cycle on Earth is a “closed system.”
Water is very, very rarely lost to outer space. This means that the same water falls as rain over and over again. So, water that fell as rain when the dinosaurs existed could be falling on you today. Rain that just fell on you may fall on someone in Spain someday and vice versa. How is it possible that the same water keeps 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? Find out by following the journey of one raindrop. As you do that, keep in mind the rule that water always seeks the lowest point to which it can go. ▼
A raindrop bounces off your head and falls onto the ground.
The raindrop seeps through the surface soil to meet with the underground water (aquifer).
The raindrop joins an underground river. It flows downhill between the sloping layers of rock beneath the ground.
The raindrop goes above ground again. 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 entire river flows downhill and empties 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 a person’s head and falls to the ground.
Earth, the Water Planet
Rivers flow even in the driest deserts. However, the water is often buried deep under the ground. Oases are fertile areas in the desert. They go deep enough below the sand surface to reach the underground water. Trees and other life-forms, including humans, can thrive in these low-lying islands of moisture. ▶
◀ The Earth is often called a “water planet” or “blue planet.” Hydrologists (people who study the Earth’s water) generally put water into two types: surface water and groundwater. Surface water is in rivers, lakes, ponds, glaciers, and the oceans. But more water is stored 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, or any water that soaks into the ground and is trapped between layers of rock. This trapped water recharges (or keeps filled) these 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. ▶
◀ Most rainbows form when sunlight passes through water droplets that stay in the air after a storm.
A Waterlogged Nation
Bangladesh is a low-lying country on the Indian Ocean. On the coast and at such a low level, it’s a wet and often dangerous place to live. In May or June, heavy rains called monsoons flood the lowest places, even cities. Typhoons create large ocean waves that wash over much of the coast. Typhoons are called “hurricanes” in the Western Hemisphere. In 1970, a tsunami (a gigantic ocean wave) hit Bangladesh. It killed more than 200,000 people there and left millions homeless.
In the nineteenth century, so-called rainmakers told farmers they could make it rain—for a big price. Many of these traveling rainmakers were crooks. But today, farmers truly can make it rain—sometimes. Airplanes drop small particles (crystals) of a pale yellow solid, called silver iodide, into clouds. The supercooled, vaporized water particles in the clouds stick to the silver iodide. Then they fall as rain. However, to make rain, there must be supercooled clouds—not blue skies. Dropping these crystals from airplanes into clouds to make rain is called cloud-seeding. At right, the “father of cloud-seeding,” Vincent J. Schaefer, does cloud-seeding experiments in 1948. ▶
