Well, there’s always a certain amount of water vapor in the atmosphere. It comes from the Earth’s surface water, like oceans, rivers, and lakes.
Water from these sources evaporates, changing from a liquid to a gas.
Down here on the ground, you usually can’t see the water vapor that’s all around us. But higher up in the atmosphere, gaseous water cools down and condenses into clouds.
When water molecules in clouds collect into big enough drops, they fall on us as rain. Or as snow, sleet, or hail, depending on the temperature.
Water is constantly cycling from land to air and back again.As you might have guessed, this process is known as the water cycle.
Scientists study it because water’s such an important part of the ecosystem.
Every living thing on the planet depends on a supply of clean water!
Well, you can think of the water cycle as the Earth’s circulatory system.
Just like there’s a fixed amount of blood circulating through your body, there’s a fixed amount of water cycling around, over, and inside our planet!
All that water is known as the hydrosphere. The sun is the heart that pumps water around the hydrosphere.
Its heat energy breaks the bonds between water molecules, causing them to evaporate from oceans, lakes, rivers… even plants!
You’re right, water returns to the earth through precipitation, also known as rain, snow, sleet, and hail.
Sometimes, water flows across the surface of the Earth as something called runoff. Runoff adds to the flow of streams and rivers!
Or water can soak into the ground, in a process called infiltration.
Water that’s infiltrated the ground is called groundwater. It can stay beneath the surface anywhere from a few days to thousands of years!
This groundwater feeds wetlands, lakes and streams, and supplies us with water from drinking, farming, and lots of other stuff!
The water cycle’s been going on for billions of years, circulating the same supply of water over and over and over again.
The same exact water molecule a dinosaur drank from a pond 100 million years ago… Could be inside the glass of water you drink today!
And check this out: even though water is constantly moving through the hydrosphere, the amount in any one part of the cycle never changes!
Well, take the ocean, it contains about 97% of all the water in the hydrosphere.
It loses a huge amount to evaporation every day, but it makes up that same amount from groundwater, precipitation, and rivers.
The same goes for the other reservoirs, or storage places.
So, the amount of water in the atmosphere and the amount of water on the Earth’s surface stays perfectly balanced all the time!
It’s kind of like a big, beautiful machine!