Your turn
Each lab is one part of the video. Tap, slide and see what happens.
Each lab isolates one part of the cycle. The models are simplified teaching sketches, not to scale.
Tap the buttons and slide the sliders.
Slide back in time. When did the dinosaurs live? Was the water already here?
Large amounts of water have likely flowed on Earth for about 3.8 billion years; dinosaurs lived from about 245 to 66 million years ago.
Slide back in time. The water was there with the dinosaurs!
Tap each place. How many of the 100 drops does it hold?
Compare the reservoirs: ocean, ice, groundwater, and everything else (under 0.1%).
Tap the sea, the ice and the ground. Where is most of the water?
Slide the sunshine up and down. How fast does the water rise?
More solar energy means faster evaporation. The rising, cooling vapor condenses into a cloud.
Make the Sun bigger. Watch the water float up to the cloud!
Speeds here are a simple teaching model.
Tap the cloud. Where does the rain go: into rivers, or into the ground?
Follow the water after it lands: runoff to rivers, infiltration to groundwater, and slow flow back to the sea.
Tap the cloud to make rain. Watch where the water goes!
Choose where your drop goes next. How long does the trip take?
Add up the typical residence times along any route you choose.
Pick where your drop goes next!
Times are typical stays, rounded.
Switch each change on and off, and warm the world. What happens?
A qualitative sketch: pumping, dams, paving and warming (about 7% more vapor per °C).
The bars are a simplified sketch model, not measurements.
The water on Earth goes round and round. It goes up into the sky, and it comes back down again. We call this the water cycle.
The warm Sun heats the sea. Tiny bits of water float up into the air. They are too small to see! The salt stays in the sea. Trees and plants let water out of their leaves, too.
High up, the air is cold. The tiny bits of water come together and make a cloud.
In the cloud, little drops bump and join. When they get heavy, they fall. That's rain, or snow when it is cold. Most rain falls back into the sea.
On land, rain runs down the hills into rivers. Some soaks into the ground, like water into a sponge. Slowly, the water goes back to the sea. Then the trip starts again!
We use the same water again and again, so let's take care of it.
The water cycle is the nonstop movement of water on, above and below Earth's surface. It has no starting point.2 Two things power it: the Sun, whose heat lifts water into the sky, and gravity, which pulls it back down and makes it flow downhill.1
In one sentence: the Sun evaporates water into invisible vapor, the vapor rises, cools and condenses into clouds, the clouds release precipitation (rain, snow, sleet or hail), and that water runs off into rivers or soaks into the ground until it reaches the sea and starts again.213
The surprising part is that it's the same water every time. Large amounts of water have likely flowed on Earth for about 3.8 billion years.4 Dinosaurs only appeared about 245 million years ago,18 so some of the water in your glass has almost certainly been around since before them. NASA puts it simply: rain falling today will someday fall again somewhere else.16
Textbooks list the stages in a neat order. Here they are, with what actually happens in each one.
| Step | What happens | What drives it |
|---|---|---|
| Evaporation | Liquid water at the surface of oceans, lakes and rivers turns into invisible water vapor.5 | Sun |
| Transpiration | Plants pull water up through their roots and release vapor through tiny pores (stomata) in their leaves.4 | Sun |
| Condensation | Rising air cools, and vapor turns back into tiny liquid droplets on specks of dust, salt and smoke. Billions of droplets make a cloud.6 | Cooling |
| Precipitation | Cloud droplets bump together and grow. Once heavy enough, they fall as rain, snow, sleet, freezing rain or hail.7 | Gravity |
| Runoff | Water flows over the ground, downhill, into streams and rivers.9 | Gravity |
| Infiltration | Water soaks into the soil and can seep deeper to refill aquifers.2 | Gravity |
| Groundwater flow | Underground water creeps slowly down and sideways and seeps into streams, lakes and springs.8 | Gravity and pressure |
There are also two shortcuts: sublimation, where ice or snow turns straight into vapor, and deposition, the reverse, which is how frost forms.12
Before water can move, it has to be somewhere. Picture all of Earth's water as 100 drops:34
Only about 2.5% of all Earth's water is fresh, and most of that is locked in ice or underground.3 The rivers and lakes that supply most of the water people use hold only about 0.007% of it.2 That's why the cycle matters so much: it keeps refilling this tiny, precious supply.
When the Sun warms the sea, some water molecules gain enough energy to escape into the air as vapor. The salt can't come along, so it stays behind in the sea. That's why rain is fresh even though most of it started in the ocean.517
About 90% of the moisture in the air comes from evaporation off oceans, seas, lakes and rivers. Most of the remaining 10% comes from plants, and a tiny bit from sublimating ice.45 Plants move more than you'd think: one acre of corn can release up to 4,000 gallons of water a day.4
Warm, moist air rises. As it rises it cools, and cooler air can hold less vapor, so the vapor condenses onto floating specks and becomes a cloud.64 A cloud is not vapor. Vapor is invisible, and clouds are made of liquid droplets or ice crystals.6
Cloud droplets are so small that they float. They collide and join, and it takes millions of them to build one raindrop. When drops get heavy enough, gravity wins and they fall.7
Most of that water lands right back in the ocean. Only about 10% of the water that evaporates from the ocean is carried over land and falls there.5 Over land, more water falls than evaporates, and over the ocean it's the other way round, which is why rivers keep flowing to the sea.4
On land, water takes one of two main routes. Some becomes runoff, flowing downhill into streams and rivers. Some infiltrates, soaking into the soil and sometimes seeping deep enough to recharge an aquifer.29 Groundwater isn't an underground lake: it fills pores and cracks in rock and soil, and it moves slowly, often feeding rivers and springs for months or years after the rain.8
The water cycle isn't one loop that every drop follows at the same speed. Each stop has its own typical "residence time":
| Where the water is | Typical stay |
|---|---|
| Atmosphere | About 9–10 days511 |
| Lake | Days to weeks (for a water molecule)2 |
| Ocean | About 3,000 years14 |
| Deep groundwater | Thousands of years2 |
| Ice caps | Thousands of years2 |
Because air holds water so briefly, all the water in the atmosphere is replaced about 40 times a year.4 Meanwhile a drop that sinks into the deep ocean or an ancient aquifer can be out of the loop for longer than recorded history.
The water cycle is Earth's delivery service for fresh water. Evaporation turns salty seawater into fresh vapor, and precipitation drops it onto land, refilling the rivers, lakes, soils and aquifers that people, farms, plants and animals rely on.15
It also moves heat. Evaporation cools the surface, and when vapor condenses into clouds it releases that heat higher up, which helps power weather.4 And the USGS points out that the water we can use depends on three things the cycle controls: how much there is, when it arrives, and how clean it is.1
People don't just use water; we change where it's stored and how it moves.1
The Sun warms water so it evaporates into invisible water vapor (plants add more by transpiration). The vapor rises, cools and condenses into cloud droplets. Droplets join into drops heavy enough for gravity to pull down as precipitation. On land the water runs off into rivers or infiltrates the ground to become groundwater, and both routes eventually lead back to the sea, where it can evaporate again.
A common list is evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater flow (sometimes called collection or storage). Real water doesn't follow one fixed order, though. A drop can skip stages, go round a short loop in days, or wait thousands of years in the ocean or in ice.
Two things drive it: energy from the Sun, which evaporates water and lifts it into the sky, and gravity, which pulls rain and snow back down and makes rivers and groundwater flow downhill.
The Sun lifts water into the air as vapor, the vapor cools into clouds, the clouds rain, and the rain flows back to the sea. Then it starts again. It is the same water going round and round, which is why it's called a cycle.
It is how fresh water gets delivered to land. The ocean is salty, but evaporation leaves the salt behind, so rain and snow are fresh. That refills the rivers, lakes and groundwater that people, plants and animals depend on. It also moves heat around the planet and shapes weather.
People pump groundwater from wells faster than rain refills it in some places, build dams and redirect rivers, and cover land with roofs and roads so rain runs off instead of soaking in. A warming climate also lets the air hold about 7% more water vapor per degree Celsius, which brings heavier downpours to some places and drier soils to others.
Large amounts of water have likely been flowing on Earth for about 3.8 billion years. NASA notes that much of it first arrived at the surface through ancient volcanoes. Since then, the same water has mostly been recycled by the water cycle.