Earth's water is always on the move, between the oceans, the air, the land and living things. This endless loop is the water cycle, and two things power it: energy from the Sun and the pull of gravity.
- Evaporation: sunlight warms water, which turns into invisible water vapor. Plants also give off vapor through their leaves (transpiration).
- Condensation: rising air cools, and vapor condenses into tiny droplets or ice crystals: clouds.
- Precipitation: droplets grow and fall as rain, snow, sleet or hail.
- Runoff flows downhill into streams, lakes and the sea; infiltration soaks into the ground to become groundwater.
Key idea
The Sun's energy lifts water into the air by evaporation; cooling turns the vapor into clouds; gravity brings it back down as precipitation and pulls it downhill as runoff and groundwater.
Why it works
Turning liquid water into vapor takes a lot of energy, and sunlight supplies it, mostly over the oceans. Vapor rides up with rising, convecting air and cools, so it condenses into droplets. Gravity does the rest: it pulls drops out of clouds and pulls water downhill over and under the land. A water molecule spends on average about 9 days in the air, but about 3,000 years in the ocean.
Worked example: A puddle disappears
A puddle dries up on a sunny afternoon. Where did the water go, and what powered it?
- The water evaporated into the air as invisible water vapor.
- The energy came from sunlight warming the puddle.
Answer: Into the air as vapor, powered by the Sun
Worked example: Sun or gravity?
Which steps are driven by gravity: evaporation, precipitation, runoff, transpiration?
- Evaporation and transpiration lift water up as vapor: powered by the Sun.
- Precipitation falls and runoff flows downhill: pulled by gravity.
Answer: Precipitation and runoff
Common mistake
Thinking clouds are made of water vapor. Water vapor is an invisible gas, part of the air. Clouds are tiny drops of liquid water or ice crystals, which is why you can see them. Check: the "steam" above a kettle is visible because the vapor has already condensed into droplets; right at the spout, where it is still vapor, the air looks clear.